We can't witness the birth of our own Solar System, but the Atacama Large Millimeter/submillimeter Array (ALMA) is providing a picture of how it may have happened. ALMA spotted signs of three giant planets forming around a young star in our cosmic neighborhood. The technique astronomers used to study these planets is one that can be used to find other newborn worlds, and see exactly how clouds of gas and dust turn into something like the Solar System.
ALMA image of the protoplanetary disk around the star HD 163296. The gaps in the disk are where... [+]
The star, which astronomers gave the memorable name HD 163296, is only about 4 million years old, which in cosmic terms makes it a baby. Researchers used ALMA to take detailed images of the disk of dust and gas surrounding the star, which showed three gaps. By studying the motion of carbon monoxide gas within the disk, the astronomers showed it was being moved by massive objects living in those gaps — a telltale sign of newborn planets. These findings were published in a pair of articles in Astrophysical Journal Letters.
We've known for a long time that star systems form out of protoplanetary disks: flat, spinning platters made of gas and dust around a newborn star. Lumps inside those disks collect matter and grow into planets, which explains why all the planets in the Solar System orbit the Sun in the same direction and on more or less the same plane. That much is uncontroversial, but the details aren't clear, especially since most other star systems we've observed don't look much like the Solar System. So, finding newly forming planets might help resolve some of the difficulties.
That's where ALMA comes in. It's an observatory in the high-elevation Atacama Desert in Chile, made up of 66 individual telescopes that observe light around one millimeter in wavelength, at the border between infrared and microwave. This kind of light is particularly useful for studying newborn planets, because it shows where the dust and certain types of molecules live.
In this case, two groups of astronomers around the globe used ALMA to study the flow of carbon monoxide (CO) molecules in the protoplanetary disk around HD 163296. While carbon monoxide is a poisonous component of air pollution on Earth, it's also a very common part of many interstellar clouds of gas (along with comets in the Solar System). These astronomers measured the speed of the CO using the Doppler effect, which is familiar from the shift in the sound of ambulance sirens as they pass by. In this case, the motion of the molecules produced a shift in the wavelength of the light emitted by the CO, which the researchers used to measure how fast it was moving.
They found the gas wasn't just orbiting HD 163296: it was being pulled toward objects hidden in the gaps in the disk. The researchers were even able to estimate the mass needed to pull the CO out of its ordinary orbit: each gap contained something a few times the mass of Jupiter. In other words, the most likely suspect is three giant planets forming inside the disk. (Even giant planets aren't very bright, so it's rare for astronomers to be able to photograph them.) They're much farther out from HD 163296 than Jupiter is from the Sun, but astronomers think many planets may form that far out and migrate inward.
The great thing is how precisely ALMA was able to measure the Doppler effect: to a few meters per second. In perspective, that's measuring the motion of gas 300 light-years away to within about 10 miles per hour. Since CO is a common molecule in protoplanetary disks, this method is likely to be useful for studying other young star systems. It might not quite be our Solar System's baby pictures, but it's about as close as we can get.
