A new weighted exoplanet has astronomers baffled.
After measuring a very small Jupiter-sized exoplanet called HD-114082b, scientists found that its characteristics didn’t quite match either of the two popular models of gas giant planet formation.
Simply put, it’s just too heavy for its age.
“Compared to currently accepted models, HD-114082b is two to three times too dense for a young gas giant only 15 million years old,” he said. legt astrophysicus Olga Zakhazy uit from the Max Planck Institute for Astronomy in Germany.
The exoplanet orbits a star called HD-114082 about 300 light-years away and has been the subject of an intense data-gathering campaign. At just 15 million years old, HD-114082b is one of the youngest exoplanets ever discovered, and understanding its properties may provide clues to how planets form — a process that is not fully understood.
Two types of data are needed to fully characterize an exoplanet, based on its effect on its host star. Transit data is a record of how a star’s light dims when an exoplanet in orbit passes in front of it. Knowing how bright the star is, this faint dimming could reveal the size of an exoplanet.
Radial velocity data, on the other hand, is a record of how much the star wobbles in place in response to the gravity of the outer planets. Knowing the mass of the star, the amplitude of its oscillation can give us the mass of the exoplanet.
For nearly four years, researchers have been collecting radial velocity observations from HD-114082. Using the collected transit and radial velocity data, the researchers determined that HD-114082b has a similar radius Jupiter – But the mass of Jupiter is 8 times greater. This means that the exoplanet’s density is nearly double that of Earth and about 10 times that of Jupiter.
There is also a very small density range in rocky exoplanets. Above this range, the body becomes more intenseAnd the planet’s gravity begins to trap a significant atmosphere of hydrogen and helium.
HD-114082b significantly exceeds these parameters, which means it is a gas giant. But astronomers don’t know how this happened.
“We think giant planets could form in two possible ways,” says astronomer Ralph Lönnhardt mpia. “Both occur within a protoplanetary disk of gas and dust scattered around a young, central star.”
Both methods are called “cold start” or “hot start”. At the cold start, the exoplanet would form, pebble by pebble, from debris in the disk orbiting the star.
The pieces attract each other, first electrostatically, then by gravity. The more mass, the faster it grows, until it becomes massive enough to trigger a runaway buildup of hydrogen and helium, the two lightest elements in the universe, creating a huge gaseous envelope around a rocky core.
Since the gases lose heat as they fall to the planet’s core and form the atmosphere, this is seen as a relatively cool option.
A hot start is also known as disk instability and is believed to occur when a swirling region of instability in the disk collapses directly on itself due to gravity. The resulting object is a fully formed exoplanet with no rocky core, because the gases trap more of their heat.
Exoplanets experiencing a cold start or a hot start should cool at different rates, resulting in different features that we should be able to observe.
The researchers say that the features of the HD-114082b do not match the hot-start model. Their size and mass are more consistent with primary accretion. But even then, it’s still pretty massive for its size. Either it contains an unusual core or something else is going on.
“It’s too early to give up the idea of a hot start,” says Lönnhardt. “All we can say is that we still don’t understand the formation of the giant planets very well.”
The exoplanets are one of three planets known to be less than 30 million years old and for which astronomers have obtained radius and mass measurements. So far, all three seem incompatible with the disk instability model.
Three is clearly a very small sample size, but three out of three indicates that primary accumulation is probably the more common of the two.
“While more such planets are needed to confirm this trend, we believe theorists should reassess their calculations.” says Zakhozai.
“It is exciting how our observational results contribute to the theory of planet formation. They help improve our knowledge of how these giant planets grow and tell us where the gaps in our understanding lie.”
Research published in Astronomie en astrofisica.