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A second planet for Beta Pic confirmed!
Astronomers have confirmed the existence of a second planet orbiting the young nearby star Beta Pictoris! This is extremely cool. Evidence for the planet was first found in 2019, but it was indirect and not quite good enough to be called proof.
The new observations just announced show an image of the planet, confirming its existence. This is the first time a planet was found indirectly and then directly, and that's kindof a big deal. Here's why.
Beta Pic (as it's usually called) is a star so young — about 18 million years old or so — that it's still surrounded by the disk of dust and gas it formed from. The first planet discovered orbiting it, called Beta Pic b, was found in 2008. It can be seen in direct images, literally meaning you can see it in photos of the star. Since its discovery it's made nearly one complete orbit around the star, and its motion is seen as well.
Beta Pic b is about 9 times the mass of Jupiter, and still glowing with the heat of its formation (planets form by drawing in matter around them, so imagine getting whacked by millions of giant asteroids all the time, and you can appreciate why the planet is hot). It takes about 23 years to orbit the star once, at a distance of about 1.5 billion kilometers — roughly the same distance of Saturn from the Sun.
Beta Pic c was detected, barely, by its effect on the star. The star and the planet orbit their mutual center of mass, and as the star moves toward us and away from us there's a Doppler shift in its light. This is called the reflex velocity method, and the detection was marginal, but promising.
So in February and March of 2020 astronomers used the massive Very Large Telescope Interferometer, which combines the power of the four giant 8.2-meter telescopes that make up the Very Large Telescope, together with an extremely sensitive camera called GRAVITY, to observe Beta Pic. They were able to detect the planet in their first observations, and follow-up observations showed it as well.
It's not an image like we usually think of them, because interferometry combines the incoming waves of light from each telescope to create an interference pattern, like waves in your bathtub can combine to make bigger crests and lower troughs as you scootch around. But the information extracted from these patterns reveals the planet.
Spectra of the planet show that it's hot, about 1,000° C, due to it being young. It orbits the star every 3.4 years at a distance of about 400 million kilometers (the distance of the main asteroid belt from the Sun, between Mars and Jupiter). From the earlier observations the mass of the planet was found to be about 8 times that of Jupiter, so a little less than Beta Pic b.
One cool thing about this is that the reflex velocity method of finding planets works best for ones close in to their host stars, maximizing the effect of the star's motion. But direct imaging works best for ones farther away from the glare of the star, so it's pretty nifty Beta Pic c was found using both. It's in a bit of sweet spot for that.
But having the mass and the temperature from the different observations is interesting, too. Planets can form in two different ways. One is that the disk of junk around the star can fragment, with those clumps then collapsing directly to make the planet (called the disk instability method; think of that as top-down). The other is for the core of the planet to grow as smaller pieces crash into each other (called the core accretion method; think of that as bottom-up). It's thought that planets as massive as Beta Pic c form through the first method, but the disk near the star isn't unstable enough to do so in this case. So it may have formed the second way, which is unusual for a planet this massive.
There are some ideas how the core accretion method might be able to make bigger planets, but it's not clear. This discovery could really boost that understanding.
Beta Pic was one of the very first stars discovered to have material around it that could be forming planets, and when the first planet was found it was really exciting. Now a second one only adds to that. The star is relatively close by, just 63 light years, so in general it's easier to study than many other similar stars. And with bigger telescopes coming online soon, including James Webb Space Telescope, you can bet that now more than ever it'll be an early target for them.
