Astronomers have so far found more than 4,000 exoplanets – alien planets orbiting other stars.
There are different ways to find it, most of which use indirect methods, but one of the coolest is very direct: to get real images of the planets near their host stars. Called direct image, this technique was used to find dozens of planets.
A team of astronomers has targeted 70 nearby stars in search of such exoplanets, and has just announced a new one: YSES 2b, a giant planet orbiting just 360 light-years.
We have seen similar to this before, but in this case, this planet is special. First, it’s about a star that will one day be much like the Sun. For another, it turns out at least 16.5 billion kilometers from the star, 110 times farther from its star as the earth is from the sun!
It’s a very, very long road, and what it’s done so far is a mystery.
Direct imaging works best to find many young planets, up to several dozen million years old. Planet formation is a violent, energetic process, so are these young planets hot. They glow strongly in the infrared (IR) part of the spectrum, so astronomers use IR cameras on large telescopes to find it. It has a side benefit that stars are usually dimmer in the IR than in visible light, making it easier to see any 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 cluster of young stars called the lower group Centaurus Crux, part of a much larger loose group stars called the Scorpius-Centaurus Society. These stars are very young, about 14 million years old (the sun is 4.6 billion years old by comparison, so these stars are babies) and only 350 or so light years away, close enough that an exoplanet separated from its star can be enough to spot images.
We know that stars more massive than the Sun tend to have more massive planets, so to avoid biases like these, the survey only looks at stars with masses like the Sun. They are aiming for 70 such stars in the group.
JA 2* is the second star they looked at where they found a planet (they observed about 45 others, but they are still working on finding planets). It has a mass of 1.1 times the sun, so it’s very similar, although it’s cooler at the moment (over time it’s likely to get warmer as it settles and becomes stable). It’s 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 any orbital motion (it will take many more years before it moves noticeably). To confirm that an exoplanet is a companion of a host, the team takes a year apart from each star. Stars are all moving as they orbit the center of the galaxy, and the targeted stars are close enough to Earth that this motion looks large (like when you sit in a car and sound trees past while a distant mountain barely moving at all). If the candidate planet is truly a companion, it will move with the star. If it’s a background star, it will not.
Images taken a year apart show that the object is really moving with the star, so it is a companion. Using physical models of how planets cool after formation, they find that it has a mass of between 5 and 8 times that of Jupiter, with a most likely mass of about 6 Jupiters, making it a true planet. So they called it YSES 2b.
Thing is, what does it do so far from the star? There are two ways to make a massive planet. The one is what is called direct collapse, where it forms due to the collapse of a part of a gas cloud, just as a star does. It can actually make something out of a star (for example how binary stars form), but the weird thing is that it’s hard to make something so lightweight. A planet that forms like this should be much more massive than YSES 2b.
The other method is called core wash, where small particles in a disk around the star stick together, become larger and then become large enough to draw in material via gravity. However, the disk around a star is quite rare as far as YSES 2b is concerned, so in that case it is too massive to form.
The probable explanation then is that it, like most gas giants, was formed by nuclear growth closer to the star where the disk is thicker, and then thrown to its current distance after encountering another giant planet orbiting the planet. star orbits; the gravity of the planet can push it up to that distance.
The problem there is that no other planet is seen in the observations. It is possible that it is so close to the star that the glare finds it too difficult to find. This seems likely to me, since we know that such things can happen, while the other two possibilities are less likely. It would be nice to know that.
Therefore, YSES 2b is an important capture site. As the team finds more, they hope to see trends in the planets that will help them understand how planets form around sunny stars, and how some form at such distant distances. Although our own solar system may not have such a planet (although we may), it helps us to understand how our own system also formed. There are still many things we do not know about planetary formation processes, and every young planet found is a step towards the advancement of knowledge.
*Many surveys tend to name the objects they find after the particular survey. The star has a more formal name of 2MASS J11275535-6626046, which can be easily searched in databases, but YSES 2 is good enough for this.