Astronomers identified a second possible exomoon-which was about??

Abstract

Exomoons represent a crucial missing puzzle piece in our efforts to understand extrasolar planetary systems. To address this deficiency, we here describe an exomoon survey of 70 cool, giant transiting exoplanet candidates found by Kepler. We identify only one exhibiting a moon-like signal that passes a battery of vetting tests: Kepler-1708 b. We show that Kepler-1708 b is a statistically validated Jupiter-sized planet orbiting a Sun-like quiescent star at 1.6 au. The signal of the exomoon candidate, Kepler-1708 b-i, is a 4.8σ effect and is persistent across different instrumental detrending methods, with a 1% false-positive probability via injection–recovery. Kepler-1708 b-i is ~2.6 Earth radii and is located in an approximately coplanar orbit at ~12 planetary radii from its ~1.6 au Jupiter-sized host. Future observations will be necessary to validate or reject the candidate.

   Dubbed Kepler 1708 b i, the satellite has a radius about 2.6 times that of Earth, and circles a Jupiter-sized exoplanet that orbits its parent star about once every two Earth years, the team reports January 13 in Nature Astronomy. That sunlike star lies about 5,700 light-years from Earth.

To find this nugget, the team sorted through a database of more than 4,000 exoplanets detected by NASA’s now-retired Kepler space telescope. Because large planets orbiting far from their parent star are more likely to have moons large enough to be detected, the team focused on a subset of 70 exoplanets.

Each of these planets is between half and twice the size of Jupiter. They all either take more than 400 Earth days to orbit their star or have an estimated average surface temperature less than 300 kelvins (around 27° Celsius), slightly higher than that of Earth.

After further screening, including tossing out exoplanets that don’t have near-circular orbits (which are statistically less likely to host moons), the team identified a strong candidate for an exomoon. It, like its host planet, caused detectable dimming of the parent star’s light when moving across the face of the star.

Discovery of the first possible exomoon, dubbed Kepler 1625 b, has faced a lot of skepticism (SN: 4/30/19). Both proposed exomoons need to be confirmed by further observations by other instruments, such as the recently launched James Webb Space Telescope, the team notes (SN: 10/6/21).

But fresh observations will need to wait: The newfound exomoon candidate and its planet won’t pass in front of the parent star again until March 24, 2023, the researchers calculate.

the last three decades, more than 4,000 planets around stars other than the Sun, exoplanets, have been discovered. These worlds display remarkable diversity, from highly eccentric Jupiters1 to compact, coplanar systems of terrestrial planets2. In an effort to understand the formation and evolution of such systems, more detailed knowledge about their environment and properties is sought3—such as the existence and nature of potential satellites4. Given the abundance of moons in our Solar System, it is reasonable to presume that exomoons will reside around some exoplanets—which has motivated efforts to detect them5,6.

One of the most promising strategies for seeking exomoons focuses on transiting planets7,8,9: worlds that periodically eclipse their stars and make up the majority of the discovered exoplanets. However, the observational bias of transit surveys10 leads to an under-representation of long-period, cool planets—precisely the type of planet where moons are thought to be most likely due to dynamical considerations11,12. Nevertheless, a small sample of long-period planetary candidates was discovered by Kepler13,14,15,16,17—worlds with orbits greater than that of the Earth around the Sun. The Jupiter-sized planets amongst these are of particular interest, as satellite formation is thought to be a natural outcome of how such planets form18.

To date, very little is known about the prevalence and properties of exomoons. Initial surveys largely focused on planets interior to 1 au (ref. 19), since this was broadly the only sample available at the time. Around these relatively close-in planets, large moons appear uncommon, with the abundance of Galilean-like satellite systems measured to be <38% to 95% confidence20. However, amongst the longest periods of these worlds, the ~1 au Jupiter-sized planet Kepler-1625 b was reported to exhibit a timing variation and transit signature consistent with a large Neptune-sized/mass moon using Hubble Space Telescope photometry21. Both of these were independently recovered in one study22, but only one (the timing) in another23—shown later to be possibly due to higher systematics in their photometric reduction24. Much like hot Jupiters, such large moons were not widely anticipated in the literature. However, subsequent theoretical work has shown that the candidate exomoon could form through a capture scenario25 or a massive circumplanetary disk26.

With no published exomoon surveys for planets at ≳1 au, and the intriguing hint of Kepler-1625 b-i, the aforementioned Kepler sample of long-period giant planetary candidates represents one of the most promising unturned stones. To address this, we here present a survey of Kepler’s cool, gas giants.