By Wes Traub
Science Update: Kepler, TESS, and the 2.4
By Wes Traub and Steve Unwin
Kepler, as you know by now, lost its second reaction wheel (of four total) on May 14 (http://www.nature.com/news/the-wheels-come-off-kepler-1.13032). If the current recovery efforts do not bring one of the two failed wheels back to life (three are needed for precision pointing), then Kepler’s planet-hunting days are over. This is disappointing, because the Extended Mission just begun was set to significantly improve our estimate of eta-sub-Earth from those extra years. However, we do have more than four years of excellent data, so Kepler has been quite successful in meeting its original requirement, and we celebrate that.
TESS (Transiting Exoplanet Survey Satellite) was selected in April for a planned launch in 2017 as an Explorer mission. TESS is led by George Ricker at MIT and Dave Latham at the CfA, with many Co-Is and collaborators as well as friends cheering it on. TESS will search for transiting exoplanets down to Earth-size, around 500,000 nearby FGK dwarf stars, in the visible magnitude range from 4 to 12, from a high Earth orbit (HEO, 13.7-day period), covering about 90% of the sky, with 27-day observations per sector. For more, see http://web.mit.edu/newsoffice/2013/nasa-selects-tess-for-mission-0405.html.
The exoplanet migration-halting mystery was addressed by Peter Plavchan and Christopher Bilinski (http://iopscience.iop.org/0004-637X/769/2/86/), who find evidence in Kepler data and confirmed exoplanets to show that hot Jupiters halt their inward migration because of planet-star torques rather than planet-disk torques.
The uncertain origin of water on terrestrial planets is an ongoing problem. Now Hamano et al. (http://www.nature.com/nature/journal/v497/n7451/full/nature12163.html) show that water of hydration in the rocks of the initial buildup of the planet may be trapped instead of being evaporated from the magma as the planet solidifies. The gatekeeping mechanism is a steam atmosphere that insulates the cooling magma: for a planet at 1 AU, say, the steam atmosphere dissipates after about 4 Myr, and the water remains trapped in the magma, later contributing to a liquid-water ocean, as on Earth; however, for a planet at 0.7 AU, say, the stellar heat input delays the dissipation of the steam atmosphere to perhaps 100 Myr, giving water in the magma a better chance to come to the surface and escape the magma and atmosphere, giving rise to a dry planet, like Venus.
Future exoplanet science took a big step forward when NASA officially decided to reserve the 2.4-meter telescopes donated by the National Reconnaissance Office for astrophysics use, with the WFIRST mission being a prime contender (http://science.nasa.gov/media/medialibrary/2013/06/04/AAS_TownHall_2013-06_V7A_for_posting.pdf.) In addition, the coronagraph instrument ow appears to be seen as a valuable addition to the mission, raising hopes for a direct-imaging coronagraph capability in space in the early 2020s dedicated to imaging giant planets, dust disks, and possibly some well-placed terrestrial planets. For the story, see http://www.spacenews.com/article/civil-space/35628only-nasa-astrophysics-remains-in-running-for-donated-nro-telescope-%E2%80%94-for#.UbUI6Nh4JGo.
Science magazine ran a special section on exoplanets (http://www.sciencemag.org/site/special/exoplanets/) featuring detection methods, instruments, and types of exoplanets, including reviews by Andrew Howard and Sara Seager, a biographical article on Bill Borucki (Kepler PI), and a research paper on Kepler-62 with its five terrestrial-size planets, including two in the habitable zone, led by Borucki. Subtle variations of the intensity of light from an orbiting exoplanet led to the discovery of Kepler-76b, a hot Jupiter, by Faigler et al.; see http://arxiv.org/pdf/1304.6841v3.pdf. The variations arise from three effects: relativistic (~v/c) forward-beaming of light from the orbital velocity of the planet, the ellipsoidal shape of the tidally-distorted and tidally-locked planet, and the non-symmetric brightness of the planet caused by an equatorial super-rotating jet in the upper atmosphere.