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January 04, 2010

Beyond the First 5 Exoplanets of the Kepler Telescope


Sky and Telescope reports: Kepler has already logged more than 100 transiting-planet candidates, but he said most of these will probably prove to be false alarms once followups are done by other methods

Kepler is watching, nearly continuously, a selection of about 156,000 stars from 9th to 15th magnitude (out of 4.6 million in its field of view) in patch of sky covering 100 square degrees a little north of the plane of the Milky Way between Vega and Deneb. The satellite will keep watching these stars nearly 24/7 for at least 3½ years, in order to catch at least three transits of any luckily aligned planets that are in wide, Earth-like, 1-year orbits. Any such planet has only a 1-in-200 chance of being in an orbit that's oriented just right to cross a Sun-sized host star as seen from our viewpoint.





Another is statistics. Kepler is intended not just to identify a few individual exo-Earths. It was designed, Borucki stressed, to watch enough stars to give a firm statistical reading on the abundance — or rarity — of terrestrial-size planets generally, throughout the galaxy and the universe.

Kepler is routinely achieving 1-part-in-40,000 brightness precision (0.000025 magnitude) for measurements of 12th-magnitude stars. That is good enough to find transits of worlds as small as Earth, as planned.

• Kepler’s measurements are so precise that most “false positives,” such as an eclipsing binary star blended with the image of another star, can probably be weeded out upfront, without tedious and expensive radial-velocity measurements from the ground. This is very good news. Eclipsing binaries are the main source of false “transits.”

• For a few stars, Kepler has measured slight surface oscillations like those on the Sun. These arise from low-frequency sound (pressure) waves resonating through a star’s body. These resonant oscillations reveal a star’s diameter, mass, and state of evolution with very high precision — refining, in turn, the diameter, mass, orbit, and age of any planet it may have.

• Two very hot objects that Kepler found orbiting white, type-A stars appear to be too hot to be planets but too low-mass to be stars. These may be an odd class of "not-really-white-dwarfs" — in which a normal white dwarf has shed all but about 0.15 solar mass to its companion star, and has thus enlarged beyond normal white-dwarf size due to its lower mass, gravity, and degree of compression.

Kepler data will be released publicly on a regular basis starting in June 2010, around the one-year anniversary of the start of its regular science observations.

What we’ll learn from Kepler five years from now will be astounding — what percentage are rocky planets, steam planets, or Jupiter-like." So, she urged the reporters present, please be patient.

One important piece of Kepler news, she said, is the finding that Sun-like stars are generally quiescent, showing only very small brightness variations. That's good news for astrobiology, and it's also good news for us. It tells us that solar-type stars spend most of their time in quiescent states, so we ourselves are in a good, safe place, not just enjoying a lucky quiet time next to a star that may act up.




• For hot Jupiters — large bodies dazzlingly lit by their stars — Kepler can see the "secondary eclipse": the planet's light being blocked by the star when the planet passes behind it, as shown


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