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The first exomoon may have been spotted

3 October 2018

Data from Kepler and Hubble suggest that a giant planet in a faraway solar system has an orbiting companion.

Columbia University astrophysicists Alex Teachey and David Kipping may have seen the telltale signs of an exomoon, a satellite orbiting a planet in a distant solar system. Initial indications came from photometry conducted aboard the Kepler spacecraft, which continuously monitored a portion of the Milky Way. A subsequent observation with the Hubble Space Telescope significantly strengthened the case. Teachey and Kipping announced their results in a press conference on Monday of this week; Science Advances is publishing their paper today. Here’s what you need to know.

Fly me to the exomoon
Rendition by Dan Durda

The stats: The putative exomoon orbits a planet called Kepler-1625b, about the size of Jupiter and most likely a few times as massive. The moon itself is extraordinarily large, about the size and mass of Neptune. Planet and moon orbit the star Kepler-1625, which is 8000 light-years from Earth, as determined by parallax measurements made in the Gaia survey. The period of the planet’s orbit of Kepler-1625 is about 9.5 months.

The evidence: When a planet passes in front of its host star, it briefly blocks some of the stellar light; intensity returns to normal as the planetary transit is completed. When a planet is unaccompanied, the transits are periodic. A moon, however, will gravitationally tug a planet to and fro as moon and planet together orbit their host. As a result, the transits will not quite be regular. Initial evidence for the exomoon came from three transits observed by the Kepler spacecraft. Significantly more precise data were provided by two days’ follow-up observations with the Hubble Space Telescope in October 2017. The transit observed by Hubble began about 75 minutes earlier than it would have, absent some perturbing influence. Moreover, Hubble photometry revealed a second, smaller dip in stellar intensity after Kepler-1625b completed its transit. Together, Teachey and Kipping argue, the two-dipped Hubble structure and Kepler triplet are best explained as arising from a planet–moon system.

A quiescent host star: Sunspot activity and other stellar dynamics could conceivably mimic the effects of a transit. To help rule out that possibility, Teachey and Kipping viewed the event in two wavelength bins. Stellar activity would typically manifest itself differently in the two bins, but the Columbia researchers saw no evidence of different behavior in the two channels.

Hopefully, a follow-up: The next transit of Kepler-1625b will be in May 2019, and Teachey and Kipping have already requested Hubble time to observe it. Their planet–moon model makes a testable prediction: The 2019 planetary transit will be preceded by a lunar transit.

An irony: The Kepler data that inspired Teachey and Kipping to request Hubble time was updated between the researchers’ initial analysis and their Hubble observations. The updated data, in isolation, make a weaker case for an exomoon than did the original data. Nonetheless, the updated data in conjunction with the 2017 Hubble follow-up clearly favor the planet–moon model above all others considered by the Columbia team.

A strange moon: The large size of the exomoon is a surprise. Such a large satellite does not comfortably fit any of the standard stories of moon formation—condensation from a protoplanetary disk, production from a planetary impact, or being captured by a planet—but it is not ruled out in such scenarios. Teachey and Kipping say their model suggests that the lunar orbit is tilted by 45° with respect to the planetary orbit, though the evidence is not strong. If the exomoon’s existence is confirmed, they say, it “will certainly provide an interesting puzzle for theorists to solve.”

The paper: A. Teachey, D. M. Kipping, Sci. Adv. 4, eaav1784 (2018).

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