ALMA discovers moons in the making

Every child on the planet knows the moon: a ball of rock orbiting the Earth. Most of the other planets in our solar system have moons, too. The giant planet Jupiter even has a whopping amount of 79 satellites – four large ones and dozens of tiny moonlets.

So, what about the planets in other solar systems? Over the past 25 years, we have discovered over 4,000 of these so-called exoplanets, orbiting other stars than our own sun. Do they also have moons?

So far, no single ‘exomoon’ has ever been found. But astronomers using ALMA have now made an extraordinary discovery. They found a concentration of dust around a young exoplanet. In the future, this dust will probably clump together into a number of moons.

The central star of the system is known as PDS70. It is about 10 million years old. That’s actually very young for a star. PDS70 is still surrounded by a large ring of gas and dust – material that can clump together into planets.

In fact, two giant planets have already formed around PDS70. They’re about as large and massive as Jupiter. The two planets (PDS70b and PDS70c) were first seen by the European Very Large Telescope (VLT). The VLT is also in northern Chile, just like ALMA.

New observations with ALMA have now revealed a concentration of small dust particles, close to the inner edge of the ring. The dust particles are about one-tenth of a millimeter across. Because of their temperature, they emit invisible radio waves that ALMA can detect.

When the ALMA astronomers compared their results with the observations of the VLT, they were happily surprised. The concentration of dust particles is at precisely the same location as the outer planet, PDS70c.

That can mean only one thing. The dust particles must orbit the young planet. So if these dust particles will also clump together in the future, we will be left with a giant planet with its own system of moons.

Four and a half billion years ago, our solar system was as young as PDS70 is now. Back then, a similar disk of dust probably surrounded the newly born planet Jupiter. Jupiter’s four large moons were formed from that disk. So the new discovery may also help astronomers to better understand the formation of moons in our own solar system.


PDS70 is a young star in the southern constellation Centaurus, at a distance of some 370 light-years. It was born just 10 million years ago. The star is a bit less massive than our own sun. In 2006, astronomers discovered that the star is surrounded by a planet-forming disk of gas and dust. In 2018, a giant planet was found: PDS70b. One year later, a second planet was discovered: PDS70c. Both planets are a few times larger and more massive than Jupiter. PDS70c orbits the star at a distance of just over five billion kilometers, about the same distance as Neptune is from the sun. At this considerable distance, it takes almost 250 years to complete one orbit. ALMA has now discovered that moons are forming around this second planet.


The concentration of small dust particles surrounding the planet PDS70c was discovered by a group of scientists led by Andrea Isella. Andrea is an astronomer from Rice University in Houston, Texas. In studying PDS70 he worked together with Myriam Benisty and Laura Pérez from Chile, Richard Teague and Jaehan Bae (USA), and Miriam Keppler and Stefano Facchini (Germany). The new results have been published in Astrophysical Journal Letters.

Check this in ALMA site