1st planet-forming disk found in another galaxy
Our Earth and the other planets in our solar system were born from a giant disk of dust and gas around our sun. And astronomers have found many such disks in our Milky Way galaxy. They call them circumstellar disks, or protoplanetary disks. It’s in these disks that planets are being born. On November 29, 2023, scientists said that – for the first time – they’ve found a circumstellar disk around a massive young star in another galaxy.
The discovery shows that such planet-forming disks are likely common around stars in other galaxies besides our own.
Researchers from the U.K., Germany and the U.S. used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to make the discovery.
1st extragalactic accretion disk
The circumstellar disk is a type of accretion disk. In accretion disks, things are both orbiting a central body – a sun, or, for example, a black hole – and small bodies are also accreting, or sticking together, to make larger bodies. Thus planets are born. The disk that ALMA discovered is the first accretion disk ever detected in a galaxy outside of our own Milky Way. As Anna McLeod, lead author at Durham University in the U.K., stated:
When I first saw evidence for a rotating structure in the ALMA data I could not believe that we had detected the first extragalactic accretion disk. It was a special moment. We know disks are vital to forming stars and planets in our galaxy, and here, for the first time, we’re seeing direct evidence for this in another galaxy.
This video summarizes the discovery of the 1st planet-forming disk in another galaxy. Video via ESO/ YouTube.
Previous hints of a planet-forming disk
The ALMA observations are the first to confirm the circumstellar disk, but there were previous hints as to its existence. In 2019, the European Southern Observatory (ESO) released an image from its Multi Unit Spectroscopic Explorer (MUSE) instrument on the Very Large Telescope (VLT) of a jet of material being emitted by a young star. That star, HH 1177, is deep inside a gas cloud in the Large Magellanic Cloud. This is the same star ALMA observed the circumstellar disk around. McLeod said:
We discovered a jet being launched from this young massive star, and its presence is a signpost for ongoing disc accretion.
Now, ALMA has confirmed that the astronomers’ suspicions were correct; there really is a planet-forming disk around this star.
Detecting accretion disks
So, how do astronomers detect accretion disks? A major clue has to do with speed. The material – gas and dust – around a young star is pulled in toward the star. But it doesn’t hit the star. Instead, it forms a broad, flattened, spinning disk. Not all the material is moving at the same speed, however. The closer the material in the disk is to the star, the faster it rotates. Astronomers can measure that difference in speed and deduce that there’s an accretion disk around the star. This also affects the frequency of light coming from the disk. Co-author Jonathan Henshaw at Liverpool John Moores University in the U.K. explained:
The frequency of light changes depending on how fast the gas emitting the light is moving towards or away from us. This is precisely the same phenomenon that occurs when the pitch of an ambulance siren changes as it passes you and the frequency of the sound goes from higher to lower.
The discovery will help astronomers learn more about how planets form, both in our galaxy and well beyond. As McLeod said:
We are in an era of rapid technological advancement when it comes to astronomical facilities. Being able to study how stars form at such incredible distances and in a different galaxy is very exciting.
Bottom line: For the first time, astronomers have discovered a planet-forming disk around a young star in another galaxy outside our own Milky Way galaxy.