Wednesday, February 02, 2011

Planets everywhere

Image Credit: NASA/Wendy Stenzel

Today, NASA and the Kepler Mission team announced the most up-to-date results of the Kepler mission's search for planets around other stars.  Today's haul was nothing short of astounding (though, dare I say, mostly expected): 1235 candidates, 68 of which are Earth-sized.  54 planet candidates (not necessarily the same ones that are Earth-sized) are the right distance from their parent star that they could have liquid water.  170 of these 1200 candidates also show some evidence of being in multiple-planet systems, and one has at least six planets!

Originally, I was going to miss the press conference, because I am supposed to be teaching a class at that time.  Then our classes were canceled due to an ice storm, so I intended to watch the press conference.  However, I was unable to watch the press conference when my apartment was hit by the 5th in an ongoing series of rolling blackouts to hit the Dallas area.  So, I have to pick up the highlights from the web.

NASA's Kepler Mission's main goal is to find Earth-sized planets in Earth-like orbits around sun-like stars.  Kepler works by watching for planets whose orbits carry them directly between their parent star and the Earth.  This means that Kepler will not see most of the planets that are out there, as this alignment is rare.

Before today, we mostly had heard about planets from Kepler that were very close to their parent star and absolutely, positively, undoubtedly real.  Today, roughly 1200 new planet candidates were announced, and perhaps 20% of them may not be real.  More on that in a minute.

There are many reasons so many of the first planets were close to the parent star.  First, Kepler has to see a planet go in front of its parent star at least three times (in other words, complete at least three orbits)  to know that it is real.  One passage alerts the mission that this star is interesting, a second passage gives astronomers a predicted orbital period, and a third pass confirms that period.  The closer a planet is to its parent star, the more quickly it completes each orbit, and the sooner we've seen those magical three orbits.

But there are many other astronomical objects that can look like planets that really aren't planets.  So much of the early work on candidate Kepler planets is to follow them up with ground-based telescopes.  These telecopes can measure how fast the parent star moves in response to the planet's gravity, and confirms if we are seeing a planet, or if it is something else.  These observations require a lot of time, so positive planet identifications are slow in coming.  The closer a planet is to its parent star, the easier it is to make these measurements.  So, the most certain planets in the early data tend to be big (their larger gravity tugs more on their parent star) and they tend to be very close to their home star.

As Kepler finds smaller planets further from their parent star, this follow-up work becomes prohibitive.  The motion of the Sun due to the Earth's gravity cannot be measured with current telescopes, and each orbit of the Earth takes a year.  So if we find a candidate Earth-sized planet in an Earth-like obit around a sun-like star, we have few options for proving it is real beyond all doubt.

However, all the work that goes in to proving the close-in planets also tells the Kepler team how likely any one detection is to be real or not.  And it seems that at least 80% of the Kepler planet candidates that pass some initial straightforward tests are real.  So, while some of those 1200 planet candidates are probably not real, most of them probably are.  If we know what percentage of the planets are likely false detections, we can still analyze Kepler's treasure trove of planets using statistics.  This has been the plan all along, and this is Kepler's main mission goal -- to see how common planets are.  The results: they're everywhere.  Which we already suspected. 

One of the coolest planetary systems Kepler scientists announced today is that of Kepler-11.  This star has at least six planets around it, and five of these are closer to the parent star than our planet Mercury, the closest planet to the Sun.  There are so many planets close in to this star that they sometimes blend together, with more than one planet passing in front of the star at the same time.  This is horribly difficult to disentangle!  Here's an artist's conception of the Kepler-11 system:

Kepler 11 has at least six planets
Image Credit: NASA/Tim Pyle

The Kepler mission will continue for several more years.  It needs at least another 18 months before that desired trophy of an Earth-sized plaet in an Earth-sized orbit around a Sun-like star will have had a chance to complete those all-important three orbits.  We are getting closer all the time to knowing how common our Solar System is (or is not).

The last warning, which I've made before.  Kepler only tells us the size (diameter) of the planets, not what they are made out of.  So any Earth-sized planet in an Earth-sized orbit around a sun-like star that Kepler finds could be as desolate as the Moon, as hellish as Venus, or as pleasant as the Earth.  We don't have the ability to see how livable these planets might be.  Yet.

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