The newly discovered exoplanet has left astronomers completely baffled.
After taking measurements of the Jupiter-bound exoplanet, named HD-114082b, scientists discovered that its properties do not neatly match either of the two popular models of gas giant planet formation.
Clearly, he is too heavy for his age.
“Compared to currently accepted models, HD-114082b is about two to three times too dense for a young gas giant with only 15 million years of age,” explains astrophysicist Olga Zakhozhay of the Max Planck Institute for Astronomy in Germany.
Orbiting a star named HD-114082 about three hundred light years away, the exoplanet is the subject of an intensive data collection campaign. Only 15 million years old, HD-114082b is one of the smallest exoplanets ever discovered, and understanding its properties could yield insights into how planets form—a process not fully understood.
Two types of information are required for a comprehensive account of an exoplanet, depending on the effect it has on its host star. Transit data is an index of the way a star’s light darkens when an orbiting exoplanet passes in front of it. If we know how bright the star is, that dim dimming can reveal the size of the exoplanet.
The given radial velocity, in turn, is an indication of how much the star wobbles in place in response to the exoplanet’s gravitational pull. If we know the mass of the star, the size of its wobble can give an estimate of the mass of the exoplanet.
For nearly four years, researchers have collected observations of the radial velocity of HD-114082. Using a combination of transits and radial velocities, the researchers determined that HD-114082b has the same radius as Jupiter – but is 8 times the mass of Jupiter. That means an exoplanet is roughly twice the density of Earth, and roughly 10 times the density of Jupiter.
This young exoplanet’s size and mass means that it is very likely to be a large rocky superplanet; their upper limit is about 3 Earth radii and 25 Earth masses.
It is also the least dense in rocky exoplanets. Above this extent, the body becomes denser, and the planet’s gravity begins to retain a significant atmosphere of hydrogen and helium.
HD-114082b is in the extreme excess of those parameters, which means it is a gas giant. But astronomers just don’t know how it got here.
“We think that giant planets can be formed in two ways,” says astronomer Ralf Launhardt of the MPIA. “Both occur within a protoplanetary disc of gas and dust distributed around a central nova.”
The two methods are called ‘cold start’ or ‘hot start’. In the cold beginning, the exoplanet is thought to form as a pebble, from debris orbiting the star’s disk.
The parts are attracted, first electrostatically, then by gravity. The more mass it achieves, the faster it grows, until it is huge enough for the accretion of fugitive hydrogen and helium, the lightest elements in the Universe, in a huge gaseous envelope around the rocky core.
Given that they lose heat as they fall towards the planet’s core and form an atmosphere, it is seen as a relatively cooling option.
A hot start is also known as an instability disk, and is thought to occur when a region of instability in the disk collapses directly in on itself under gravity. The resulting body is a fully formed exoplanet that does not have a rocky core, where gases retain most of their heat.
Exoplanets that experience a cold or warm start and must cool at different rates have distinct efficiency characteristics that we must observe.
The properties of HD-114082b do not fit the hot start model, the researchers say; size and mass become more constant with core accretion. But even then, even now, it is too great a size. Either it has the usual core, or something else is going on.
“It’s too early to abandon the idea of a hot start,” says Launhardt. “All we can say is that we still don’t understand the formation of the giant planets very well.”
One of three known exoplanets younger than 30 million years old, for which astronomers have obtained radius and mass measurements. So far, all three students seem to reject the pattern of inconsistency.
Obviously, three is the smallest sample size, but three instead of three suggests that perhaps core accretion is the more common of the two.
“While more such planets are needed to confirm this trend, we believe that theorists should re-evaluate their calculations,” Zakhozhay says.
“It raises the question of how our observations feed into the theory of planetary formation. They help our understanding of how these giant planets grow and will tell us where the gaps in our understanding lie.”
The research was published in Astronomy & Astrophysics.
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