A Menagerie of Exoplanets

Many announcements at the first day of the American Astronomical Society’s 217th meeting in Seattle revolved around extrasolar planets (or more concisely, “exoplanets”).  I have the pleasure of attending and soaking up the gestalt, so let me fill you in.

The headline story?  Kepler-10b, the first rocky planet discovered around another star. Other planets (such as CoRoT-7b) might qualify as rocky, but Kepler-10b definitely fits the bill, which is a first.  The planet has an estimated diameter 1.4 times that of Earth, but its density could be more than twice that of our lovely home planet!  The estimated density clocks in at 8.8 grams per cubic centimeter:  for comparison, water is one gram per cubic centimeter, and Earth’s density averages out to about five grams per cubic centimeter (lets’s hear it for a nickel-iron core).

In case you glaze over at those numbers…  Imagine picking up a gallon jug of water.  Kinda heavy-ish, right?  In fact, it weighs more than eight pounds.  A jug filled with material the average density of Earth?  About 42 pounds.  And a gallon of Kepler-10b would weigh more like 73 pounds.

Before you start planning a visit, however, you should note that the planet orbits its parent star in about 20 hours (Earth takes 365 days) and its surface probably reaches temperatures hot enough to melt iron.  It most assuredly does not lie in the so-called habitable zone. NASA even produced a roller coaster of a video that would make more than a few people nauseous if we played in the planetarium (heck, even if you put your face a little too close to the computer monitor…).  “Artistic license,” I think they call it.  But the cracked, lava-covered surface depicted in the video is certainly evocative, if somewhat obviously computer-generated.

Ed Guinan from Villanova University described an exoplanet that actually causes its parent star to spin faster! The planet follows a death spiral of an orbit, getting closer and closer to the star, and in the process, it feeds angular momentum to the star, causing it to spin more and more quickly.  Tidal forces (such as those that cause, well, tides on Earth due to the gravitational influence of the Moon and Sun) can’t pull this off:  it would take 500 billion years for the planet to spin up its star tidally, and that’s about 35 times longer than the Universe has existed, making it an unlikely explanation.  Perhaps magnetic fields link the planet and the star, tranferring energy between them.  Hmmm…

Speaking of magnetic fields, another announcement by Adam Kowalski from the University of Washington suggests that small stars might flare up too energetically for life to remain comfortable nearby.  In an exhaustive survey, he observed a significant fraction of stars that increased in brightness by 10% or so for brief periods of time.  So if you live on a planet orbiting a small star subject to such flare-ups, watch out!  Quiet, middle-aged stars with a little extra mass (such as the Sun) make better parents than these cantankerous characters.

Academy Fellow Geoff Marcy closed the first day with an extended presentation on the current state of exoplanet science. With more than 500 known exoplanets, we can begin to formulate clearer ideas about the variety of worlds astronomers are discovering.  (Marcy pointed out that 15 years ago to the day, he gave a review talk on the one known exoplanet and two recent discoveries.  My, how times change!)  A few highlights…

We would certainly love to find an Earth-sized exoplanet, and we have recent, encouraging news on that front:  smaller planets (three-to-ten times the mass of Earth) seem to occur with greater frequency than larger ones, according to a study by Marcy himself. And other studies have bolstered that result.

We’re also finding utterly bizarre exoplanets, some of which orbit their stars basically backward!  In our Solar System, for example, all the planets orbit in the same direction the Sun rotates, which seems reasonable if the kit and kaboodle formed together 4.6 billion years ago.  But we have found many oddball exoplanets that orbit their stars in the reverse direction, or at weird angles.  Marcy quickly pointed out that our current  theories of planet formation don’t explain this very well—indeed, they don’t explain the encouraging news about smaller planets I just mentioned in the previous paragraph!  More work is needed.

(At this point, I should point out the good-natured goading that happens between theorists and observationalists.  The Universe never fails to surprise us, but researchers nonetheless try to concoct theories that describe observations as accurately as possible…  Often with remarkable success!  But the scientists like Marcy who spend hour after hour after hour at, say, a telescope making difficult observations can’t resist pointing out when the Universe comes up with another bolt from the blue.  And they usually do so with glee, ordering their theorist colleagues back to the drawing board!)

Marcy also touched on the more unusual details related the two planets in the Kepler-9 system. Great that the Kepler scpacecraft discovered two planets orbiting the same star, but the weirdness starts when you look carefully at the stars’ orbital periods.  They change slightly over time!  And not in a simple, steady way: they vary back and forth, and these variations hint at gravitational interactions between the planets.  Astronomers can even tease out constraints on the masses of the planets in this way.  And that is super spiffy science.

Looking for the place to go for up-to-date and reliable exoplanet information?  Marcy says Exoplanets.org will satisfy your yen.

For now, it’s time to sign off from Seattle.  More news tomorrow.