As the Kepler mission shifts into its new mode of operations, multiple new searches for exoplanets are in the works.
Change is in the air.
For the past several years, every winter meeting of the American Astronomical Society has heralded vast discoveries of exoplanet candidates, largely from the Kepler mission. At the meeting this month, Kepler announced some icing for that cake: 554 new planet candidates, 6 of them in their stars’ habitable zone. Another 8 planets previously in the “candidate” column have moved to “confirmed” status.
For those keeping count, that brings the candidate total to 4,175 and the number of confirmed planets to 1,013. (The candidate total includes the confirmed planets.) Of the newest confirmations, three were no more than double Earth-size and in their star’s habitable zone, bringing the number of confirmed Earth-size planets in “Goldilocks” orbits to eight.
That’s pretty impressive considering that Kepler was forced to halt its original mission in May 2013 when the second of its four reaction-control wheels failed. The new discoveries come largely from continued analysis of the treasure trove Kepler collected over its first four years.
Yet real work remains to be done. As impressive as these numbers are (and even more discoveries are yet to come), Kepler’s goal has always been not to merely find some planets, but to find vast numbers of them, especially Earth-size ones.
Only with large, statistical samples can astronomers begin to answer how many stars host planets, and how many of those planets have a chance of being habitable. That analysis is still under way, and Fergal Mullally (NASA Ames) says astronomers expect to spend one or two more years teasing the subtleties out of the data.
Kepler’s mission now continues in modified form as K2, which has already discovered four super-Earths passing in front of red dwarf stars — including three super-Earths in a single system. Meanwhile, a deluge of exoplanet-finding missions are in the works, and the sampling below is by no means exhaustive.
The European Southern Observatory recently announced “first light” for the Next-Generation Transit Survey, which will look for transiting planets around bright (i.e. nearby) stars in the coming years. The Transiting Exoplanet Survey Satellite (TESS), slated for launch in August 2017, aims to do the same from space. By targeting nearby stars unlike Kepler’s largely distant stellar sample, these missions both hope to find exoplanets that will be easier to follow up on.
Another exoplanet hunter, known as Evryscope, is in the construction phase. Led by Nicholas Law (University of North Carolina at Chapel Hill), the team proposes a vast telescope array to continuously image the entire sky. The prototype array places 27 individual telescopes into a common mount; an attached 780 million-pixel detector records a continuous movie of the entire night sky in 2-minute exposures.
Evryscope will see first light this spring or summer at the Cerro Tololo Inter-American Observatory in Chile. Among many possible applications, this instrument could detect transiting giant planets or, around nearby M dwarfs, rocky planets in the habitable zone.
Direct-Imaging Is the New Black
While transit-based searches may still bring in most of the goods, direct-imaging technology is also coming into its own, and for good reason.
Imaging planets directly as they orbit their (very bright) stars naturally leads to the discovery of planets on large orbits, rather than the tight orbits typical of transit-based finds. In addition, planets found by direct imaging can also yield direct spectra that shed light on their atmosphere’s composition.
So far only 26 planets have been imaged directly (excluding brown dwarfs), but that should change soon. The Gemini Planet Imager (GPI, pronounced gee-pie) is perhaps the best-publicized project, but the instruments SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) and SCExAO (Subaru Coronagraphic Extreme AO) are also on the direct-imaging hunt.
During GPI’s first week of observations, the instrument targeted a known system, imaging three planets (c, d, and e) in the HR 8799 system, and capturing spectra for two (c and d). The observations, to be published in the Astrophysical Journal and announced at the AAS, show that planets c and d, once thought to be similar in size and composition, might actually be quite different.
It’s not clear yet whether the differences lie in chemical composition or cloud cover. Marshall Perrin (Space Telescope Science Institute) suggested at a AAS press conference that the differences could be explained if planet c had patchy clouds that allow for a deeper look into its atmosphere.
The results are intriguing, says Jonathan Fortney (University of California, Santa Cruz), but he cautions that detailed analysis might be affected by systematics. After all, the observations were taken as part of GPI’s first light, as astronomers and technicians were still working out the instrument’s kinks, and under less than ideal weather conditions.
Indeed, new observations of HR 8799 are already planned for this summer, says Thayne Currie (Subaru Telescope / NAOJ). And, now that GPI has entered full operations, it will begin its survey of 600 carefully selected stars — the search for new, directly imaged exoplanets is on.
Find the answers to the most pressing questions about our galaxy's enormous population of exoplanets in Sky & Telescope's August 2013 issue, featuring exoplanet guru Sara Seager.