Best view ever of the birth of a solar system
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How did our Solar System look like when it was still forming? We don’t have a time machine to go back 4.6 billion years and take a look. But there’s another way to find out. We can look at another solar system that is forming right now. That will probably give us a good idea of our Solar System’s birth.
There’s one problem, though. Other solar-systems-in-the-making are usually very far away. It will be hard to study them in detail. Now, ALMA has set its sights on one of the nearest systems we know. It hosts a star called TW Hydrae. The star is just 10 million years old – very young for a star. It is still surrounded by a flat, rotating disk of gas and dust. In the future, the material in the disk will clot together into planets. And the best news is: TW Hydrae is ‘only’ 175 light-years away.
175 light-years may be nearby in cosmic terms, but it’s still an enormous distance of some 1,700 trillion kilometers. Nevertheless, ALMA astronomers were able to image the disk of TW Hydrae in surprising detail. That was possible by placing the individual ALMA antennas as far away from each other as possible, at mutual distances of up to 15 kilometers.
ALMA captured the faint radio waves from millimeter-sized dust particles in the disk. These particles represent the very first step toward the formation of full-fledged planets. In fact, much larger bodies may already have formed. At distances of 3 and 6 billion kilometers from the star, the astronomers detected dark gaps in the disk. Here, smaller dust grains may already have been swept up by larger planets.
The two circular gaps are at the same distance from TW Hydrae as Uranus and Pluto are from the Sun. But the very center of the disk also appears to be empty. That central hole is about the size of Earth’s orbit around the Sun. So maybe Earth-like planets have already formed in the disk, too!
TW Hydrae is a very faint, young star in the constellation Hydra, the Water Snake. It is part of larger group of some thirty newborn stars, called the TW Hydrae Association. This young stellar family was born some 10 million years ago. It’s the nearest star forming region in our Milky Way galaxy, at a distance of only 175 light-years. Some 4.6 billion years ago, our own star, the Sun, was probably born in a similar group.
The ALMA observations of TW Hydrae were carried out by a large international group, led by Sean Andrews. Sean is an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. ALMA observed the star on three days in the fall of 2015: 23 November, 30 November and 1 December. Sean and his colleagues described their result in an article in Astrophysical Journal Letters – a professional astronomical magazine.