Exomoons: Satellites Orbiting Planets Beyond Our Solar System

An artist’s illustration of the exoplanet Kepler-1625b with its hypothesized moon, which is thought to be about the size of Neptune. This candidate moon, and others like it, could host satellites of their own, researchers say. Credit: Dan Durda
Extrasolar moons, or exomoons, are moons that orbit a planet outside our solar system, called an exoplanet. Although nearly 4,000 exoplanets have been discovered by space telescopes like NASA’s Kepler and TESS since 2009, only one exomoon has been described — and it’s still unclear if it’s actually a moon.

By Jeremy Rehm | SPACE.com

Astronomer David Kipping and his graduate student Alex Teachey at Columbia University in New York first reported the possible exomoon in October 2018 in the journal Science Advances.

Using NASA’s Hubble space telescope, the pair watched as Kepler-1625b — a Jupiter-size exoplanet — passed in front of its star, causing a slight dip in the amount of light visible from Earth.

This „transit method“ is how thousands of exoplanets have been discovered so far. But watching Kepler-1625b transit in front of its star came with two surprises. First, the exoplanet completed its transit about 1.25 hours earlier than expected, which suggested that something was gravitationally tugging on it. Then there was a shallow dip in light shortly after the planet finished passing in front of the star, possibly indicating a satellite trailing behind Kepler-1625b.

„We’ve tried our best to rule out other possibilities such as spacecraft anomalies, other planets in the system or stellar activity, but we’re unable to find any other single hypothesis which can explain all of the data we have,“ Kipping said during a teleconference with reporters.

The alleged exomoon is the size of Neptune — around a third the size of Kepler-1625b. That’s enormous for a satellite, which is usually much smaller than the planet it orbits. Such moons are predicted to be quite rare, if not impossible, by current models of planet-moon system formation.

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