To trace a gas giant’s birth, take its temperature

Giant gassy planets such as Jupiter and Saturn are born from, well, gas. But how are they created and how do they evolve?

Researchers from ETH Zürich and the Universities of Zürich and Bern simulated different scenarios to find out how they exactly this. And in two papers published in the Monthly Notices of the Royal Astronomical Society this month, Judit Szulágyi and colleagues found astronomers might be able to determine how an exoplanet was born just by taking its temperature.

Astronomers have two main theories explaining how gas giants arise around a young star.

A bottom-up formation mechanism called core accretion states that a solid core, roughly 10 times the size of the Earth, attracts gas to it and keeps it.

The top-down scenario – disc instability – states that the gaseous disc around a young star is so massive, gravitational instabilities trigger formation of spiral arms of gassy clumps strung together.

These clumps themselves collapse, thanks to their own gravity, into a gaseous planet – a bit like how stars are born.

Both allow for a disc of gas to form around the giant planet, called the circumplanetary disc, from which moons are born.

Szulágyi and colleagues, using supercomputers running for months, found a huge difference between the two formation scenarios. In the disk instability situation, the gas in the planet’s vicinity remained very cold (around 50 Kelvin or -222 °C).

But in the core accretion case, the circumplanetary disc was heated to several hundred Kelvin. 

“When astronomers look into new forming planetary systems, just measuring the temperatures in the planet’s vicinity will be enough to tell which formation mechanism built the given planet,” Szulágyi explains.

In the core accretion model, the researchers had a closer look at the circumplanetary disc around planets with masses three to 10 times bigger than Jupiter’s.

Computer simulations showed that gas falling on the disc from the outside heats and created a bright shock front on the disc’s upper layer – significantly changing the appearance of young forming planets.

“When we see a luminous spot inside a circumplanetary disk, we cannot be sure whether we see the planet luminosity, or also the surrounding disc luminosity,” says Szulágyi.

This may lead to an overestimation of the planet’s mass of up to four times, “so maybe an observed planet has only the same mass as Saturn instead of some Jupiter masses”.