A newly weighed exoplanet has left astronomers baffled.
After taking measurements of 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 of just 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 about 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 about how planets form – a process not fully understood.
Two types of data are needed to comprehensively characterize an exoplanet, based on its effect 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, is a record of how much the star wobbles in place in response to the gravitational pull of the outer planets. If we know the star’s mass, the amplitude of its wobble can give us the mass of the exoplanet.
For nearly four years, researchers have been collecting radial velocity observations of HD-114082. Using the collected transit and radial velocity data, the researchers determined that HD-114082b has a similar radius Jupiter – But Jupiter’s mass is 8 times greater. This means that the density of the exoplanet is almost twice 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 hold an important atmosphere of hydrogen and helium.
HD-114082b significantly exceeds these parameters, which means that it is a gas giant. But astronomers don’t know how this happened.
“We think that giant planets could form in two possible ways,” says astronomer Ralph Lönnhardt MPIA. “Both occur within a protoplanetary disk of gas and dust distributed around a young, central star.”
Both methods are referred to as “cold start” or “hot start”. In the cold start, the exoplanet is thought to form, pebble after 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 runaway buildup of hydrogen and helium, the two lightest elements in the universe, creating a huge gaseous envelope around a rocky core.
Given that the gases lose heat as they fall toward the planet’s core and form the atmosphere, it’s seen as a relatively cool option.
A hot start is also known as disc instability, and is thought to occur when a swirling region of instability in the disc collapses directly in on itself by gravity. The resulting object is a fully formed exoplanet with no rocky core, as the gases retain more of their heat.
Exoplanets that experience a cold start or a hot start must cool at different rates, resulting in distinct characteristics that we should be able to observe.
The researchers say that HD-114082b’s characteristics do not fit the hot-start model. Their size and mass are more consistent with primary accretion. But even then, it’s still quite massive for its size. Either it contains an unusual nucleus or something else is going on.
“It’s too early to give up on the idea of a hot start,” Lönnhardt says. “All we can say is that we still don’t understand very well the formation of the giant planets.”
The exoplanets are one of three planets that we know are younger than 30 million years old, and for which astronomers have obtained radius and mass measurements. So far, all three seem to be incompatible with the disk instability model.
Three is clearly a very small sample size, but three for 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 that theorists should begin to reassess their calculations.” Zakhozai says.
“It’s exciting how our observational results feed into the theory of planet formation. They help improve our knowledge about how these giant planets grow and tell us where the gaps in our understanding lie.”
Research published in Astronomy and astrophysics.
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