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Wednesday, September 14, 2011

Another way to find planets

Exoplanets are now almost mundane.  Hundreds have been found in just the past few years.  What makes Kepler-19c unique is not that it orbits a star other than our sun, but the way it was found.  Its presence slightly alters the orbit of a fellow planet that had already been discovered.

The planet Kepler-19b was discovered orbiting the star Kepler-19 by, wait for it, NASA’s Kepler space telescope. The Kepler telescope locates planets using the ‘transit’ method.  That is, it looks for the periodic dimming in a star’s light as a planet passes directly in front of that star.  Kepler-19b was found by this method.  However, Kepler-19b doesn’t transit its star exactly on time.  Instead, it crosses its star up to five minutes early or late each time it goes around.  That doesn’t sound like a lot, but something has to account for the discrepancy.  That something is an unseen second planet (Kepler-19c) whose gravity is ever so slightly tugging on Kepler-19b.

The "invisible" world Kepler-19c, seen in the foreground of this artist's conception, was discovered solely through its gravitational influence on the companion world Kepler-19b - the dot crossing the star's face. Kepler-19b is slightly more than twice the diameter of Earth, and is probably a "mini-Neptune." Nothing is known about Kepler-19c, other than that it exists.
Credit: David A. Aguilar (CfA)

Other than the fact of its existence, little is known about Kepler-19c.  It doesn’t transit Kepler-19 from our vantage point, and it has no detectable gravitational effect on that star. 

As Daniel Fabrycky of the University of California, Santa Cruz says:

[Kepler-19] could be a rocky planet on a circular 5-day orbit, or a gas-giant planet on an oblong 100-day orbit.

By the way, the planet Neptune was discovered in a similar fashion when astronomers attempted to explain aberrations in Uranus’s orbit.  

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