1200 New Worlds

I have been reading as much as I could get my hands on in the days since NASA’s press conference on February 2nd announcing that over 1200 new exoplanets (planets orbiting stars other than our sun) had been discovered by the Kepler spacecraft. This announcement is an analysis of just the first four months of observations—from May to September 2009. There is an absolute flood of data coming from the spacecraft, and scientists are happily struggling to keep up.

It is amazing how such momentous events pass without comment in what we call the news. On the day of the press conference, I eagerly awaited a report on the national news. Instead, the big news of the day was that it snowed in the Midwest in February! Some of us know that scientist Debra Fischer had it right when she called this “an incredible, historic moment.” Absolutely correct.

Here are some take-home bullet points:

• 1235 planetary candidates (probably planets but need further confirmation).
• In decreasing order of size: 19 larger than Jupiter, 165 Jupiter-sized, 662 Neptune-sized, 288 bigger than Earth, 68 roughly Earth-sized.
• Bigger planets are easier to find, so this distribution is probably still a little skewed toward bigger planets. As more data are gathered, even more small planets should be found.
• There are a lot of small planets out there!
• There are a lot of planets (54) with the right conditions to support life as we know it.
• This sampling strongly suggests millions of planets in our Milky Way galaxy that are capable of supporting life.

And here are some details for those of us who like such things!

How do we know there are planets around other stars?

In almost all cases, we know this because of the planet’s influence on the star. The planet tugs the star back and forth as it orbits around it, and this shifts the wavelength of light we see from the star in a regular pattern. If the planet passes directly in front of (transits) the star, the star’s light will be dimmed, also in a regular pattern. Kepler detects planets by this transit method, as illustrated below.  Note: clicking on any of the images in these posts will bring up the full-resolution version.

Do we actually have direct images of any exoplanets?

Remarkably, yes, but not very many. Here are three examples:

Can we detect any planets that orbit around another star?

No.  The planetary orbit has to be lined up so that it is edge-on to us for the planet to transit its star.  But it’s easy to determine how likely that is, and from a sampling of many stars, to determine how many more planets are there that we cannot detect.  It’s important to understand how important the huge sample size is to the Kepler mission.  If we’re trying to learn something about the population of exoplanets, the more members of that population we can detect, the better our information.

Why are Kepler’s results so significant?

Previous methods of exoplanet detection are strongly biased toward large planets that are close to their parent stars.  None of these planets is Earth-like—imagine Jupiter, a large gaseous planet, orbiting closer to our sun than tiny Mercury.  That describes most of the planets discovered before Kepler.  Kepler is showing that there are indeed many stars with smaller planets around them, planets that are much more like our own Earth.  Here is the key graphic from the press conference:

Are any of these planets habitable for life as we know it?

Yes—54 of them!  Habitability is here defined as having a surface temperature between the freezing and boiling points of water, allowing liquid water to exist, and a planet small enough to be rocky instead of gaseous like Jupiter.  Since the data analyzed so far are from only 4 months of data, only planets with short orbital periods can have transited during that time period.  Such planets will orbit close to their stars.  For the planets to be habitable, those stars must be smaller and cooler than our sun.

So…the habitable planets found so far are not quite Earth-like since they orbit stars that are not quite sun-like.  But…we should not discount another possibility, and that is habitable moons of gas giant planets like Jupiter that orbit close to their suns.  Fans of the movie Avatar may recall that it took place on such a world.

Kepler will continue to monitor these stars for a total of at least three and a half years, and probably longer with a mission extension.  The longer it goes, the more planets it will be able to detect, particularly small planets far from their stars.  Imagine detecting the Earth, which Kepler could do from over 1000 light years away.  It would take at least a year to observe two transits (one at the very beginning and one at the very end of your observing period), and you would really like to observe three just for confirmation.  Three and a half years of observation would probably give you those three transits.

In other words, as Kepler continues, the habitable planets it finds will orbit stars increasingly like our own sun.

 Can we tell if any of these planets actually do have life on them?

Not yet.  There are proposed space-borne telescopes that would be able to do so, however, by detecting the presence of telltale gases in a planet’s atmosphere.  Gases like oxygen and methane are quickly removed by chemical processes unless they are somehow replaced by other processes.  On Earth, that “other process” is biological.  Plants take in carbon dioxide and produce oxygen; if plants were to disappear from the planet, oxygen would disappear from its atmosphere.  I’ve told my students that such detection on exoplanets will almost certainly happen in their lifetimes.  I’d like to live long enough for it to happen in mine!