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SYFY WIRE Bad Astronomy

Wait. *HOW* fast do giant planets form?

By Phil Plait
Artwork of a planet forming in a young star’s disk. Credit: NAOJ

Back in the day, when I worked on supporting astronomers’ Hubble observations, one of my favorite projects was one involving observing very young stars. These stars, just a few million years old, were still surrounded by a swirling disk of gas and dust which, at the time, we assumed were forming planets.

These observations were cutting edge, and were giving us our first glimpses at these disks in detail. I worked hard to write software that allowed the team to analyze the shape of the disks and the way they reflected their star’s light, all to learn more about the process of planetary formation in its earliest stages.

That wasn’t all that long ago, but oh my, how things have changed. These days, we get images of protoplanetary disks that look like this:

Rings of dust around the very young star AS 209 indicate a planet forming there. Credit: ALMA (ESO/NAOJ/NRAO)/ D. Fedele et al.


That is AS 209, an extremely young star that is no older than about a million years. If that still sounds like a long time to you, remember that the Sun is 4.6 billion years old: 4,600 times older than AS 209!

It’s a star similar to the Sun in that it has about 90% of the Sun’s mass and is about 1.5 times more luminous; young stars tend to be overly luminous for their mass, and as it ages it’ll settle down a bit. AS 209 is part of a stellar nursery about 400 light years from us toward the constellation of Ophiuchus. The star itself is enshrouded in dust and isn’t really visible in this image — it was taken with the Atacama Large Millimieter/submillimeter Array, or ALMA, which is sensitive to warm dust. So what you’re really seeing here is that dust cocooning the star.

But what you also see is that this isn’t really a disk: It’s a set of rings! Around the inner cloud is a faint ring with a gap separating them, then a wider gap separating it from a much larger ring. It’s too faint to see here, but outside the second ring is the actual disk of dust. The inner ring is about 11 billion kilometers out from the star, and the outer one nearly 20 billion. Our entire planetary system could fit comfortably inside the inner ring, for scale.

So why are there rings? Planets! Or more likely, a planet. That’s the first of two very interesting things going on here.

Schematic of the dust rings around the young star AS 209. The axes are directions on the sky (vertical = north/south, horizontal = east/west) and 1 AU is 150 million km. Credit: Fedele et al.

You might think we’re seeing evidence of two planets, one each in the two gaps, with the gravity of those planets eating up material and carving the gaps in the disk to create the rings. But the astronomers who took and analyzed these data have another idea: There’s only one planet, and it’s in the outer gap. It alone may be responsible for everything you see here.

The planet would be a gas giant, probably about 70% the mass of Saturn (or 4 times the mass of Neptune if you prefer; a middlin’ to big gas giant), orbiting the star about 15 billion kilometers out in the outer gap. That gap is pretty well devoid of dust, so the planet is forming from that material there and has eaten most of it. The gravity of the planet also causes the dust outside of its orbit to pile up a bit, creating the outer ring (which is then really just the inner edge of the disk that reaches out to even greater distances).

The inner gap, at about 9 billion km out from the star, is at a special place: If you were a dust grain orbiting the star there, you would go around the star twice for every one time the planet orbits. This is called a resonance, and in circumstances like that it makes the dust grain’s orbit unstable. The planet’s gravity tugs on it cyclically, pulling it out of that location. Some time in the past the disk was smooth there, but over time the planet’s gravity yanked out most of the dust there, creating that gap. In fact the data show there is still some dust there, which is consistent with a planetary resonance more than a planet physically in that gap hoovering up material.

We see this same thing in Saturn’s rings; the big gaps and divisions are in resonance with moons like Mimas and Titan, which tug on the ice particles there. Some things we see locally are, quite literally universal.

So that’s cool. But there’s an implication to this that’s even more amazing, and when I realized it I gasped out loud: There’s a planet there, bigger than nearly all the planets in our solar system, and it formed in less than a million years.

Holy planetesimal accretion! That’s very, very fast. We’re talking about a staggeringly massive object, something like 70 times more massive than Earth, forming that rapidly. Wow.

There’s been an ongoing argument in astronomy about how quickly planets form. I remember being taught it likely took tens of millions of years, but — like my work on Hubble — things have changed a wee bit since then. When I was in grad school we had the fuzziest views of young stars, and the data we needed to get a better handle on how planets formed were severely lacking (heck, I graduated with my PhD the year before the first planet was found orbiting a Sun-like star).

Now you can look at an image and point to where the planet is gathering itself.

Some people don’t like change, and fear progress. I am not one of these people. Show me more! I cannot wait for the next innovation, the next leap in engineering and science, that allows me to see the Universe ever more clearly, to understand it that much better.

Show me more! There’s so much more to see.