Although astronomers have discovered thousands of extrasolar planets, images of very young, newly formed exoplanets are still a rarity. A research team has now succeeded in observing one of the youngest and smallest young planets to date. With the help of infrared optics from the W. Keck Observatory in Hawaii, they discovered a young red dwarf around 400 light-years away, a gas planet weighing three to five Jupiter and a maximum of 2.5 million years old. It is one of the few young planets that have been photographed directly so far and the lightest of its age group, as the team explains.
From almost star-sized gas giants to rocky earth twins: in the past few years, astronomers have discovered thousands of extrasolar planets, including many grouped in planetary systems similar to ours. But how planets are formed is still fraught with many questions. One of the reasons for this: So far, astronomers have only very rarely succeeded in observing planets that are just emerging. In some cases, only gaps in the protoplanetary gas and dust disks reveal that a planetary embryo is growing there. In other cases it was possible to directly depict a young planet, in these very few lucky cases the young planets were often extremely large and far from their stars. Most of the time, the picture was successful because these young planets are still relatively hot and therefore emit infrared radiation.
Infrared spot turns out to be a young planet
Astronomers working with Eric Gaidos from the University of Hawaii in Manoa have now tracked down such a young planet. They investigated an abnormality discovered in 2018 while observing the red dwarf 2M0437, which is around 400 light-years away. The star is only 2.5 million years old and lies in the Taurus star formation region and is very small with only 0.15 to 0.18 solar masses. During infrared observations with the Subaru telescope in Hawaii, astronomers discovered another bright spot in the vicinity, the nature of which, however, remained unclear. Gaidos and his team have now examined this spot more closely with the powerful adaptive optics of the W. Keck telescopes on Mauna Kea. Over a period of three years, they analyzed the movement of the star and its companion.
“Thanks to the excellent data from the Keck Observatory, we were able to prove that this weakly shining neighbor moves through space together with the star and is therefore a real satellite,” explains co-author Adam Kraus from the University of Texas in Austin. More detailed analyzes showed that the exoplanet, baptized 2M0437b, weighs three to five times the mass of Jupiter and orbits its central star at a distance of around 100 astronomical units. For comparison: Neptune in our solar system is around 30 astronomical units from the sun. “This discovery extends the elite list of the few exoplanets that we can observe directly with our telescopes,” says Gaidos. “By analyzing the light from this planet, we can learn something about its composition and perhaps also where and how it formed in the long-vanished gas and dust disk around its central star.”
Smallest in its age group
From the observations it appears that the young gas giant is round
1120 to 1220 degrees and therefore still radiates a strong initial heat. This fact and the young age of its central star suggest that the planet 2M0437b must also be very young, as the team explains. They estimate that the young planet is no more than 2.5 million years old, but probably even younger. 2M0437b is thus the smallest exoplanet that has been directly observed at such a young age. “So far there are only four young planets directly depicted whose masses are comparable to or less than that of 2M0437b – and all four are older,” write the astronomers. The observation of such young planets is particularly interesting because they can be used to understand how such gas giants are formed.
According to common doctrine, planets can be formed in two ways: During accretion, planetary nuclei grow slowly through the gradual accumulation of first smaller, then larger chunks. The earth and other terrestrial planets, but possibly also the solid cores of the gas planets, owe their formation to this process. However, the accretion takes some time and is more protracted the further out a planetary embryo orbits, because there the material density in the primeval cloud is lower. The second path of formation is the local collapse of the primordial cloud: parts of the gas collapse under the pressure of its own gravity and material condenses in the center to form a planet. This fragmentation of the protoplanetary disk takes place relatively quickly and is discussed above all for the formation of large gas giants.
However, the recently discovered young planet 2M0437b does not fit either of these two theories. “It is a challenge to both of the leading theories on planet formation,” explain the researchers. It is too young and circling too far outside to have been created by accretion. On the other hand, the gas and dust disk around its little star was probably not massive enough to have formed it through a collapse. ”So how this young planet came into being is still a mystery.
Source: Eric Gaidos (University of Hawaii, Manoa) et al., Monthly Notices of the Royal Astronomical Society, accepted; arXiv: 2110.08655