Planetary systems consist of large clouds of dust and gas that form disks around young stars. Over time, these discs coalesce to form planets of various sizes, compositions, and distances from their parent star.
In the past few decades, observations at mid- and far-infrared wavelengths have led to the discovery of debris disks around young stars (less than 100 million years old). This allowed astronomers to study planetary systems in their early history, providing new insight into how systems formed and evolved.
This includes the SpHere INfrared Survey for Exoplanets, or SHINE, an international team of astronomers dedicated to studying forming star systems.
Using the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile, the SHINE collaboration recently observed the debris disk of a nearby star, called HD 114082, in visible and infrared wavelengths, which is an F-type star (yellow white dwarf).
Combined with data from NASA’s Transiting Exoplanet Satellite (TESS), they were able to image a Jupiter-sized gas giant embedded directly within the disk, dubbed HD 114082 b.
Scientists point out that the newly discovered planet, although its diameter is similar to that of Jupiter, its mass is eight times the gas giant, which gives it twice the density of Earth, despite the fact that it mostly consists of gas.
The properties of this “superjupiter” have not only left astronomers baffled, but may challenge current theories about planet formation.
The outer planet, located about 310 light-years outside the solar system in the constellation Centaurus, orbits a sun-like star that is only 15 million years old, making it a relative “baby” in cosmic terms, also when compared to our 4.6 billion-year-old planet.
It is common for astronomers to discover gas giant planets similar to or larger than Jupiter, but it is unusual to discover a planet of this density and heaviness.
These statistics are outlandish, Olga Zakuzi, an astronomer at the Max Planck Institute for Astronomy in Germany and lead author of the new study, said in a statement: “Compared to current models, the exoplanet HD 114082 b is two to three times as dense for the young gas giant. which is only 15 million years old.
HD 114082 b’s diameter and mass give it a density twice that of Earth – which is amazing given that it’s a gas giant composed mostly of hydrogen and helium gas, the lightest elements in the universe.
The outer planet revolves around its star half the distance between the Earth and the sun, and completes an orbit every 110 Earth days, an orbit similar to the orbit of Mercury, the closest planet to the sun.
And if the measurements of the mass of this planet are correct, that would make it twice as dense as Earth (Earth is already a dense planet, being a rocky type and having a metallic core). This could be because the planet is very young.
There are two possible ways a gas giant like HD 114082 b could form, and both occur in the protoplanetary disk, which is a disk of gas and dust that collapses to form planets.
The first formation mechanism involves the primary accretion model: a protoplanet begins life as a rocky solid core into which more and more material accumulates. Once this core reaches a critical mass, its gravitational effect pulls the surrounding gas into it, causing hydrogen and helium to build up in the core in a process that generates a giant planet.
The second mechanism, the disc instability model, involves dense, gravitationally unstable patches of the protoplanetary disk collapsing and growing to form gas giants lacking a rocky core.
These formation models differ in the rate at which the accretion gas cools, leading astronomers to describe planets as starting out “hot” (nucleus accretion) or “cold” (disk instability).
Scientists currently prefer the hot-start model, but the two approaches should lead to notable differences, thus pointing scientists towards the correct formation model.
In gas giants, this key property is size: since hot gas occupies more volume than cold gas, smaller gas giants may have been from a “cold” start, while larger gas giants, such as HD 114082 b, were most likely formed by pulp buildup.
The difference in size caused by the two possible origins should be particularly pronounced between the younger worlds, becoming less measurable over hundreds of millions of years as the planet cools and the gas shrinks.
Although a hot start is generally the expected model, the density of HD 114082 b appears to defy what astronomers would expect for the core superposition model, favoring instead the cold start or disk instability model.
Alternative explanations for HD 114082 b’s small size and large mass include the idea that the exoplanet simply has an exceptionally large rocky core buried in its core or that astronomers don’t yet have an accurate picture of how quickly the gas is cooling an infant gas giant.
“It’s too early to abandon the idea of a hot start,” Ralf Lönnhardt, an astronomer at the Max Planck Institute for Astronomy and co-author of the new study, said in the statement. “All we can say is that we still don’t understand very well the composition of the giant planets.”