Just a few years back (well, a coupla decades) it wasn’t well understood how planets form. There were theories, sure, and the best one of the lot was that when a cloud of gas and dust collapsed (due to a collision with another cloud, or maybe the wind from a nearby star, or getting slapped around by a supernova blast), it formed a flat disk. The star formed in the center, and the planets formed farther out.
This theory has been validated many times, and astronomers now accept it as true (I’ve written about this here, here, and here, for example). In fact, we’ve gotten pretty good at finding young stars still surrounded by their disks, and Hubble is particularly good at imaging them. The disk is faint, and can generally be seen because it reflects the light from its parent star, but that star can be thousands of times brighter than the disk. Hubble is good at high-contrast objects like that; the camera I worked on for years was awesome at imaging them (I wrote about one particular star/disk combo on my Bitesize pages).
And now, just a few years into this field of science, it’s gotten to the point where we can catalog oddities; objects that make us scratch our heads and say, "What’s going on here?"
Enter HD 15115.
HD 15115 is a young star, maybe 12 million years old, and is located about 150 light years away from us. It’s pictured at the top of this post in a new Hubble image released today (a paper with more detail was published in April). The gray circle is what’s called an occulting mask — no, not something you wear at Halloween, but a piece of metal that blocks the overwhelming light of the star so fainter objects can be seen near it. This is precisely like trying to see an airplane flying in the sky when it gets near the Sun– you hold your hand to block the Sun, and you can see the fainter airplane. The gray stripe on the left of the image is another occulting bar, called a finger.
The disk of the star is seen almost exactly edge-on. This is a fairly typical looking disk at first glance. In this false-color image, it’s bright near the star, and fades away farther out. But look again: you’d expect the disk to be symmetric around the star… in other words, the left and right sides should be about the same brightness and size.
But they’re not! On the right of the star, the disk is much brighter and longer than on the left! In fact, the left (east) side of the disk stretches out to about 47 billion kilometers from the star, while the right (west) side goes out to about 83 billion kilometers. For comparison, Pluto is about 5 billion miles from the Sun, so this is not only a huge disk, but it’s really lopsided. This result has also been confirmed using the monster 10 meter Keck telescope in Hawaii. What gives?
Well, clearly the disk isn’t symmetric. There must be more material on the right side, which makes it look brighter (it can reflect more starlight). But it’s also longer, indicating that it’s physically longer on one side than the other — meaning the disk is probably highly elliptical.
We’ve seen asymmetric disks before; usually it’s due to a planet tugging on the disk. Dust clumps up on one side, making it brighter. It’s certainly possible that’s what’s happening here.
But there is another possibility, and it’s intriguing to say the least. HD 15115 is part of a cluster of young stars that were all born at the same time. After 12 million years the cluster is breaking apart (sniff– it’s tough when they leave the nest!) and the stars are starting to scatter. Located 10 light years away from HD 15115 is another star, HIP 12545 (HD = Henry Draper catalog; HIP = Hipparcos catalog). It’s possible that HIP 12545 passed close enough to HD 15115 to disturb the dust disk, causing the asymmetry. The gravity of the passing star stretched and distorted the disk.
That’s pretty cool, if true. It’s pretty rare in general for stars to get close enough to one another to cause any real effects — the Sun can go billions of years without it happening — but the odds get stacked when the stars form in clusters. Encounters are more common, and so it’s possible a stellar interloper whacked up the disk around HD 15115.
I expect we’ll see more of these as we get better at observing young stars. I also expect we’ll get more surprises, too. One of the most fun parts about a young field of science like this one is that just as you start to get comfortable and think you understand what’s happening, bam! The Universe tosses something like this at you and you have to rethink your ideas. It distorts your disk, so to speak.
But it also keeps you on your toes. Scientists who study objects like this need to be ready to reshape their ideas when they get a curveball, and it’s one of the reasons science is so strong. Mental exercise and science: they’re good for your brain.