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Lhc essay

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.

Duh. I mean, sure, that’s why we call it space. But what’s so very interesting about this is what you see when you look on very large scales; scales so huge that galaxies become mere dots. You might expect matter to be strewn evenly throughout the Universe, but it isn’t. Over these vast vistas, matter in the Universe is clumped, falling along huge filaments and sheets. These in turn are curved, closing in on themselves.

The Universe is foamy! It looks like a sponge, with matter clumping along the outside of the bubbles. Shortly after the Universe formed, dark matter clumped up, creating those filaments. These acted like gravitational scaffolding. its gravity attracting normal matter, which then fell onto the filaments like Spanish moss hanging from tree branches. This material formed galaxies and clusters of galaxies.

Matter is still falling into those filaments today. As it does, the voids—the bubbles of the sponge—get bigger. We can predict how they grow using Einstein’s Theory of Relativity, which describes how the Universe expands and how the matter and energy in it behave as it does. This video from the Max Planck Institute for Astrophysics shows that growth in a computer simulation:

But there’s more to this: Dark energy, the weird stuff pervading space that’s causing the Universe to expand faster every day, is also in there, inflating the voids. Relativity, as originally formulated, doesn’t include that. If we can measure just how voids grow we can use that as a test of relativity and also understand better how these gigantic pockets of nothing get bigger. To do this, traditionally, astronomer measure that growth by examining the galaxies along the bubble edges.

But that turns out to be hard, because galaxies aren’t just falling onto the filaments. They also have what’s called peculiar motion, sideways velocity which makes measuring their precise velocity difficult. What to do?

A group of astronomers did something clever. Instead of looking at the galaxies, they looked into the voids themselves. Using observational data that shows distances to galaxies in the Universe—and therefore the locations of voids—they compare how these voids change in shape over time to what’s expected by computer models using relativistic calculations.

Not surprisingly, they find relativity to be pretty robust. The behavior of the Universe appears to obey the rules laid down by relativity, which is reassuring. I’ll note relativity has been tested approximately a bazillion times. and always comes up looking good .

So while that’s not exactly shocking, what I find interesting about this is that by looking at the voids instead of the galaxies around them, the astronomers who did this work were able to improve on previous methods dramatically, with uncertainties (that is, statistical accuracies) four times smaller than previous models!

That’s pretty dang good. We’re still learning just how the Universe behaves on the very largest scaled; heck, dark energy was only discovered in 1998! Research like this will help us understand what the Universe itself is doing as it ages and grows, and that to me is simply stunning. Even if you don’t understand the details of the work or the math behind it, know this: Astronomers are trying to understand literally everything, across all space and time.

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