For the first time, astronomers have obtained a direct image of a young, still-forming exoplanet in ultraviolet light, allowing them to measure just how fast it's growing. The number is surprisingly low, meaning the planet may be at the tail end of its growth spurt.
PDS 70 is a very young star, only about 5 million years old (the Sun is a thousand times older than that), located 370 light years away in the constellation of Centaurus. In the early 2000s astronomers determined PDS 70 was still surrounded by the disk of material it was born from, and was still accreting (that is, gathering) material from it. It turns out there are two disks around the star; one very close in to it and another much farther out, with a gap between them.
Suspecting there might be a planet or planets in that gap, creating it as they ate up material themselves, astronomers took a look and indeed found two planets there. One, called PDS 70b, is probably about the same mass as Jupiter, and orbits the star 3.5 billion kilometers out (roughly the distance of Uranus from the Sun). The other PDS 70c, a little over 5 billion km out (about the distance of Neptune from the Sun) and has over 4 times Jupiter's mass.
Getting a planet to be as massive as Jupiter or more in only 5 million years means they must've grown rapidly. These are hefty planets! But the rate at which planets grow is difficult to measure. One way is to measure the warm, low-density hydrogen gas, which glows at a characteristic wavelength (color) of around 0.656 microns (red light) — this is called H-alpha. This gas is flowing down to the planet, and how bright it is in H-alpha corresponds to how rapidly the planet accretes gas.
However, the theoretical relationship between how bright the H-alpha is and how rapidly the planet grows is not exact and depends on a lot of factors that are difficult to measure. However, a team of astronomers thinks they've found a better way. As this material flows from the disk around the star onto the planet it comes in so rapidly that it slams into the planet and creates a shock wave in the gas. This heats the gas up, causing it glow in a different way, most strongly in the ultraviolet part of the spectrum. If you can measure that from a young planet, you can figure out the accretion rate more accurately.
This is still pretty dang tough to do, since stars are bright in UV, and you have to observe it from space, or else Earth's atmosphere will absorb all the light you're trying to see.
So this team used Hubble. It's in space, and it has sensitive UV cameras. They used the Wide Field Camera 3 on Hubble to observe the star, and rotated the telescope between observations. This means they see the star rotated in their images, which helps understand the shape of the light from the star on the camera, allowing them to use an innovative technique to subtract it away and leave behind light from anything that's not from the star.
When they did so, out popped the planet PDS 70b! This is the first time such a planet has ever been detected in UV images. Note: PDS-70c, the other planet, is too faint in UV to be seen in their images.
Using the planet's brightness, they find it's eating matter at a rate of about 0.00000001 Jupiters per year — or one Jupiter's worth of mass every hundred million years. That's pretty slow (slower than some recent measurements of its growth using H-alpha). If it grew to about the mass of Jupiter in only 5 million years, that means it must've been growing much more rapidly in the past, and has now slowed down. The star is also growing pretty slowly, so this likely indicates the entire system is at the tail end of its growth period, and may soon (well, in a few million years) settle down to become a more normal-looking mature system.
The team also took H-alpha measurements along with their UV ones, and found that the planet's brighter there than they expected. It's possible that the planet's growth rate changes over time, but it would have to change very rapidly, since it was also measured in 2018 and found to be significantly dimmer. They claim that this shows that using H-alpha isn't as much of a lock on growth rate as first thought, and that using ultraviolet imaging is better.
While it's still tough to observe ultraviolet light from these young stars, this shows it's possible, and we can see planets close in to the star with sufficient quality to measure and analyze their light. That's a big deal. Not long ago we thought it took tens of million of years to get planets the size of Jupiter, but now we're seeing it can happen a lot faster, and ends earlier, too.
As a reminder, the first exoplanet was only found in 1992. Now we are not only seeing them being born, but we're able to measure with some confidence how fast they're growing. It's an astonishing accomplishment, and a lovely demonstration of how science progresses.