Kepler-1625b is a super-

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

orbiting the Sun-like star Kepler-1625 about away. The large gas giant is approximately the same radius as

and orbits its star every 287.4 days. In 2017, hints of a Neptune-sized exomoon in orbit of the planet was found using photometric observations collected by the Kepler Mission. Further evidence for a Neptunian moon was found the following year using the

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...

, where two independent lines of evidence constrained the mass and radius to be Neptune-like. The mass-signature has been independently recovered by two other teams. However, the radius-signature was independently recovered by one of the teams but not the other. The original discovery team later showed that this latter study appears affected by systematic error sources that may influence their findings.

Characteristics

Mass and radius

Kepler-1625b is a Jovian-sized gas giant, a type of planet several times greater in radius than Earth and mostly composed of hydrogen and helium. It is 11.4 times Earth's radius, approximately equal to that of the planet

. However, it is up to 11.6 times more massive (about 3,700

Earth masses An Earth mass (denoted as M_\mathrm or M_\oplus, where ⊕ is the standard astronomical symbol for Earth), is a unit of mass equal to the mass of the planet Earth. The current best estimate for the mass of Earth is , with a relative uncertainty ...

), based on radial velocity observations. This puts it just below the deuterium-fusing limit, which is around 13

Jupiter masses Jupiter mass, also called Jovian mass, is the unit of mass equal to the total mass of the planet Jupiter. This value may refer to the mass of the planet alone, or the mass of the entire Jovian system to include the moons of Jupiter. Jupiter is b ...

. Any more massive and Kepler-1625b would be a

brown dwarf Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most ...

. However, this mass value only corresponds to an 3-sigma upper limit and the mass of the planet remains undetected at this time.

Orbit and temperature

Unlike the gas giants in our Solar System, Kepler-1625b orbits much closer, slightly closer than the orbital radius as the Earth around the Sun. The planet takes to orbit Kepler-1625, as a result of the star's slightly greater mass than the Sun. Kepler-1625b receives 2.6 times more insolation than the Earth, meaning it lies at the inner edge of the habitable zone. However, as the planet has likely no solid surface, bodies of liquid water are impossible.

Candidate exomoon

In July 2017, researchers found signs of a

Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...

-sized exomoon (a moon in another solar system) orbiting Kepler-1625b using archival Kepler Mission data. In October 2018, researchers using the

published new observations of the star Kepler-1625 which revealed two independent lines of evidence indicative of a large exomoon

Kepler-1625b I Kepler 1625b I, a possible moon of exoplanet Kepler-1625b, may be the first exomoon ever discovered (pending confirmation), and was first indicated after preliminary observations by the Kepler Space Telescope. A more thorough observing campaign ...

. These were a 20-minute

Transit Timing Variation Transit-timing variation is a method for detecting exoplanets by observing variations in the timing of a transit. This provides an extremely sensitive method capable of detecting additional planets in the system with masses potentially as small as ...

signature that indicated an approximately Neptune-mass moon, and an additional photometric dip that indicated a Neptune-radius moon. The relative phasing of the two signatures was also consistent with that which a real moon would cause, with the effects in anti-phase. The study concluded that the exomoon hypothesis is the simplest and best explanation for the available observations, though warned that it is difficult to assign a precise probability to its reality and urged follow-up analyses. In February 2019, a reanalysis of the combined Kepler and Hubble observations recovered both a moon-like dip and similar

transit timing variation Transit-timing variation is a method for detecting exoplanets by observing variations in the timing of a transit. This provides an extremely sensitive method capable of detecting additional planets in the system with masses potentially as small as ...

signal. However, the authors suggested that the data could also be explained by an inclined hot-Jupiter in the same system that has gone previously undetected, which could be tested using future Doppler spectroscopy radial velocity measurements. A second independent reanalysis was published in April 2019, which recovered one of the two lines of evidence, the transit timing variation, but the not the second, the moon-like dip. The original discovery team responded to this soon after, finding that this re-analysis exhibits stronger systematics in their reduction which may be responsible for their differing conclusion.

References

{{reflist Giant planets in the habitable zone Exoplanets discovered by the Kepler space telescope Exoplanets discovered in 2016 Cygnus (constellation)