Almost 200 new potential planets have been spotted by the Kepler spacecraft, of which 104 have been confirmed. Although not as large as some previous Kepler announcements, the latest haul is the largest since damage to the telescope's balancing mechanism ended the initial mission. The announcement also includes a number of planets that may lie in their star's habitable zones, including one system containing at least four rocky worlds.
The original Kepler mission discovered more than 1,000 planets around other stars (exoplanets) that have since been confirmed by other means, as well as many further candidates. But then two of the reaction wheels responsible for pointing the telescope failed.
This halted the original mission, but engineers proposed a mechanism, known as K2 or “Second Light”, that could keep Kepler operating if its targets were restricted to moderately bright stars, generally those that make up in closeness to Earth what they lack in intrinsic brightness. As before, Kepler finds planets by detecting their drop in brightness as they pass across their star's face.
Although a handful of planets had been announced from K2 prior to this week, the forthcoming publication of 104 discoveries in the Astrophysical Journal Supplement Series (preprint available in arXiv) marks a dramatic escalation, increasing by 30 percent the number of planets known around 9th-13th magnitude stars, bright enough to see with a medium-sized amateur telescope.
After examining the radial velocities of stars associated with 197 K2 candidate planets, a team led by Dr Ian Crossfield of the University of Arizona concluded that 104 are real, 30 false alarms, and 63 still undetermined. Surprisingly, false positives are most common for worlds large enough to be gas giants.
Of the confirmed finds, 37 have radii less than double that of Earth, and five probably have Earth-like temperatures.
"An analogy would be to say that Kepler performed a demographic study, while the K2 mission focuses on the bright and nearby stars with different types of planets," Crossfield said in a statement. "The K2 mission allows us to increase the number of small, red stars by a factor of 20, significantly increasing the number of astronomical 'movie stars' that make the best systems for further study."
The study's highlight is adding three new planets to the one Kepler previously discovered around star K2-72, all with diameters 20 to 50 percent larger than Earth's, making them almost certainly rocky planets, rather than small gas giants. Their orbital resonance offers hope for determining their masses.
All four planets orbit much closer to their star than Mercury does to the Sun. Allowing for the fact that K2-72 is a faint M2 red dwarf, the outer two planets probably get similar radiation to Earth. This doesn't necessarily make them suitable for life. There is a lot of debate as to whether planets around red dwarfs would be subject to lethal stellar outbursts. Nevertheless, at 181 light-years from the Sun, these look like priority targets for further exploration.
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