Finding new worlds with Kepler is an absorbing occupation, but the one thing missing from most exoplanet news is proximity. While we continue to search for planets around the Alpha Centauri stars, the closest candidate I know about is the gas giant thought to orbit Epsilon Eridani, some 10.5 light years out. If you’re looking for potential habitability, you have to extend all the way out to Gliese 581 (almost twice the distance), where planets are plentiful and there is at least the chance (GL 581d) that one skirts the edge of the habitable zone. There are probably many planets closer than 20 light years, but we don’t have the tools in space to find them easily.
Kepler, you’ll recall, studies a field of stars in Cygnus, Lyra and Draco, the goal being to develop a statistical approximation of the prevalence of Earth-sized planets in the galaxy. Looking out along the Orion arm, Kepler is watching stars anywhere from 600 to 3000 light years away. In fact, fewer than 1 percent of the stars Kepler sees are closer than 600 light years. We have missions on the drawing board like TESS — Transiting Exoplanet Survey Satellite — which are designed to study 2.5 million of the brightest stars in the sky, looking for nearby transiting planets, and the European Space Agency’s PLATO — PLAnetary Transits and Oscillations of stars — would likewise go to work on relatively bright stars near our Solar System.
Will these missions ever fly? While we wait for the inexorable review process to grind its way forward, it’s gratifying to see the discovery of another planet that’s not, in astronomical terms, all that far away. The candidate exoplanet, UCF-1.01, is 33 light years from us orbiting the red dwarf GJ 436, already known to be home to a hot Neptune designated GJ 436b. Working at the University of Central Florida, Kevin Stevenson and colleagues went to work on what appeared to be the signature of a small planet, going through observations from Spitzer as well as the Deep Impact spacecraft, the Very Large Telescope and the Canada-France-Hawaii Telescope on Mauna Kea. The find marks the first time Spitzer has been used in a transit discovery.
Image: GJ 436, a red dwarf in the constellation of Leo with a ‘hot Neptune’ and two new exoplanet candidates. Credit: ESO Online Digitized Sky Survey.
It’s an interesting little world, this UCF-1.01, with a diameter estimated at 8400 kilometers, or about two-thirds that of the Earth, quite a catch for Spitzer, which some project scientists believe may be able to discover exoplanets no larger than Mars. UCF-1.01 would orbit its star in a snappy 1.4 days, with surface temperatures reaching close to 600 degrees Celsius. Due to its size and proximity to the host star, the planet is unlikely to have retained its original atmosphere, though the paper does note that impacts or tidal heating could create a transient atmosphere.
Most likely, this is a dead, cratered world, conceivably a place covered in magma, according to Joseph Harrington (UCF), a co-author on the paper. If confirmed, the new world will be called GJ 436c, but the UCF team is not done yet. There is also evidence for a third world in this system, dubbed UCF-1.02. Confirming the new worlds will involve additional work. From the paper:
To deﬁnitively establish UCF-1.01 as a planet (to be called GJ 436c), we require only a few hours of additional observations, preferably from another telescope or at least at a different wavelength. Establishing UCF-1.02 as a planet (to be called GJ 436d) would likely require an extended observing campaign to constrain its period then successfully predict a transit.
We only have one Kepler world smaller than the two Spitzer candidates, though there will surely be more as the data analysis continues. Meanwhile, we’re still waiting for information about flight possibilities for PLATO, which will have a larger field of view than Kepler, while TESS is involved in a concept study that could lead to selection as a NASA Explorer-class mission. One way or another, through space-based resources or ground observation, the list of nearby exoplanets is going to grow, though we can only speculate on when — and where — we’ll find the first true Earth analog. Interesting, conceivably habitable M-dwarf planets are probably more common.
The paper is Stevenson et al., “Two nearby sub-Earth-sized exoplanet candidates in the GJ 436 system,” accepted for publication in The Astrophysical Journal (abstract).