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Astronomers Think They’ve Spotted a Moon Forming Around an Exoplanet

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Image of the PDS 70 system made from ALMA data. The splotch on the right might be the first-ever observed circumplanetary disk.
Image of the PDS 70 system made from ALMA data. The splotch on the right might be the first-ever observed circumplanetary disk.
Image: A. Isella, ALMA (ESO/NAOJ/NRAO

Scientists have detected a signal around an exoplanet that might be the signature of a circumplanetary disk—a disk of debris that could one day form into exomoons.

Astronomers theorize that in Jupiter and Saturn’s early days, the massive planets would trap debris into these orbiting circumplanetary disks. The mass from the disks could then do one of two things: fall onto the planet or clump up into moons. New results from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile show off what might be evidence for one of these disks around a young exoplanet that’s 370 light-years away from Earth.


The exoplanet at hand is called PDS 70c, the second-closest known exoplanet to the parent star PDS 70. It orbits far from its young (approximately 5 million-year-old) parent star, further than the distance between the Sun and Neptune and orbiting slightly closer than a vast ring of dust. Scientists first spotted evidence of PDS 70c earlier this year using the Very Large Telescope (VLT). The researchers interpreted their results not only to be the mark of an exoplanet but one that was gobbling up some of the gas and dust in its vicinity.

A team of researchers from the United States, Chile, France, and Germany followed the VLT observations up by reanalyzing and recalibrating past ALMA data on the star system. They were hunting for submillimeter light, a wavelength that falls between the infrared and the microwave part of the spectrum, and an especially good one for observing clouds of dust. They spotted a fuzzy submillimeter source in the same spot as the PDS 70c.


The scientists claim that they have located a cloud of dust surrounding the large young planet, and inferred that the cloud was a circumplanetary disk. They estimated the disk’s mass at between .002 to .0042 times the mass of the Earth, or somewhere around a quarter the mass of our own Moon.

These dust clouds are theorized to serve important roles in planetary formation, according to the paper published in the Astrophysical Journal Letters. Astronomers think that Jupiter and Saturn’s moons formed out of a ring like this, and that the properties of the ring might determine the properties of the resulting moons. These days, astronomers really care about moons like the ones that formed around Jupiter and Saturn because some of these moons might harbor the conditions for extraterrestrial life.

They also found a mysterious source of submillimeter emission somewhat close to PDS 70b, but couldn’t offer a strong guess as to what that emission was.

These are still inferences based on fuzzy splotches, and there’s still a lot that astronomers don’t understand about how planets form. But if these results hold up to further scrutiny, then PDS 70c could be a planet just finishing up its formation phase and could be a good source for understanding how planets—and maybe their moons—form.