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Comment by toomuchtodo | Hacker News Reader

> This is because Kepler can only detect planets which lie in the same plane as earth and the star so the planet occults the star.

What would be necessary for a more comprehensive survey?

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If the planet is not transiting there are two other ways to detect it: radial velocity measurements, and direct imaging. In the radial velocity technique you measure the wobble of the star due to the gravitational pull of the planet. This also requires that the planet be somewhat edge-on, but the requirement isn't nearly as strict as with the transit method. The difficulty is that Earth size planets have very little mass, and so impart a very small wobble to the star, on the order of only ~10 cm / s.

In the second method, you just try to block as much light from the star as you can and go look for the reflected light of the planet. This is much, much more difficult and so far has only been done for very large, distant planets.

What kind of resolution is possible with direct imaging at 1400 ly away? I would imagine that it's probably milliseconds of arc within the field of vision of any telescope, so how close could we actually look at the planet?

With adaptive optics it's possible to achieve resolutions down to ~30 milliarcseconds. At 1400 ly that would subtend a distance of 13 AU, so you wouldn't be able to find Earth-like planets. But even if it's theoretically possible to resolve 13 AU separations at 1400 ly, in practice the light from the star is so much brighter than the planet that it bleeds all over the image and completely washes out the reflected light of the star.

To do direct imaging you really need to look at systems near the Earth, maybe a few tens of ly away. Then you can separate the planet from the star by enough that you can hope to mask the light from the star and still be able to detect the planet.

To get some idea of the techniques required to do this, take a look here: http://astrobites.org/2013/06/09/lowest-mass-exoplanet-disco...

Likely something facilitating direct imaging, which would require canceling out almost all of the light from the parent star (diminishing it by a factor of at least 10^6:1).

While there are some techniques that could facilitate that (such as enormous star shades coupled with space telescopes) they wouldn't enable the sort of shotgun survey approach that Kepler has been able to do, but it would enable us to survey and extensively study stars one at a time for planets.

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