Bad Astronomy | Exoplanet YSES 2b is much further from its star than expected

Astronomers have so far found more than 4,000 exoplanets – alien planets orbiting other stars.

There are several ways to find them, most of which use indirect methods, but one of the coolest is quite direct: getting real images of the planets near their host stars. Called direct imagingthis technique has been used to find dozens of planets.

A team of astronomers has focused on 70 nearby stars in search of such exoplanets, and has just announced a new one: YSES 2b, a giant planet orbiting a star just 360 light-years away.

We’ve seen some similar to this before, but in this case, this planet is special. For one thing, it orbits a star that will one day look very much like the sun. In any case, it works for someone else 16.5 billion kilometers from the star, as much as 110 times as far from its star as Earth from the sun!

That’s a very, very long road, and what it does this far is a mystery.

Direct imaging works best to find very young planets, up to tens of millions of years old. Planet formation is a violent, energetic process, so these young planets are too hot. They glow strongly in the infrared (IR) part of the spectrum, so astronomers use IR cameras on large telescopes to find them. This has an added benefit that stars are typically fainter in the IR than in visible light, making it easier to see planets.

The astronomers who found this new planet are conducting a survey called the Young Suns Exoplanet Survey, or YSES, and they are looking at a nearby group of young stars called the Lower Centaurus Crux subgroup, part of a much larger loose group of stars called the Scorpius-Centaurus association. These stars are very young, about 14 million years old (for comparison, the sun is 4.6 billion years old, so these stars are babies) and are only 350 light years away, so close that an exoplanet can be separated enough to be seen in pictures.

We know that stars heavier than the sun generally have heavier planets, so to avoid such biases, the survey only looks at stars with masses similar to the sun. They are aiming for 70 such stars in the group.

YES 2* is the second star they looked at where they found a planet (they’ve observed about 45 others, but they’re still looking for planets around it). It has a mass 1.1 times the mass of the Sun, so it is very similar, although it is cooler at this point (it will likely get hotter over time once it descends and becomes stable). It is about 360 light years from us.

The planet looks like a background star and is so far from the star that it is not possible to see orbital movement (it will take many years for it to move noticeably). To confirm that an exoplanet is a companion to a guest star, the team takes photos of each star one year apart. Stars are all in motion as they orbit the center of the galaxy, and the targeted stars are so close to Earth that this motion appears great (such as when you are in a car and trees are whizzing past while a distant mountain hardly seems to move at all). If the candidate planet is truly a companion, it will move with the star. If it’s a background star it won’t.

Photos taken a year apart show that the object really moves with the star, so it’s a companion. Using physical models of how planets cool down after formation, they discover that it has a mass somewhere between 5 and 8 times that of Jupiter, with a most likely mass of about 6 Jupiters, making it a real planet. So they called it YSES 2b.

The point is, what’s it doing so far from the star? There are two known ways to make a huge planet. One of them is what is mentioned immediate collapse, where it forms through the collapse of part of a cloud of gas, much like a star. This can, in fact, create something so far from a star (it’s how binary stars are formed, for example), but the weird thing is that it’s hard to make something so lightweight. A planet that forms like this should be much heavier than YSES 2b.

The other method is mentioned nuclear accretion, where small particles stick together in a disk around the star, expand and then become large enough to draw in material by gravity. However, the disk around a star is pretty thin as far as YSES 2b is, so in that case it’s too massive to have formed that way.

The likely explanation, then, is that, like most gas giants, it moved closer to the star where the disk formed thicker through nuclear accretion, and then was thrown to its present distance after encountering another giant planet orbiting the star; the gravity of that planet could throw it to that distance.

The problem there is that no other planet like this one is seen in the observations. It is possible that it is so close to the star that the brilliance makes it too difficult to find. That seems likely to me, since we know these things can happen, while the other two possibilities are less likely. Still, it would be nice to know for sure.

That’s why YSES 2b is a major catch. If the team finds more, they hope to see trends on the planets that will help them understand how planets form around sun-like stars, and how some form at such distant distances. While our own solar system may not have such a planet (although we could), it helps us understand how our own system has formed as well. There are still many things we do not know about planetary formation processes, and each young planet found is a step toward advancing that knowledge.


*Many studies tend to name the objects they find after the study in question. The star has a more formal name of 2MASS J11275535-6626046, which is easier to look up in databases, but YSES 2 is good enough for this.

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