The Physics Police

The Physics Police

Monday, March 3, 2014

Kepler Past, Present, and Future

The Kepler mission is one of most interesting things that has happened in science in my lifetime. The spacecraft was launched in 2009 and spent its first 4 years staring, unblinking, at a tiny patch of stars in the constellation Cygnus.

Before this mission, we didn't know for sure that planetary systems were common around other stars. True to the Copernican principle, once it started looking, Kepler caught all sorts of planets transiting in front of their host stars. These transits cause the stars light to dim slightly, and the Kepler team reports these as candidate exoplanets.

Follow up on these candidates is performed by ground-based telescopes, which carefully measure the redshift of the starlight from the candidate's star. Changes in the redshift can betray small changes in velocity due to the gravitational tug of the planet on its star.

The sensitivity of these ground-based telescopes is quite impressive. For example, the HARPS spectrograph in Chile can identify radial-velocity shifts as small as 0.3 m/s. For comparison, the Earth's gravity perturbs the Sun by only 0.1 m/s. This puts us right on the edge of being able to confirm Earth-like planets.

Last week NASA announced the confirmed identification of 715 "confirmed" exoplanets. This announcement is huge because, before it was made, the list of confirmed exoplanets numbered only 975. In fact, Kepler now accounts for 57% of all confirmed planets.

However these 715 exoplanets were not confirmed by spectroscopy. The paper in which this list was published calls them "confirmed" citing a false-positive rate less than 1%. They were able to rule out false positives by looking for systems with multiple planet candidates.

Because planetary systems tend to form in a tight disk, we sometimes see the whole system edge on. This means that we're likely to see multiple planets transiting in front of the star. This fact helps distinguish multiple planet systems, busy with lots of transit events, from false positives, where spurious data look like single, isolated transit events.

Part of what makes this technique possible is how common are multiple planetary systems, like ours. In other words, the Copernican principle makes hunting exoplanets a little bit easier.

One of the greatest things about this new planet list is that it includes over 100 Earth-size planets.
Geoffrey Marcy has predicted that the Milky Way galaxy contains something like 40 billion Earth-sized planets within their star's habitable zone.

You may ask, how do you get 40 billion Earth-like planets from just 100 Earth-sized planets? One thing to keep in mind is that, by chance, we only see a few of the Kepler stars edge on to their planetary ecliptic. Most planets orbit in an ellipse that doesn't eclipse their star in our line of sight.

Also, the Kepler viewing area is only a tiny fraction of the sky. I couldn't find this number anywhere else, so I calculated it myself:

115.6 / 41,253 = 0.28%

So there are still plenty of planets out there, just waiting to be discovered! And visited...?

Sadly, last year Kepler suffered failure the second of four reaction wheels. With only two working reaction wheels, it can no longer keep oriented in space, which requires three wheels, one for each dimension of space. This mechanical failure caused an early end to it's planet-finding mission.

Or so it seemed, until NASA came up with what they're calling the K2 mission. By keeping the sun in the X-Y plane, they can do a pretty good job keeping the space craft on balance using just two reaction wheels. The catch is, Kepler will be limited to the field of stars in our ecliptic. It will also have diminished precision, but will still be able to discover lots of new planets.

Let me explain why I think this is a good idea!

So far, we've been finding exoplanets in a patch of the sky where, if there was anybody home, they couldn't easily see our planetary system, because our ecliptic is pointed in a different direction.

But Kepler will now be pointed at stars who see our ecliptic edge on.

If anybody's home around these stars, they could detect Earth by the same transit method.

Neighbors worth knowing something about, right?

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