Astronomers revealed the second exomoon candidate in history. Credit: Helena Valenzuela Widerström

Astronomers Spot Super Moon Orbiting Distant Alien World

This is only the second exomoon candidate in history.


Astronomers have presented the results of a search for exomoons in 70 cold long-period exoplanets. They found a new exomoon candidate, Kepler-1708b-i, which has a mass 2.6 times that of Earth and orbits the gas giant at an average distance of 1.6 astronomical units from its star.

Exoplanets and Exomoons

More than 4,000 exoplanets have been discovered over the past three decades, showing an amazing diversity, from gas giants in very elongated orbits to compact coplanar systems from terrestrial planets.


Given the abundance of natural satellites of planets in the solar system, it is reasonable to assume that exomoons are also widespread in our galaxy, but so far these objects are extremely difficult targets to search for and are only candidates for them. In Earth’s case, it is believed that the collision of the Earth with the hypothetical planet Thea led to the formation of the Moon – it was formed from debris thrown into orbit.

Most often, exomoons are searched for in transit exoplanets that periodically pass through the disks of their stars, with long-period gas giants considered to be the most interesting targets.

When was the first exomoon candidate discovered?

The first-ever candidate for exomoons was discovered by scientists in the summer of 2017 near the planet Kepler-1625 b, about four thousand light-years from Earth.

The satellite signal was recorded during three transits of Kepler-1625 b across the disk of the mother star – this is a small statistic, and therefore astronomers could not unambiguously say whether this object was the first exomoon known to mankind. Moreover, there were scientists that cast doubt on the original conclusions.


Overall, current theories cannot explain how the planet Kepler-1625 b could have received such a satellite. And more importantly, Earth’s version does not work for this object either.

New research attempted to find more exomoon candidates

A team of astronomers led by David Kipping (David Kipping) from Columbia University published the results of a search for exomoons in 70 cold (equilibrium temperature less than 300 kelvin), long-period (orbital period more than 400 days) gas exogiants, with radii greater than two Jupiter radii.

The scientists compared the light curves of the selected exoplanets’ parent stars with model light curves where the exoplanets might have satellites. In the end, the researchers identified only one interesting result, which can be interpreted as a candidate exomoon with a statistical significance of 4.8 sigmas.

Observational and calculated light curves for Kepler-1708b transit events across the star's disk. Credit: David Kipping et al. / Nature Astronomy, 2022
Observational and calculated light curves for Kepler-1708b transit events across the star’s disk. Credit: David Kipping et al. / Nature Astronomy, 2022

What did astronomers find?

We are talking about the exogiant Kepler-1708b, which has a mass of 4.6 Jupiter masses and is located at an average distance from its star of 1.6 astronomical units. It is assumed that within 12 planetary radii of Kepler-1708b is its exomoon, designated Kepler-1708b-i, which has a mass of 2.6 Earth masses.

Scientists emphasize that they cannot find any reason to refute the status of Kepler-1708b-i as a candidate for an exomoon, but call for a cautious interpretation of the results. In their opinion, more detailed observations are needed using the Hubble, James Webb, or PLATO space telescopes to confirm or refute the discovery.

Perhaps the exomoon candidate Kepler-1708b-i could result in being the first reliable discovery of such an object outside the Solar System.

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Bartels, M. (2022, January 13). The Hunt is on for exomoons around alien planets and scientists may have just found one.
EurekAlert! (n.d.). Astronomers find evidence for a second supermoon Beyond our solar system.
Kipping, D., Bryson, S., Burke, C., Christiansen, J., Hardegree-Ullman, K., Quarles, B., Hansen, B., Szulágyi, J., & Teachey, A. (2022, January 13). An exomoon survey of 70 cool giant exoplanets and the new candidate kepler-1708 B-i. Nature News.
O’Callaghan, J. (2022, January 13). Astronomers have found another possible ‘exomoon’ beyond our solar system. Scientific American.

Written by Vladislav Tchakarov

Hello, my name is Vladislav and I am glad to have you here on Curiosmos. As a history student, I have a strong passion for history and science, and the opportunity to research and write in this field on a daily basis is a dream come true.

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