Exomoon the size of Neptune

An Exoplanet that is several times the mass of Jupiter has a moon that is the size of Neptune. The moon is estimated to be only 1.5 percent the mass of its companion planet. They are both 8000 light years away.

The exoplanet is Kepler 1625b. The planet and its satellite are both gaseous.

The host planet and its moon lie within the solar mass star’s (Kepler 1625) habitable zone, where moderate temperatures allow for the existence of liquid water on any solid planetary surface. “Both bodies, however, are considered to be gaseous and therefore unsuitable for life as we know it,” Kipping said.

Future searches will target Jupiter-sized planets that are farther from their star than Earth is from the Sun.

Science Advances – Evidence for a large exomoon orbiting Kepler-1625b

Abstract
Exomoons are the natural satellites of planets orbiting stars outside our solar system, of which there are currently no confirmed examples. We present new observations of a candidate exomoon associated with Kepler-1625b using the Hubble Space Telescope to validate or refute the moon’s presence. We find evidence in favor of the moon hypothesis, based on timing deviations and a flux decrement from the star consistent with a large transiting exomoon. Self-consistent photodynamical modeling suggests that the planet is likely several Jupiter masses, while the exomoon has a mass and radius similar to Neptune. Since our inference is dominated by a single but highly precise Hubble epoch, we advocate for future monitoring of the system to check model predictions and confirm repetition of the moon-like signal.

The search for exomoons remains in its infancy. To date, there are no confirmed exomoons in the literature, although an array of techniques has been proposed to detect their existence, such as microlensing, direct imaging, cyclotron radio emission, pulsar timing, and transits. The transit method is particularly attractive, however, since many small planets down to lunar radius have already been detected, and transits afford repeated observing opportunities to further study candidate signals.