A far-flung exoplanet may have thrown its sibling under the bus

With nearly 4,000 exoplanets (and counting!) having been discovered so far, a few of them are going to be, well, weird. While HIP 65426 b may not be the weirdest, it's still pretty odd.

OK, to be fair, the planet seems like a fairly normal Jupiter-like planet. What's peculiar isn't what it is, but where it is: Apparently, it's at least 14 billion kilometers from its star! That's three times farther out than Neptune is from the Sun.

How did it get out there?

First, a bit of an intro. The European Space Agency mission Hipparcos was a star-counter: It logged the positions, motions, and distances of about 100,000 stars. It created a catalog of all those stars, and the star in question here was the 65,426th entry. HIP 65426 is what we call an A-type star, roughly twice the mass of the Sun, a bit less than twice the size, and a few dozen times more luminous.

It's relatively close to us, about 360 light years away. It's not quite bright enough to see with the naked eye, but with binoculars you could pick it out (and lived in the southern hemisphere; it's in the constellation of Centaurus).

It's a young star, too, probably about 14 million years old. Its age was found because it's part of a "moving group" of stars, young stars that formed together and are moving as a group through the galaxy. Various methods exist to get the ages of stars in groups like this, which makes them nice labs for us to study. Stars change as they age, so knowing whether they are young or old is important.

Since HIP 65426 is young, that opens up an interesting possibility: If it has massive planets, they could be easier to spot. That's because planets are hot for millions of years after they form, and give off lots of infrared light. Stars don't emit as much IR light as they do visible light, so that makes the planets easier to directly image: Literally, take pictures of them and see them directly.

SPHERE is a powerful camera on one of the Very Large Telescope, um, telescopes (it's actually four telescopes that can be connected to operate as one). It uses some complex physics to reduce the amount of light from a bright star while letting other light through, which means a fainter planet is easier to spot. The SpHere INfrared survey for Exoplanets uses this camera to monitor 600 nearby young stars, looking for gas giants still glowing from their birth heat.

The exoplanet HIP 65426b can be seen to the lower left of the star (whose much brighter light has been blocked) in these images in two different infrared colors. Credit: Chauvin et al.

Photo: Chauvin et al.

HIP 65426 turned out to be a good target: They found one! In the image you can clearly see the star and planet. Of course, an image like that isn't conclusive; it could be a background star. But the astronomers took images like this over several months, long enough for the star's motion through space to be detectable. If the other object were a background star it wouldn't move, but in fact what they saw was it trailing along with the star, making it almost a certainty it's a planet orbiting the star.

Even better: It's the very first exoplanet directly seen using SPHERE! I'm guessing it won't be the last.

Looking at the colors of the planet in detail (and knowing its age, which tells you how much it's cooled since birth) allows a measurement of its temperature, an estimate to be made about its mass and size, and even some insight on its atmosphere (probably cloudy and dusty). HIP 65426 b is hot: about 1300 – 1600 Kelvins (roughly 1000 – 1300° C, so far hotter than an oven set to broil). It has something like 6 – 12 times the mass of Jupiter, so it's almost certainly a planet, and not a brown dwarf (which have from 13 – 77 times Jupiter's mass). It's also about 1.5 times the diameter of Jupiter.

All of that is pretty much what you'd expect for an infant plane… or at the least there's nothing really out of the ordinary there.

But then that distance, 14 billion kilometers. That's a long way out. We're pretty sure a planet that massive wouldn't be able to form that far out from the star; there just wouldn't be enough material to make a planet that big out at that distance.

So… maybe it formed farther in, where there was more stuff to build planets. If true, how did it get out so far?

This part I love: Maybe there used to be another massive planet in that system. If the two got too close together (which happens all the time in newborn planetary systems) then it's possible the gravitational interaction threw HIP 65426 b out into a long, elliptical orbit. The other planet, in that case, would have dropped into the star.

Ouch.

But here's a funny thing: The star HIP 65426 is pretty unusual in one respect: It's a super fast rotator. The Sun takes about a month to spin once. HIP 65426 spins all the way around in something like 8 hours. Remember, it's also a lot bigger than the Sun, so this is really something. The star must be seriously flattened by centrifugal force! It's one of the fastest spinning stars of its type known. It actually couldn't spin too much faster without flying apart!

And that sets up an interesting idea. What if the second planet dropped in to the star after flinging the other out, and as it fell in it sped up the star's rotation? A planet spiraling into a star ha s a lot of angular momentum, which would transfer to the star and ramp its rotation up a lot. We know that can happen. This would be an extreme case, but it's not out of the question.

Interestingly, too, the star lacks a debris disk, a huge disk of material from which planets are formed. Stars its age generally still have that disk. Perhaps the planetary collision may have helped eject the material in the disk, or something else happened… but in my experience, when you see a bunch of weird stuff all in one place, they're related. It's not for sure, but it seems likely. It's not clear how all these things add up, so right now this is little more than speculation.

But there's a way to find out: If we can observe the planet HIP 65426 b long enough we can determine the orbit its on. If it's highly elliptical, and drops much closer in to the star, that supports the planetary collision scenario. If it's circular, well, then we'll still have a mystery, won't we? Those are fun too.

Unfortunately, it'll be a while before we have that info; the planet has an orbital period of over 600 years, so tracing its orbital motion could take decades. But as usual, let me point out we're just starting out here. This is literally the first exoplanet SPHERE has found, and we only have direct images of a dozen or so others. As we find more we'll be able to categorize them better, study them in more detail, and figure out better just what is normal behavior and what isn't .

A new field of science is opening up as we watch. Besides understanding, that's one of the most exciting things science gifts us.