Astronomers photograph giant exoplanet in unusual large orbit
Astronomers have discovered many giant planets, similar to Jupiter or Saturn, orbiting other stars. Some of these – called hot Jupiters – are unlike any planets in our solar system, however, circling much closer to their stars than any of our sun’s planets orbit the sun. Hot Jupiters appear to be quite common, even though none exist in our own solar system. This spring (April, 2021), astronomers at Leiden University announced an opposite planet of sorts, a giant world called YSES 2b, orbiting much farther out from its star than has typically been seen before. The researchers captured a direct image of this planet, something not easy to do. The planet orbits its star a whopping 20 times farther than Jupiter orbits our sun. That’s the equivalent of 110 times the distance from Earth to the sun.
YSES 2b is located 360 light-years away, in the direction to the southern constellation Musca the Fly. It is a young gas giant six times more massive than Jupiter. Other similar gas giants have been found before, but this one is a bit different.
The researchers don’t yet know why the planet is so far from its star. Scientists normally have two models – core accretion and disk instability – to explain the formation of such large planets, but this giant world doesn’t seem to fit either of them.
The first, core accretion, says that the planet formed where it is, due to planetesimals collecting together to form a rocky core, heavy enough to collect gas around it. But if that were the case, it is far too heavy. This is because there is generally too little material that far out from a star to make a planet that big.
The other theory, disk instability, is that the planet formed by a gravitational instability in the original circumstellar disk of material that surrounds a young star (and this star is only 14 million years old, still a baby star if you will). But, the planet we see now isn’t heavy enough for that process to have created it.
What other possibilities are there? The scientists think that it is possible the planet first formed by core accretion closer to the star, but then migrated outwards to a much more distant orbit. For that to work, however, the gravitational influence of a second planet would be needed, and such a planet hasn’t been found yet.
YSES 2b was discovered as part of the Young Suns Exoplanet Survey (YSES). Researchers will continue to study this peculiar world as well as search for more planets orbiting young sun-like stars. Lead author Alexander Bohn at Leiden University stated that:
By investigating more Jupiter-like exoplanets in the near future, we will learn more about the formation processes of gas giants around sun-like stars.
At the moment, only large planets like YSES 2b can be directly imaged, and even then still just look like a bright dot. Distant Earth-sized worlds are too small for telescopes to observe, but that will change in the years ahead as the technology advances.
In 2020, the telescope used for YSES, the Very Large Telescope (VLT) in Chile, also imaged a multi-planet system around the sun-like star TYC 8998-760-1, which is 300 light-years away. This was the first multi-planet system ever imaged directly, with the first detections made in 2018 and 2020. A special planet finder instrument, called SPHERE, on the telescope was used to obtain the images. This instrument can capture both direct and indirect light coming from exoplanets. Bohn said at the time:
This discovery is a snapshot of an environment that is very similar to our solar system, but at a much earlier stage of its evolution.
Co-author Matthew Kenworthy added:
Even though astronomers have indirectly detected thousands of planets in our galaxy, only a tiny fraction of these exoplanets have been directly imaged. Direct observations are important in the search for environments that can support life.
The discovery of YSES 2b provides a challenge to astronomers in terms of how it formed, and will help scientists better understand planetary formation processes.
Bottom line: Astronomers have directly imaged a giant gas exoplanet that has an unusually large orbit.