A Quarter of Sun-Like Stars Host Earth-Size Worlds 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.'"
Re:Great! (Score:3, Insightful)
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?
Selection Bias? (Score:4, Insightful)
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:Seems like useless info (Score:4, Insightful)
What are you basing that guess on?
Re:But... (Score:5, Insightful)
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:Great! (Score:5, Insightful)
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.