This image may be the first look at exomoons in the making

It offers some of the best evidence yet that planets around other stars have moons, or exomoons

Observations by the ALMA telescope array in Chile show the young star PDS 70 surrounded by a ring of dust. The bright dot just inside that ring is a smaller disk of potentially moon-forming material. It surrounds a young planet called PDS 70c.

ALMA/ESO, NAOJ and NRAO, M. Benisty et al

New telescope images may provide the first views of moons forming outside the solar system.

The images come from the ALMA telescope array in Chile. It glimpsed a dusty disk around an exoplanet that orbits a star roughly 370 light-years away. That disk could have enough material to make up to 2.5 moons the size of Earth’s.

Researchers described their findings online July 22 in the Astrophysical Journal Letters.

Observing this system could offer insight into how planets and moons form around stars.

ALMA observed PDS 70 in July 2019. This star is circled by two planets. Like Jupiter, both are giant worlds. The planet closer to the star is named PDS 70b. The other is PDS 70c. Unlike most other known exoplanets, these two are still forming. They’re gobbling up bits of dust and gas that swirl around their star. As they form, planets are expected to wrap themselves in their own debris disks. These disks are thought to control how planets pack on material and build moons.

ALMA spotted a disk of dust around PDS 70c. It’s about as wide as Earth’s orbit around the sun. This disk offers some of the best evidence yet that planets around distant stars form moons, known as exomoons. Other astronomers claim to have spotted fully formed exomoons. But those observations are only tentative.

close-up of potential moon forming inside ring of debris around planet PDS 70c
Just inside a ring of dust surrounding a young star is the planet PDS 70c. That exoplanet is surrounded by its own disk of possibly moon-forming material (bright dot at center).ALMA/ESO, NAOJ and NRAO, M. Benisty et al

Unlike PDS 70c, the planet PDS 70b does not appear to have a moon-forming disk. That may be because it has a smaller orbit. PDS 70b is about as far from its star as Uranus is from the sun. PDS 70c is nearly as far as Pluto is from the sun. That puts PDS 70c closer to an outer ring of dust around the star. PDS 70c can easily steal some material from the ring to feed its moon-forming disk.

“C is getting all the material from the outer disk,” says Jaehan Bae. “B is getting starved.” Bae is an astrophysicist. He works at the Carnegie Institution for Science in Washington, D.C.

“In the past, b must have gotten some material in its [disk],” Bae says. “It could have already formed moons.” But to make the new images, ALMA observed wavelengths of light emitted by sand-sized dust grains, not large objects. So any such b moons would not be visible.

Maria Temming is the assistant editor at Science News for Students. She has bachelor's degrees in physics and English, and a master's in science writing.

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