Baby Exoplanets Carve Grooves Into Their Nursery

Contributed by
Dec 14, 2016

HD 163296 is a very young star about 400 light-years from Earth. That’s pretty close as these kinds of objects go, making it a ripe target for astronomers to observe. And by young I do mean young; it’s only about 4 million years old—the Sun is literally a thousand times older than that!—so astronomers chomp at the bit to observe it. It’s a perfect opportunity to see how stars form.

And not just stars, but planets! We’ve known for a while that HD 163296 has a dusty disk surrounding it, and that this is exactly the sort of thing we expect planets to form in. And now new observations show pretty convincing evidence that this baby star has at least two baby planets orbiting it!

The image above reveals the disk around HD 163296 as observed by ALMA, the Atacama Large  Millimeter/submillimeter Array. ALMA sees “colors” of light well outside what our eyes can, wavelengths longer even than infrared. Warm dust around a star emits this kind of light, and the image above shows just that. You can easily see the dust is not just in a disk, but in a set of rings around the star.

Using physics and math, we can predict that forming planets will carve gaps just like that in the disk, their gravity drawing in the material around them as they grow. And there those gaps are!

But there’s an issue: There are also processes in the disk itself that can cause gaps without planets. For example, material closer in to the star orbits it faster than stuff farther out. This can cause turbulence in the disk, mixing the material, a little bit like how stirring your coffee mixes the cream into it. This is complicated by the presence of a magnetic field as well. If you do the math—and it’s pretty fierce to calculate—you find that at a certain distance from the star turbulence will cause a gap to form, and it looks a lot like the gaps carved by planets. How can you tell the difference?

ALMA to the rescue again! It can see different wavelengths of millimeter light (think of them as different colors), which is critical: Dust emits at one wavelength, but gas like carbon monoxide (which we know is present in the disk as well) emits at another. Why is this important? Because a forming baby planet will suck up all the material around it, gas and dust. But the turbulence in the disk affects the dust but not the gas.

ALMA observed HD 163296 to look at both gas and dust, and what the data show is that the two outer gaps are empty of both, but the inner gap still has gas in it. That means the outer two gaps are indeed being created by planets, both probably about the mass of Saturn, but that inner gap is probably due to the internal physics of the disk itself, and is not from a planet.

How about that? We can point a series of big metal dishes at a star 4 quadrillion kilometers away, and not only see where planets are forming but also be able to tease out how its disk is stirred, not shaken. Amazing.

But there’s a little bit more. Another thing that really gets me about these images is how clean they are! The disk looks so sharp, the gaps carved out so well defined. Years ago, I worked on Hubble observations of this very star and disk. I did everything I could to get the data into shape, and at the time the image we put together was the sharpest ever made of the star and disk.

The odd black shapes are artifacts of how I processed the data; the star is millions of times brighter than the disk in visible light, so we aimed Hubble to put it behind a metal “occulting bar” in the camera that blocked the star’s light. The individual images were rotated with respect to each other (that helps eliminate some sources of noise in the data) and when de-rotated and combined the dark shapes were left over. Based on their shapes I called the one in the middle the chameleon, and the one in the upper left the Romulan Warbird.

Anyway, you can faintly see the outer edges of the dust disk, an annulus (or ring) surrounding the star. Now get this: The entire ALMA image would fit inside the dark spot of the chameleon in the middle of our image! ALMA’s combined antennae have far greater resolving power than even Hubble, so it is able to peer more closely at the disk to see details. Also, the star is far, far fainter in the wavelengths ALMA sees, so astronomers don’t have to fuss over needing those pesky occulting bars.

But together these images give us even more information on the size, shape, and thickness of the dust in the disk, how it’s distributed, and even the size of the dust grains (different sized grains scatter starlight differently, and that can be measured in these images).

Now mind you, when I was a kid, we had no clue how planets were born. There were many ideas, some of which now seem silly in retrospect, but the actual process was unknown.

Today, right now, we have actual photographic evidence of exactly that process. Just a few decades ago we could only guess, and now we know. We point our telescopes at nascent solar systems and see for ourselves how nature makes planets. And when we do, we see in our mind’s eye our own solar system, 4.6 billion years ago, just starting to coalesce, our own planet in its birth throes.

As always, I stand in awe of science, and of our own human capacity to learn, to explore, to find out. These are among the best things we do.