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SYFY WIRE Bad Astronomy

A Tale of Three Jupiters, Part 3: Jupiter’s Brother Has Three Suns

By Phil Plait

Three recent news stories have come out, all dealing with exoplanets—alien worlds—that have something in common with Jupiter. All three stories are pretty cool, and I thought it would be fun to tell each in a separate, but related post. Not only that, I can tie the planet to the post number, too; Part 1 was on a solitary starless exoplanet, Part 2 was about a double of our own Jupiter. For Part 3, it’s about a planet in a triple-star system.

In the realm of science fiction, planets orbiting binary stars are common. Heck, if you’re young and growing ever-more dissatisfied with living on a desert planet that’s the farthest from the bright center of the Universe, you can put your foot up on a rock and contemplate the double-sunset while the wind blows through your 1970s ‘do.

It turns out reality is similar (well, minus the Luke Skywalker self-pity thing). About half of all exoplanets may orbit binary stars. In fact, we’re even finding exoplanets in triple-star systems!

And a new one just joined the list. KELT-4Ab is a Jupiter-like planet orbiting one of the stars in a trinary system. A handful of others have been found before, but this one has a pretty cool story behind it.

There are lots of ways to look for exoplanets. The easiest—though by no means actually easy—is using transits. If the planet orbits the star with its orbit edge-on as seen from Earth, than once per orbit the planet passes directly in front of the star. This is called a transit, and it causes the star’s light to dip a little bit. That dip can tell you the size of the planet, as well as the period (the year) and even the shape of the orbit.

Some telescopes observe stars one at a time, looking for transits. Others look at vast swaths of the heavens, observing thousands of stars at once. KELT is like that; but there’s a twist. You might expect astronomers to use some mighty telescope with a huge mirror that gulps down light.

Not in this case. KELT stands for Kilodegree Extremely Little Telescope. Yes, seriously. It’s more of a camera with a really nice lens than a telescope, able to see a field of view a staggering 26° on a side; that’s more than 3,000 times the area of the full Moon on the sky! It patiently snaps away, recording the brightnesses of thousands of stars. It can’t see very faint stars, but it makes up for that by observing so many at one time. If something interesting is seen, they can use bigger telescopes to take a better look.

KELT-4A (also known as HIP 51260 if you want to look for it in stellar catalogs) is a star in the constellation of Leo, and is roughly 700 light-years from Earth. It’s more massive and hotter than the Sun (technically, an F star), making it bright enough to see by KELT even at its great distance.

KELT detected a periodic dimming of the star, a dip in the light every three days, lasting for just about four hours. They followed up these observations using different telescopes and confirmed it: They had found a planet orbiting the star.

KELT-4Ab, as the planet is called, is a little different than what we have here in our own solar system. As I mentioned in a previous post, transits can’t give you the mass of the planet, but taking high-resolution spectra can. The team did just that and found the planet has a mass about 90 percent that of Jupiter.

The transit data gives the planet’s size, and they found it to be 1.7 times bigger than Jupiter. Why would a planet with lower mass be bigger than Jupiter?

Because it’s hot. Being so close to the parent star means it’s heated tremendously, to a temperature of more than 1,500° C! Many such planets have been found in fact, and we call them “hot Jupiters.” And what happens to a gas when you heat it up? It expands. That’s why KELT-4Ab is bigger than you might expect. It’s puffy.

We’ve seen planets like this before. The fun part of all this started when high-resolution follow-up images were made using the huge Keck telescopes in Hawaii. It was already known that the primary star was orbited by another star, roughly 50 billion kilometers away (more than 10 times farther than Neptune is from the Sun). But the Keck images revealed a surprise: That second star is actually a tight binary, a pair of stars orbiting each other. Both are a bit lower mass and cooler than the Sun, and orbit each other at a distance of about 1.5 billion kilometers.

So KELT-4 is actually a triple star! The primary is labeled as A, the two stars in the binary B and C, and the planet is 4Ab (by convention, the first planet in a system is given the star’s name plus a lower case b label to avoid confusion). The planet takes three days to orbit star A, stars B and C orbit each other every 30 years, and the pair together orbit star A once every 3,800 years or so.

So that’s quite a system! And here’s an interesting question: How did it form? The stars were probably born together in the same cloud of gas and dust, with the binary forming orbiting the primary. We think hot Jupiters initially form farther out from the star than where we see them now, and migrate inward. That can happen as they plow through dust and debris left over around the star after it forms, but it’s possible the distant binary might have affected that as well. They’re pretty far out, but combined have more mass than the Sun does! So they may have had an impact on KELT-4Ab, and any other possible planets in that system that haven’t yet been discovered.

We’re still getting a grip on how planets form, and what subtle forces affect them. But this is still pretty new; the first exoplanet orbiting a Sun-like star was only found in 1995, after all!

But it goes to show you that the Universe is subtle, and also efficient. It seems that wherever planets can form, they do. And that is one of the most thrilling things I know.

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