A recently weighed exoplanet has baffled astronomers.
After taking measurements of a very small, Jupiter-sized exoplanet called HD-114082b, scientists found that its characteristics don’t fit either of the two popular models of gas giant planet formation.
Simply put, it was too heavy for his age.
“Compared to currently accepted models, HD-114082b is two to three times denser for a young gas giant that is only 15 million years old,” he said. explains astrophysicist Olga Zakhazy from the Max Planck Institute for Astronomy in Germany.
Orbiting a star named HD-114082 some 300 light years away, the exoplanet 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 can provide clues to how planets form — a process that is not fully understood.
Two types of data are needed to comprehensively characterize an exoplanet, based on its influence on its host star. Transit data is a record of the way a star’s light dims when an orbiting exoplanet passes in front of it. If we know how bright the star is, this faint dimming can reveal the size of an exoplanet.
Radial velocity data, on the other hand, are records of how much a star wobbles in place in response to the outer planet’s gravitational pull. If we know the mass of the star, the amplitude of its wobble can give us the mass of the exoplanet.
For nearly four years, researchers have been collecting observations of HD-114082’s radial velocity. Using the collected transit and radial velocity data, the researchers determined that HD-114082b has the same radius Jupiter – But Jupiter’s mass is 8 times greater. This means the density of the exoplanet is almost twice that of Earth, and about 10 times that of Jupiter.
There is also a very small range of densities on rocky exoplanets. Above this range, body get more intenseAnd the planet’s gravity began to restrain its important 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 have formed in two possible ways,” said astronomer Ralph Lönnhardt mpia. “Both occur within a disk of protoplanetary gas and dust that is scattered around the center of a young star.”
These two methods are referred to as “cold starts” or “hot starts”. On a cold start, exoplanets are thought to have formed, pebble by pebble, from debris in the disk orbiting the star.
The pieces attract, first electrostatically, then gravitationally. The more mass, the faster it grows, until it becomes massive enough to trigger a buildup of hydrogen and helium, the two lightest elements in the universe, creating a huge gaseous envelope around the rocky core.
Given that gases lose heat when they fall into a planet’s core and form an atmosphere, this is seen as a relatively cool option.
Hot start is also known as disc instability, and is thought to occur when swirling regions of instability in the disk collapse directly in on themselves by gravity. The resulting object is a fully formed exoplanet without a rocky core, because the gas retains more of its heat.
Exoplanets that experience a cold start or a hot start must cool at different rates, resulting in different characteristics that we should be able to observe.
The researchers say that the characteristics of HD-114082b do not match those of hot-start models. Its size and mass are more consistent with primary accretion. But even so, it was still quite massive for its size. Either it contains an unusual core or something else is going on.
“It’s too early to give up on the idea of a hot start,” kata Lönnhardt. “All we can say is we still don’t understand very well the formation of giant planets.”
The exoplanet is one of only three planets that we know are younger than 30 million years old, and for that astronomers have obtained measurements of their radius and mass. So far, these three seem to be incompatible with the disk instability model.
Three is obviously a very small sample size, but three for three suggests that primary accumulation may be the more common of the two.
“While more such planets are needed to confirm this trend, we believe that theorists should start reassessing their calculations.” said Zakhozai.
“It’s interesting how our observations feed into the theory of planet formation. They help increase our knowledge of how this giant planet grew and tell us where gaps in our understanding lie.”
Research published in Astronomy and astrophysics.