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# A Quarter of Sun-Like Stars Host Earth-Size Worlds

#### Soulskill posted about a year ago | from the but-can-we-drill-for-oil-on-them? dept.

105

astroengine writes "Although there appears to be a mysterious dearth of exoplanets smaller than Earth, astronomers using data from NASA's Kepler space telescope have estimated that nearly a quarter of all sun-like stars in our galaxy play host to worlds 1-3 times the size of our planet. These astonishing results were discussed by Geoff Marcy, professor of astronomy at the University of California, Berkeley, during a talk the W. M. Keck Observatory 20th Anniversary Science Meeting on Thursday. '23 percent of sun-like stars have a planet within (1-2.8 Earth radii) just within Mercury's orbit,' said Marcy. 'I'll say that again, because that number really surprised me: 23 percent of sun-like stars have a nearly-Earth-sized planet orbiting in tight orbits within 0.25 AU of the host stars.'"

cancel ×

### Great! (1)

#### Anonymous Coward | about a year ago | (#43186373)

Lots of Goldilocks and no bears.

### Re:Great! (5, Informative)

#### Greg01851 (720452) | about a year ago | (#43186427)

Actually, "within 0.25 AU" puts them too close to their star to be habitable... i.e. not in the goldilocks zone :( PS 1 A.U. is the distance of the Earth to the Sun, just in case you didn't/don't know.

### Re:Great! (1, Funny)

#### sycodon (149926) | about a year ago | (#43186517)

PS 1 A.U. is the distance of the Earth to the Sun, just in case you didn't/don't know.

Doesn't that make us "earthists"? It's like the Interstellar equivalent to the Imperial system.

### Re:Great! (0)

#### Anonymous Coward | about a year ago | (#43186615)

Doesn't that make us "earthists"?

Yay. More self-flagellation material.

### Re:Great! (1)

#### sycodon (149926) | about a year ago | (#43186663)

Sorry, forgot the bemused sarcasm tags, whatever those would be.

### Re:Great! (0)

#### Anonymous Coward | about a year ago | (#43186791)

Ever wonder why the distance from north pole to equator is 10,000 km?

I recommend you get a copy of The Handy Physics Answer Book [amazon.com] before making any more attempts at science humor.

### Re:Great! (1)

#### DigiShaman (671371) | about a year ago | (#43188001)

Yes. And until we make first contact with any intelligent alien species, who cares?!

### Re:Great! (0)

#### Anonymous Coward | about a year ago | (#43189621)

PS 1 A.U. is the distance of the Earth to the Sun, just in case you didn't/don't know.

Doesn't that make us "earthists"? It's like the Interstellar equivalent to the Imperial system.

Yes, and that will cause all sorts of trouble when we start with interstellar trade. We will probably have to change to universal units when that happens.

### Re:Great! (3, Insightful)

#### Anonymous Coward | about a year ago | (#43186597)

It depends upon the size and temperature of the star - a planet that is 0.25 AU from a star half the size or half the "temperature" of the sun may very well be in the goldilocks zone of the star. (Remember the inverse square law!) But in this case, it looks like they are talking about earth-sized planets that are within 0.25 AU of sun-sized stars, and those are not in the goldilocks zone - but they are also a lot easier to find than earth-sized planets in the goldilocks zone are (the inverse square law strikes again!). So the question is, what does this finding suggest about how common terrestrial (i.e., non-Jovian) planets with relatively round orbits in the goldilocks zone are?

### Re:Great! (1)

#### mcgrew (92797) | about a year ago | (#43190463)

I suspect life is rare outside this world, but I'd love to be wrong about that. We've found evidence that our closest neighbor was once hospitable to life, but yet have not found evidence of any life ever being there (we may still, most of it is unexplored). It may be that life itself is a fluke. It's also possible that we're first, or that it only happens once in a billion years in any given galaxy. Or we may find life and not recognize that it is, in fact, alive.

We don't know a lot more than we do know.

### Re:Great! (1)

#### utoddl (263055) | about a year ago | (#43191349)

We've found evidence that our closest neighbor was once hospitable to life

Mexico?

### Re:Great! (0)

#### Anonymous Coward | about a year ago | (#43187293)

I've read an article about this last week, there they were talking about smaller suns which are close by. The smaller suns have their habitual zone at 0.25 AU and their goldilocks zone are bigger therefor have more chance of having a planet inside it.

### Re:Great! (4, Insightful)

#### thomst (1640045) | about a year ago | (#43188181)

Greg01851 noted:

Actually, "within 0.25 AU" puts them too close to their star to be habitable... i.e. not in the goldilocks zone :(PS 1 A.U. is the distance of the Earth to the Sun, just in case you didn't/don't know.

Yep. Important datum, that.

However ... since this announcement ONLY applies to 1-3 Earth-mass planets within .25 AU of G-type stars (because it's the result of occulation observations, and that's the limit of resolution for any current telescope), it says nothing whatsoever about Earth-ish planets that obit in the "Golidlocks zone". OTOH, I think it's not unreasonable to extrapolate that, if there're appropriately-sized worlds in too-close orbits around that high a proportion of G-type stars, there's a pretty good likelihood that there're just as many (or more) in the zone where life could evolve.

Perhaps we'll find out when/if the James Webb telescope is launched.

Exciting stuff, regardless.

### Re:Great! (1)

#### Shavano (2541114) | about a year ago | (#43188231)

But that does not preclude many more planets further out. Planets orbiting close to their stars are are easy to detect with Kepler. It has a harder time finding stars that are at Earthlike distances because such planets are much less likely to transit while Kepler is watching.

### Re:Great! (0)

#### Anonymous Coward | about a year ago | (#43189851)

I for one cannot wait to meet these extremely hot alien chicks!

### Re:Great! (1)

#### gatkinso (15975) | about a year ago | (#43186619)

Plenty of bears I am sure. Hopefully we are the biggest and meanest of them.

### Re:Great! (0)

#### Anonymous Coward | about a year ago | (#43188051)

The biggest and meanest bears aren't the ones you have to worry about. They don't even know how to bang two rocks together let alone throw one. Keep your distance from their cage and you'll be fine. It's the smart ones you have to be wary of. They'll figure out how to open the door and get at you when you're not looking.

### Re:Great! (0)

#### Anonymous Coward | about a year ago | (#43190001)

They'll figure out how to open the door and get at your pic-a-nic baskit when you're not looking.

FTFY

### Re:Great! (1)

#### slick7 (1703596) | about a year ago | (#43188439)

Plenty of bears I am sure. Hopefully we are the biggest and meanest of them.

It's not the bears I'm worried about but the snakes and wolves. As for the biggest, meanest and the smartest, I unfortunately doubt it.

### Re:Great! (1)

#### Sussurros (2457406) | about a year ago | (#43187827)

So any planets outside the goldilocks zone are unbearable?

There's no way to know for sure but I've often wondered if Earth being a binary plant and the warmer Earth core from the tidal force of the Moon affects where the goldilocks zone is. It'll certainly have made a difference on the volcanoes that broke snowball earth and the resultant explosion of complex life, and also on the intertidal zone that must have been so important when life later moved from water to land.

There are two binary planets in our solar system out of perhaps twenty in total so very roughly that hopefully indicates that 10% of these exoplanets might have suitable moons. That's a very promising percentage when looking for complex life in other systems.

### Why the Surprise? (2)

#### sycodon (149926) | about a year ago | (#43186385)

As Carl said, "...billions and billions..."

### Re:Why the Surprise? (2)

#### Greg01851 (720452) | about a year ago | (#43186431)

But Mr. Sagan didn't have any proof at the time... now there's much more evidence :)

### Re:Why the Surprise? (2)

#### gmuslera (3436) | about a year ago | (#43186705)

Even if Earth-sized planets were not as common as this study say, just with the raw amount of galaxies in the universe you should have billons of those planets anyway.

### Re:Why the Surprise? (1)

#### G3ckoG33k (647276) | about a year ago | (#43186525)

The submitter's "I'll say that again, because that number really surprised me: 23 percent of sun-like stars have a nearly-Earth-sized planet orbiting in tight orbits within 0.25 AU of the host stars.'" and your "As Carl said, "...billions and billions..."" don't mix well with all people.

Many people get wound up for an aware agenda or an unaware one. Why? I have no serious idea, but I think that the lack of religion today, in general, coupled with the the non-scholar yet educated wish for sensation among scientists or slashdot-readers is part of the answer.

Yes, with the media hype over a new pope the last few days I think that says it all. There are more ignorant people than ever, both in absolute numbers and in relative terms. We are living in dangerous times.

Not my two-cents, but two Euros.

There is no fundamental reason why one quarter of all sun-like stars shouldn't have Earth-size objects fairly close to them, according to any theory I am aware of.

### Re:Why the Surprise? (0)

#### Anonymous Coward | about a year ago | (#43186577)

There is no fundamental reason why one quarter of all sun-like stars shouldn't have Earth-size objects fairly close to them, according to any theory I am aware of.

No one said with any solid evidence that such planets could exist, but while we were still arguing over solar system formation models, no one was quite sure how many would exist. The enthusiasm comes not from this contrasting some previous theory. This is instead exciting because it would lead to a lot of interesting things. Turns out how many people wanted the universe to be is how it is. I don't know why you would be surprised that others would be surprised it worked out that way.

### Re:Why the Surprise? (0)

#### Anonymous Coward | about a year ago | (#43186583)

Damn...how late is it over there? Because you clearly need some sleep.

23:07...

### Re:Why the Surprise? (2)

#### bware (148533) | about a year ago | (#43187239)

There is no fundamental reason why one quarter of all sun-like stars shouldn't have Earth-size objects fairly close to them, according to any theory I am aware of.

There's no reason why they should, either. Thus, observational science. To find out.

A decade ago, we simply did not have any idea of what \eta_{earth} was. 0.01? 0.1? 1.0? No idea. Now we do. That's pretty cool.

To me, this, along with cold dark matter and dark energy, are the quantum theory and general relativity of our time. We know that we don't really understand the universe, but we have inklings of what to look for. It's a good time to be a scientist. Well, except for shrinking budgets - this kind of science, so far, requires big bucks.

### Re:Why the Surprise? (0)

#### Anonymous Coward | about a year ago | (#43188155)

...blah blah blah...
Not my two-cents, but two Euros.

We owe you quite a lot of change.

### Re:Why the Surprise? (1)

#### tqk (413719) | about a year ago | (#43189205)

I have no serious idea, but I think that the lack of religion today ...

So, you're demonstrably an idiot. Understood. Carry on.

### Re:Why the Surprise? (1)

#### gatkinso (15975) | about a year ago | (#43189779)

You mean your two cent, I think.

### Re:Why the Surprise? (1)

#### angel'o'sphere (80593) | about a year ago | (#43190287)

There is no fundamental reason why one quarter of all sun-like stars shouldn't have Earth-size objects fairly close to them, according to any theory I am aware of.
Then you should make yourself more aware about solar system forming theories.

I only know *one* the one that was taught in school or was written in science magazines in the 1970s.

According to *that* theory rocky planets close to the star should be very common. In fact every *cloud* of *dust* with similar size and composition like the cloud our solar system formed from should end up in a similar solar system. Two to four rocky planets relatively close and the rest water/gas planets relatively far out.

The reason is simple, the cloud is heating up and compressing due to gravity. Depending on density the raw elements favour a particular distance around the center. Imagine an oil refinery. The thicker and heavier/denser the material the closer it is to the sun, like the heavy tar and oil at the bottom of the refinery. The lighter/thinner/less dense the stuff is (like gases and water etc.) the farer out they are or the more at the top of the refinery, like gasoline and other light oils.

### Re:Why the Surprise? (0)

#### Anonymous Coward | about a year ago | (#43186587)

And Drake worked out the math for it.

The Drake equation states that:

N = R^{\ast} \cdot f_p \cdot n_e \cdot f_{\ell} \cdot f_i \cdot f_c \cdot L

where:

N = the number of civilizations in our galaxy with which communication might be possible (i.e. which are on our current past light cone);

and

R* = the average rate of star formation per year in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
f = the fraction of the above that actually go on to develop life at some point
fi = the fraction of the above that actually go on to develop intelligent life
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space[5]

### The Drake Equation is Stupid. (1)

#### Anonymous Coward | about a year ago | (#43187199)

One unknown represented as the product of many unknowns is not progress, nor is it math. Ockham would slit his throat.

### Re:The Drake Equation is Stupid. (0)

#### Anonymous Coward | about a year ago | (#43187611)

You're right... clarifying what you need to know to work out a problem... that has NO place in science at all.

### Re:The Drake Equation is Stupid. (0)

#### Anonymous Coward | about a year ago | (#43188331)

The point is that several of the unknowns are just as hard, if not harder to ascertain than the quantity you're looking for.

fi, fc and L can only be determined if when you know a significant number of alien civilizations, at which point you might as well just see what the ratio of civs to stars is and work from there.

Using the Drake equation is like using atomic masses to calculate the mass of your car: It's possible to do things this way, but it's downright stupid to try it as a practical matter.

### Re:The Drake Equation is Stupid. (0)

#### Anonymous Coward | about a year ago | (#43188233)

"Ockham would slit his throat."

Utilizing the principle of Occam's Razor, I'll take that to be Occam...

### Re:The Drake Equation is Stupid. (1)

#### tragedy (27079) | about a year ago | (#43188735)

While William of Ockham lived at a time when spelling was not exactly standardised, it would seem that Ockham is correct, and not Occam, no matter how many times it's been spelled that way. He is almost certainly named for the village of Ockham (name recorded as "Bocheham" nearly a thousand years ago). The "ham" at the end is from old French/German/English for "home" or "village". See, for example, the word "hamlet". So, since he's named after a village called Ockham, and the village, quite correctly, has "ham" at the end, it should be Ockham's Razor. Aside from needing to have "ham" at the end, there are some good pronunciation/spelling reasons for why it shouldn't be Okham or Occham.

### Re:The Drake Equation is Stupid. (1)

#### Patch86 (1465427) | about a year ago | (#43189451)

I believe it is usually accepted that his surname at birth was "Ockham". Most philosophers at that time would "latinize" their name when publishing writings; hence he latinized his name to "Occam". So both are correct, depending on whether you want to use the Latin or English version of his name.

### Re:Why the Surprise? (0)

#### Anonymous Coward | about a year ago | (#43186813)

that's not what he said [imdb.com]

### But... (2)

#### war4peace (1628283) | about a year ago | (#43186389)

...that's translated as "lots of stars have planets in orbits which can in no way sustain life". Dims my hopes rather than the other way around.
Also: would that not decrease the chance of planets in goldilocks range overall, since planet material in that system was partly used to give birth to close orbiters?

### Re:But... (3, Informative)

1: the majority of stars are smaller and dimmer than the sun, .25 AU is not necessarily out of the "green" zone for the most common dwarf stars.

2: earth sized planets further out from stars can not be reliable detected using current technology and processes. The fact that the earth sized planets that we can detect are plentiful does indicate that the earth sized planets we can not detect are not plentiful. Recall that the first few exo-planets were much larger than Jupiter and much closer than earth. We are constantly expanding the lower limit of mass and higher limit of distance that we can detect effectively.

### Re:But... (2)

#### CrimsonAvenger (580665) | about a year ago | (#43186629)

1: the majority of stars are smaller and dimmer than the sun, .25 AU is not necessarily out of the "green" zone for the most common dwarf stars.

TFA uses the phrase "sun-like stars" a lot. It doesn't get more specific than that.

It's certainly possible they're talking about "all dwarf stars", but it's not really a good way to bet.

### Re:But... (0)

#### Anonymous Coward | about a year ago | (#43188769)

"Sun-like stars" is not very specific at all. Do they mean all yellow dwarfs, or all companion-free yellow dwarfs, etc.? Even the more specifically defined term, solar type star, encompasses up to 10% of stars in our galaxy. Solar analog and solar twin categories place tight constraints of temperature and metallic, but still easily allow for more than a 50% difference in luminosity.

### Re:But... (0)

#### Anonymous Coward | about a year ago | (#43190083)

If you cannot detect it it isn't there? Dang, i thought ghost hunters was real.
But to ask the perenial question. One that I've thought about for ages, at least a moment or two. Telescopes are a form microscope, Are you sure you are not seeing an form of an electron many times maginified, And the quantium effect of distance, slowing the movement rate to miniscuile? After all the magical sky being would put all sorts of puzzles out there to keep you entertained. And that may be one of them.

### Re:But... (1)

Get some reading comprehension. The frankly idiotic conclusion you draw is exactly the opposite of what I wrote.

### Re:But... (5, Interesting)

#### cnettel (836611) | about a year ago | (#43186563)

Well, we do have Mercury and Venus in our system and that hasn't hurt us, has it? (Yeah, Mercury is small, but Venus is also on the too-close side even without greenhouse gases and almost Earth-size.)

I guess the point with Kepler is still that due to the methodology (repeated occlusions), shorter orbital periods will increase the chance of detection (more data points to establish significance), in addition to the fact that a planet closer to its host star will occlude a larger area and thus give a stronger signal. Just keeping Kepler going will increasingly shift the distribution of detected planets towards higher star-planet distances. The minimum detectable size will be more or less of a constant function of that distance, though, although again I guess repeated observations can sometimes bring out something that would otherwise be just at the noise floor.

For reference, Kepler has just completed 4 years of operation, but actual planet detection only started on May 12 2009. If you want three confirmed events, you could per definition not yet have detected e.g. an exo-Mars. It simply hasn't passed by three times yet. If the orbital plane is different, the planet might not pass in our line of sight every time, and then working out the period and get a detection can take even longer.

Just wait and see.

### Re:But... (3, Informative)

#### angel'o'sphere (80593) | about a year ago | (#43190331)

Most people don't understand that we can only find (with the current way how we do detection) very very few planets. Perhaps 1/300 or even less (more likely 1/900) of the systems can be observed in a way that reveals planets.

We can only detect a planet if his orbit plane is cutting the star like this: -o-
Ofc you can turn this now clockwise or counterclockwise, the cut does not need to be horizontal.

However we can not detect any planet in a solar system that looks like a cut up onion to us: the star in the middle and the planets orbiting on the rings around it (because the planerts are to dim to see directly, and they never obscure the star)

### Re:But... (1)

#### MartinSchou (1360093) | about a year ago | (#43186593)

Why does it dim your hopes? We already knew that Earth was pretty rare (1 in 8 planets in our solar system) before discovering extra solar planetary systems.

Currently we know of 861 extra solar planets, which moves our rarity to 1 in 869. With an estimated 100 billion to 400 billion extra solar planets in the Milky Way, that becomes quite a few Earth-like planets.

If an Earth-like planet is a million to one, then it's between 100,000 and 400,000. If it's a billion to one, then it's between 100 and 400.

And that's without considering the estimated 170 billion galaxies in the observable universe and the billions of years that planets and life have to develop after our observations.

### Re:But... (4, Insightful)

#### Immerman (2627577) | about a year ago | (#43187855)

Umm, what? If anything our solar system suggests Earth-like planets are very common - we have three of them here including Mars and Venus. Only Earth is firmly in the "Goldilock zone", but you can only reasonably expect one planet to fall into that zone around any given sunlike star, *maybe* two if they fall near opposite extremes.

Given that current detection methods can't yet reliably detect a planet the size and distance of Earth the fact that we're detecting lots of larger, closer planets in no way detracts from the possible commonness of exo-Earths, it just means we're detecting lots of planets that are easy to detect, and can now say that ~1 in 4 sunlike stars has something like a Venus or Mercury - if our system is at all typically I'd expect such stars to also have a good chance of having additional Earthlike planets further out, we just can't detect them yet without being extremely lucky.

### Re:But... (3, Interesting)

#### Anonymous Coward | about a year ago | (#43188263)

You might want to throw Europa in there. Not a planet, and not in the Goldilocks Zone - but it's close enough to the right size, and tidal forces contribute enough heat to possibly put it in a 'Goldilocks Emeritus' category.

### Re:But... (2)

#### Tablizer (95088) | about a year ago | (#43187493)

Close to the sun means that alien babes will be wearing really really skimpy bikinis.

### Seems like useless info (1)

#### YodasEvilTwin (2014446) | about a year ago | (#43186409)

As we all know, Mercury is not exactly hospitable to life. How many Earthlike planets are orbiting Sunlike stars in more Earth/Mars sized orbits?

### Re:Seems like useless info (1)

#### gatkinso (15975) | about a year ago | (#43186659)

I will guess 1 % of K and G type stars. Which means within 100 LY there would be about 20 such worlds.

### Re:Seems like useless info (3, Insightful)

#### 0111 1110 (518466) | about a year ago | (#43187399)

What are you basing that guess on?

### Re:Seems like useless info (1)

#### gatkinso (15975) | about a year ago | (#43189785)

Based on the fact that there are approximately 512 G and 1540 K class stars within 100 LY of earth. (You can of course Google for yourself).

512 + 1540 = 2052 *.01 = 20

### Re:Seems like useless info (1)

#### gatkinso (15975) | about a year ago | (#43189797)

Oh PS as far as the 1% goes... that is just a guess... straight from the bung hole. Which of course means the 20.5 figure is no better.

Statistically speaking there is no way for us to know what the figure of merit is, so 50% would be the only appropriate guess. I just intentionally low-balled it.

### Not an astronomer, but... (0)

#### Anonymous Coward | about a year ago | (#43186471)

Could it be that they're detecting none smaller than earth because we simply don't have the detection capability to go that small? Perhaps Earth is on the small side of rockies, despite being the largest rocky planet in our solar system? I guess I'm saying that "the mysteriosu dearth of planets smaller than earth" may simply be a detection/technology issue.

### We're only detecting the low-hanging fruit for now (2)

#### Immerman (2627577) | about a year ago | (#43187739)

Absolutely. Present technology is strongly biased towards detecting large planets orbiting close to their stars in a plane we're looking at nearly edge-on. This is a recognized weakness among astronomers, and means that planets that depart from any of those criteria will be less likely/take longer to be detected. It typically take at least 3-5 orbits worth of observations to confirm a planet detection, and smaller or more distantly-orbitting planets will be harder to detect (lower signal-to-noise ratio), so more orbits are required for confirmation. Something like an exo-Jupiter with it's multiple-century orbit won't be directly* detected for a thousand years or so using current technology, despite it's large mass. And an exo-Earth with it's small signal and longer year will take much longer to detect than say an exo-Mercury.

We can make educated guesses about what other system are actually like, but for the immediate future the only planets types we can make any sort of statistical extrapolation about are the kinds that are easiest to detect. On the bright side as the length of observation increases not only can we detect more planets directly, we can also more accurately characterize the orbits of previously detected planets, including the perturbations caused by other planets in the system too small or slow to detect directly.

* technically what I'm calling direct detection is via the Doppler shift it causes in its star's spectrum

### Mysterious Dearth? (0)

#### Anonymous Coward | about a year ago | (#43186473)

Although there appears to be a mysterious dearth of exoplanets smaller than Earth

-Our ability to detect smaller planets is improving, but is still limited.
-Many systems we have detected have Jupiter-mass or larger bodies orbiting very closely to the parent star. These huge planets are unlikely to have formed there. They could easily have "gobbled up" smaller planets when they moved inward towards the star.
-Forming solar systems are chaotic with tons of protoplanets. The smaller ones can easily combine or be smashed, absorbed, or thrown out of the system altogether.

### Is this good news or bad news? (0)

#### Anonymous Coward | about a year ago | (#43186475)

The news sounds bad... All those potential systems are now basically useless to us and probably wouldn't harbor life.

### they can't find any smal planets (1)

#### ozduo (2043408) | about a year ago | (#43186489)

Because the death star has been busy!

### I'll say that again (0, Funny)

#### Anonymous Coward | about a year ago | (#43186557)

And now you've said it four times. You said the same thing four times. Four times. The entire summary is just the same statement made four times.

### Re:I'll say that again (0)

#### Anonymous Coward | about a year ago | (#43187047)

"And now you've said it four times."

At least she said before:
'Listen carefully! I shall say this only once.'

IOW it's a fucking 'Allo, allo' reference.

### Zomg watch out (0)

#### Anonymous Coward | about a year ago | (#43186605)

Big Bird stands up to a third taller than the average adult. Should I tremble in fear?

### Artifact of our technology (2)

#### gatkinso (15975) | about a year ago | (#43186683)

In 30 years we will be able to detect planetesimals smaller than the moon orbiting stars out to 300 LY. This is of course just a guess.

### Maybe those Mormons were on to something (-1)

#### Anonymous Coward | about a year ago | (#43186717)

They claim that all the faithful (men) get their own planet in the next life. But take it with a grain of salt when it was handed to you by an angel named moroni and a guy hallucinating a prophetic book he found under a rock in upstate New Amsterdam.

### Streetlight effect? (3, Interesting)

#### gmuslera (3436) | about a year ago | (#43186755)

How much harder would be to find planets of those sizes if they were at a bigger distance from their sun?

### Re:Streetlight effect? (5, Informative)

#### bjorniac (836863) | about a year ago | (#43187161)

The answer is that it's not much more difficult, but a lot more time consuming (gleaned from going to talks on the subject, not my area of expertise).

There are two basic ways that these planets are observed: They make the stars they orbit wobble (the basic 2 body problem - each body orbits the center of mass of the pair) and they dim the light from the star when they pass in front (like an eclipse).

The time problem comes from the fact that orbits are longer for objects more distant from the star. If we make the simplification that the orbit of the planet is basically circular, the time period for an orbit increases as radius^(3/2). (Insert semi-major axis for radius for non-circular). The standard is about three events separated by equal times to count as an observation - you have to wait to see an event at least twice to know the time period and so infer the radius of orbit, and once again to remove some flukes. Hence you're having to wait a long time looking at a star to see this happen.

Now, on top of that you've got the possibility that there's more than one planet, that the earth-like planet isn't the dominant mass, etc etc. This can all be cleverly dealt with (multiple wobbles, multiple eclipses) but it adds time to the confirmation process.

To give an example: Suppose you were somewhere near Proxima Centauri, and making the relevant observation looking for Earth. It would take at least three years to detect Earth, even if your telescope was amazing. Dynamics of the system would pick up the effect of Jupiter on the sun first, for the wobble detection (you wouldn't get much eclipse given the angle between the plane of the solar system and the position of PC) and it might take quite some analysis to pick up Earth at all given the effects of all the other planets.

Anyway, I'm sure some astro people can give a much better version of all this. Suffice to say that we aren't looking for Earth like planets at Earth like radii yet, but I imagine over the next ten to twenty years there will be a lot of poor graduate students analyzing data desperately looking for Gallifrey.

### Re:Streetlight effect? (0)

#### Anonymous Coward | about a year ago | (#43188785)

It is not just the longer orbital period. The further the planet is from the star, the much smaller the possible angles the orbital plane can make relative to our view such that the planet actually goes in front of the star. A planet 1 AU from a star about the same size as our Sun would need its orbital plane to be within a half a degree range of tilts to actually go in front of the star. The Doppler shift method doesn't require direct transit, but sensitivity drops the more tilted the orbit is relative to the view too.

### Re:Streetlight effect? (2)

#### angel'o'sphere (80593) | about a year ago | (#43190427)

The answer is that it's not much more difficult, but a lot more time consuming (gleaned from going to talks on the subject, not my area of expertise). Correct and not correct.

Lets focus on the correct part first:
We can only detect planets that transit regularly their star. That means the ecliptic is somehow perpendicular to our view.

Try this experiment: take a pice of paper, draw a sun in the middle and a few rings around it.

Now hold the sheet in two hands in front of you that you only can see the edge of the paper. That means the paper is e.g. horizontal in the height of your eyes. You an turn it in any angel you want as long as you only see the edge: -- or / or \ etc.

In this situation you could find any planet, if you watch long enough, e.g. to find a pluto like planet and if you want 3 transits you ofc have to wait till the little bugger has orbited its star 3 times, this is roughly 750 years :-/

Now to the incorrect part :D (yes I'm nitpicking)
If you tilt that pice of paper slightly, so you can see the circles you have drawn and imagine you had a ping pong ball glued into the middle as the star you will notice the following:
Even tilted, the closer circles you see are obscured behind the ping pong ball. That means the part in front of the ball is obscuring (transiting) the ball/star and hence can be detected. However the bigger circles are *to big*. The part of the orbit in front of the ping pong ball will pass "below" the ball in front of you, and wont cut it. The same circles other side, wont be hidden behind the ball but will go (from your point of view) above the ball.

So, if a ecliptic plane of an observed star is just tilted a little bit, the closest planets will transit it, and the farer away planets wont.

### Re:Streetlight effect? (0)

#### Anonymous Coward | about a year ago | (#43190611)

Didn't he basically make the same point for Earth from Poxima Centauri?

Also, you're wrong about the transit method - even at an angle, there will be a change in reflection light when a planet is behind the sun as opposed to in front of it, so you still get the dimming/brightening effect.

Or do you really think that more than 23% of solar systems are edge on to us?

### Much Harder, I suspect (0)

#### Anonymous Coward | about a year ago | (#43187415)

I think there are two aspects to answering your question properly, and they depend upon the method of detection you contemplate.

The first technique that we have used to detect exoplanets has been through the laws of gravity - i.e. the mass of the exoplanet causes it's star to 'follow' it's orbit, even very slightly. This causes a minute shift in the light reaching us from the star. The force of gravity experienced between the two objects - the planet and it's star, varies inversely with the square of the distance separating them: translation - if you double the distance of separation, the gravitational effect is reduced to one quarter, and so on. From this it should be apparent that as you move further from the star, the ability of the planet to influence the star's position is reduced, and thus our ability to detect is reduced. Only very large and very massive exoplanets [or, at a pinch, other stars such as a brown dwarf] would be of use.

The second technique would be to look for occlusion events - that is, a darkening in the apparent brightness of the star as a planet transits between it's star and our point of observation. But let's think about this for a moment... The earth is positively tiny in relation to our sun, and if a remote exoplanet of similar size just happened to share an orbital plane with say a sizeable asteroid belt, then the scatter effect and variation in stellar brightness would, from a distance, have a greater random pattern. It would look like a low-variation flicker.

So - and I'm guessing - I'd suspect that planets orbiting further out, especially worlds with a gravity field compatible for humans, would be pretty darn hard to detect.

### "Sun-like stars?" (0)

#### Anonymous Coward | about a year ago | (#43186795)

"Sun-like stars?" Does that mean that they're owned by Oracle?

### Sample bias (1)

#### TheGoodNamesWereGone (1844118) | about a year ago | (#43186995)

Earth-sized planets are underrepresented because while we have the right technology, we don't have funding for spacecraft capable of detecting them.

### Prayer (0)

#### Anonymous Coward | about a year ago | (#43187111)

Dear God, please teleport me to one of these planets that is only inhabited by supermodel, nympho women. Thank you.

### Re:Prayer (1)

#### PPH (736903) | about a year ago | (#43187487)

Stand by for the snu-snu.

#### Anonymous Coward | about a year ago | (#43187147)

A Quarter of Sun-Like Stars, that we have observed, Host Earth-Size Worlds. Obviously, we haven't observed all the billions of stars in our galaxy. I happen to like Geoff, but is he up for a new grant or soemthing?

### Time to... (0)

#### Anonymous Coward | about a year ago | (#43187221)

Fire up space engine http://en.spaceengine.org/ and find them.

p.s. don't click that link unless you want to be shown how small are planet is compared to the entire universe.

### Selection Bias? (3, Insightful)

#### The Raven (30575) | about a year ago | (#43187311)

I am confused... can someone explain how this report is not selection biased against distant or small planets?

To put it another way, we started by finding huge planets. As we have gotten better methods, we have found successively smaller planets. The three factors that make a planet easy to find are its diameter (occlusion of star), gravitational effect (how much the star wobbles), and distance (how likely that the planet will occlude the star from our perspective, and also factoring into the gravitational effect).

Distant, small planets simply won't be detected from our perspective. So the report is not really saying 'Only 23% of stars have earth sized planets'. It's really saying 'We know that about 23% of stars have rocky planets that are really close. Since we have no reason to believe our solar system is extremely unique, that makes it very likely that an even greater percentage of stars have rocky planets that are farther out'.

This is probably a huge boost to the 'how many stars have possible life sustaining planets' factor in that oft derided formula, the Drake equation.

### Re:Selection Bias? (1)

#### Anonymous Coward | about a year ago | (#43187451)

Kepler observes planets making transits in front of their stars. Over short times of observation, it is easy to detect large planets close to their sun (they make more frequent transits). As the timescale of the datasample increases, we will be able to detect smaller planets further out (which cause a smaller dimming of their star less frequently). Also Kepler can only detect planetary systems who's planets transit their star in our (Kepler's) line-of-sight. So as the sample time increases, more smaller planets will be found. Kepler's mission has years to go. 23% is probably very, very conservative.

### Re:Selection Bias? (1)

#### The Raven (30575) | about a year ago | (#43192269)

This is a bit late, but I think you do not understand a simple problem with transit detection: most planetary systems do not orbit in a plane we can detect. To detect planetary transit very close to the star, the range of system planes is pretty high. But the farther out form the star, the fewer and fewer system will EVER occlude from our perspective. Our only way of detecting these systems is by measuring star wobble, and that only detects big planets.

Se we can use this information detecting nearby occlusion as a proxy to estimate how common more distant planets may be, even though they will never occlude in a plane that includes our observation.

### Re:Selection Bias? (1)

#### EnsilZah (575600) | about a year ago | (#43192445)

Alright, I'll take a crack at explaining how it's not selection-biased against distant or small planets.

The scientists making the observations are only making claims based on the scope of the evidence they have and their understanding of the science.
Now to make observations without any backing evidence or understanding, reporters are employed (or clergy but not in this specific case).
Hope that cleared things up.

### question of size & mass (0)

#### Anonymous Coward | about a year ago | (#43187377)

Look I am not a scientist but can we make a leap here and suggest that a given sun with similar mass and composition to our own, will contain planets of a similar sizes and orbits to our own due the laws of physics.

We all know that the law of gravity make things fall at the same speed so using this we can assume that a given planet will orbit around a star within a certain distance due the force of gravity.

Please let me know if I am wrong in this premise and shoot me down in flames.
So we look for earth like stars similar mass and composition, we should find earth like planets orbiting these stars.

### 23 (1)

#### Pharoah_69 (2866937) | about a year ago | (#43187635)

23 huh? There are also 23 flavors in Dr. Pepper. Is there a connection?

### John Gribbin's Book: Recommended. (2)

#### smpoole7 (1467717) | about a year ago | (#43187795)

"Alone In The Universe: Why Our Planet Is Unique" by John Gribbin. I've just finished it. Those who've always hoped to one day chat with a Wookie or a Klingon (not to mention SETI-types) will find it thoroughly depressing, but it's filled with excellent science. There's a good review of it here:

Computer geeks will like it because many of its conclusions are based on cluster-run computer simulations. :) The results of the simulations are nothing short of amazing.

Example: Earth's molten iron core is what gives us a strong magnetic field that protects our atmosphere. The only way they could get that to work out was to put a supernova(!!!) .1 light years (that's not a typo) from the solar system at a critical time while it was forming. This also helps answer why our system has an unusual mix of elements compared to other stellar systems (particularly of radioactives such as Aluminum 26 and Iron 60).

Example: we're actually a binary planet -- Earth and Moon. The moon is thought to have formed from a planet in the Langrange point, called "Theia," that would have fractured our thick crust, making continental drift possible; the moon's gravitational effects on Earth are also critical.

Read the book. Even if you disagree with it (and I know many here will, especially my good friends who love SETI), but it's an excellent read.

### Re:John Gribbin's Book: Recommended. (1)

#### a_hanso (1891616) | about a year ago | (#43188721)

The rare earth hypothesis has a lot of good points. But at the risk of being ad hominem, the author is the same guy who once supported the theory that planetary alignments could result in earthquakes: http://en.wikipedia.org/wiki/John_Gribbin#Biography [wikipedia.org]

### Re:John Gribbin's Book: Recommended. (1)

#### tragedy (27079) | about a year ago | (#43188899)

Hmmm. This book is by John Gribbin. Considering that he predicted that the alignment of the planets would trigger massive earthquakes worldwide and sink LA into the ocean back in the early 1980's, I'm going to take it with a grain of salt. His writing doesn't seem to be based on a lot of evidence.

Example:

Example: Earth's molten iron core is what gives us a strong magnetic field that protects our atmosphere. The only way they could get that to work out was to put a supernova(!!!) .1 light years (that's not a typo) from the solar system at a critical time while it was forming.

What critical time are we talking about here? I'm guessing it's going to be a window of hundreds of millions of years if not billions of years before the actual formation of the sun. Considering that stars aren't actually in fixed positions relative to each other. I'd say that affects the odds quite a bit.

Not to mention the fact that, even without as much radioactive material, any planet will still have a molten core for a very long time. The radioactive heating extends it significantly, but life arose on Earth within a billion years of its formation. The core still would have been molten then even if it were 100% dead from a radioactivity standpoint. It's pretty likely that a planet of similar size and age to Earth but with no radioactive elements might still have a molten core. It could also be twice or three times as massive and therefore keep its heat of formation even longer. Not to mention that there are all kinds of possible arrangements that don't rely on radioactive elements to keep a core hot. Such as binary planet systems converting their massive rotational energy into heat through tidal forces over time.

Our big moon is also useful to us, but lots of planets have moons. I haven't seen any hard and fast rule on how big a moon you need to have or even if you really need to have one and, even if you need one, the odds against having one don't seem to be astronomical.

I'm not saying we don't have a nice planet. We do. It's a very nice planet (arguing from an anthropocentric viewpoint, anyway), but that doesn't mean that it's the only nice planet by a long shot. It also doesn't mean that a less nice planet can't also support life, which may even develop into intelligent life.

### Re:John Gribbin's Book: Recommended. (0)

#### Anonymous Coward | about a year ago | (#43189837)

Less nice planets for us but not for them!

### Re:John Gribbin's Book: Recommended. (1)

#### smpoole7 (1467717) | about a year ago | (#43190399)

> I'm guessing it's going to be a window of hundreds of millions of years if not billions of years before the actual formation of the sun.

Read the book. (It's about a million years, and the star would first shed its outer layers -- now known to be quite common -- then explode about 100,000 years later.)

If you don't want to buy it, fetch it from the local library or ask a friend to Lend it to you in e-form. But don't criticize it until you've read it. :)

### Re:John Gribbin's Book: Recommended. (1)

#### smpoole7 (1467717) | about a year ago | (#43190413)

Interesting that two people here have brought up Gribbin's old (decades ago) prediction about the alignment of the planets ... which HE HIMSELF later said was bogus. His reputation has been excellent otherwise.

I cringe when I think of some of the stuff that I used to believe and say. :)

### Re:John Gribbin's Book: Recommended. (1)

#### angel'o'sphere (80593) | about a year ago | (#43190477)

Example: Earth's molten iron core is what gives us a strong magnetic field that protects our atmosphere. The only way they could get that to work out was to put a supernova(!!!) .1 light years (that's not a typo) from the solar system at a critical time while it was forming. This also helps answer why our system has an unusual mix of elements compared to other stellar systems (particularly of radioactives such as Aluminum 26 and Iron 60).
This is nonsense.
Even if we had no strong magnetic field, still 2/3rds of the atmosphere where still left.
A planet forming in the distance to the sun like earth: will have an molten iron core. Always! Unless there is no iron in the cloud forming the solar system.
Venus and Mars alos *have* and *had* respectively an molten iron core. So it is not "that rare"
This super nova thing is completely bollocks. When the solar system formed all the inner planets where completely molten lava blobs. And the billion years they got bombarded by asteroids and stayed molten.

Example: we're actually a binary planet -- Earth and Moon. The moon is thought to have formed from a planet in the Langrange point, called "Theia," that would have fractured our thick crust, making continental drift possible; the moon's gravitational effects on Earth are also critical.
While the Moon certainly has an great impact, we can absolutely not determine how big that impact is or if it is necessary for life unless we have a second system to compare with. Looking at Mars and Venus we can easy conclude that a moon is *not* necessary. If Venus had not so much CO2, hence not such a strong greenhouse effect, it very likely had life. Same for Mars, we are more or less certain that Mars once had life. We only need to find the final proof!

### Re:John Gribbin's Book: Recommended. (0)

#### Anonymous Coward | about a year ago | (#43190911)

You are, sir, an idiot. Of course Earth is unique and it is the center of the universe, and you know what ... common sense says that it is not unique at all. Scientists want to get popular and fools read it believe and admire.

### Re:John Gribbin's Book: Recommended. (1)

#### whodunit (2851793) | about a year ago | (#43191057)

The only way they could get that to work out was to put a supernova(!!!) .1 light years (that's not a typo) from the solar system at a critical time while it was forming

... what about tidal heating? It sounds like a fascinating scientific examination of how our own solar system came to be, but even if Earth IS unique, it does not follow that only planets identical or almost identical to Earth are capable of supporting life. In fact, an overly terran-centric viewpoint is harmful; from the possibility of life underneath Europa's oceans (enabled by tidal heating) and what we've learned of extremophile bacteria, life can survive and thrive in many more environments then we ever thought possible. Of course, what we're really interested in is environments that can support advanced, multi-cellular INTELLIGEGENT life, but I've seen nothing to make me doubt that "underwater worlds" like Europa are incapable of generating such life-forms given more favorable conditions.

### Re:John Gribbin's Book: Recommended. (1)

#### smpoole7 (1467717) | about a year ago | (#43192473)

Gribbin's point is that Earth is uniquely suited to *INTELLIGENT* life. In the very first chapter he says that, in fact, he thinks that simple single-celled life is probably common in the entire galaxy.

Sigh.

I recommended the book because it was a good read. READ THE BOOK. :)

### why do they keep making such statements? (1)

#### argStyopa (232550) | about a year ago | (#43187899)

Astronomers (and/or the reporters sending these reports) keep making comments like "... there appears to be a mysterious dearth of exoplanets smaller than Earth..."
and then, later buried in the text:
"...âoeThis is a guess, but theyâ(TM)re just harder to detect,â Marcy told Discovery News. âoeSmall planets dim the star less â" the dimming has to be greater than the noise to detect the planet...."

Well, no SHIT, Sherlock. But: Observing a lack of something doesn't mean that something is missing, particularly when we KNOW our method of observations are clumsy and limited.

We only recently developed the tech to discover planets at all, so obviously our gross methods first detected the largest planets that would either a) swing the primary around the most, or b) occlude the most light.

The Earth, transiting in front of the sun, blocks about 1% of the (2d) visible side. That's not much occlusion.

Further, when you consider that our detection methods largely rely on the faint chance (see what I did there?) that the planet passes almost DIRECTLY between Earth and the observed primary - and does so repeatedly (at least 3x), and within the span of observation - it's pretty f*cking unbelievable that we've seen ANY. And if the chance of observing something happening is vanishingly small, yet we are seeing thousands of these, then isn't it logical that the event must be happening with very high frequency?

On the contrary, given our still-new detection technologies, and the geometry working against us, I'd say the numbers of planets we've already observed strongly suggests that 'planet-building mechanisms' are common around pretty much every star.

### drunk, lost keys, lamppost. (0)

#### Anonymous Coward | about a year ago | (#43187921)

You find what you are able to see,of course.

Afghan version:

One day Mullah Nasruddin lost his ring down in the basement of his house, where it was very dark. There being no chance of his finding it in that darkness, he went out on the street and started looking for it there. Somebody passing by stopped and enquire:
- What are you looking for, Mullah Nasruddin ? Have you lost something?

- Yes, I've lost my ring down in the basement.

- But Mullah Nasruddin , why don't you look for it down in the basement where you have lost it? asked the man in surprise.

- Don't be silly, man! How do you expect me to find anything in that darkness!

### no ... no Princess Lea (1)

#### noshellswill (598066) | about a year ago | (#43189873)

Haha so  grant-grubbing astro-fellators have found 3000-odd potentially habitable planets. WOO-H00 !  Since the odds for life are  10^(-40) under EXCELLENT conditions methinks they gotta look a little harder. Fact is EARTH alone harbours life in all the universe ... or stated otherwise it takes (an) entire  BigBang and entire universe for a ghosts chance of  happenstancing  one (1) inhabited planet. Don't be pathetic and expect lifes' meaning to appear from the void ! Suck it  feckin-A down gamrduudz  and deal with it.
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