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A Quarter of Sun-Like Stars Host Earth-Size Worlds
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.'"
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.
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.
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.
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.
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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.
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