The gas giant is in a place where it could not possibly have been formed by conventional means. This confirms what researchers have suspected for some time: nature can craft planets in different ways.

All planets are born in protoplanetary disks. These are disks of gas and dust around young stars. Such dust discs can spawn a wide variety of planets, from Earth-like worlds to mighty gas giants. Researchers do have ideas about how the latter planets are formed. For example, it is believed that most see the light of day through ‘core accretion’. During this process, small objects in the protoplanetary disk – such as dust or pebbles – collide as they orbit the parent star and clump together. In this way, a planetary core is gradually created which – as soon as it has sufficient mass – begins to draw gas from the same protoplanetary disk towards itself. The result is a gas giant: a planet core surrounded by an enormously thick coat of gases.

Alternative method

So far it’s a pretty straightforward story. But researchers have long suspected that not all gas giants arise in the same way. In theory, gas giants could also see the light of day in a much more abrupt way. This is when the protoplanetary disk cools and, under the influence of gravity, disintegrates into one or more fragments, which subsequently develop – much faster than in the manner described above – into Jupiter-like protoplanets. However, there was no evidence for this alternative method of formation for a long time. But the Hubble Space Telescope now seems to be changing that.

AB Aurigae b

The telescope has spotted a planet that could not actually have formed in the conventional way – i.e. by clumping together small particles and subsequently collecting gases. It concerns the protoplanet AB Aurigae b, which is still in the protoplanetary disk around its very young (only 2 million years old) parent star. The planet is about 9 times more massive than Jupiter and, crucially, a whopping 13 billion kilometers from its parent star. That means the distance between the planet and its parent star is more than twice the distance between Pluto and our sun!

That a gas giant orbits such a great distance from the star is significant, according to the researchers. Because at such a great distance from the parent star, it would take an awfully long time — if at all — to form a gas giant in the conventional way. “Nuclear accretion requires different masses of solid materials to clump together over a period of time (several millions of years),” said study researcher Thayne Currie. Scientias.nl† And at a greater distance from the parent star, all of this simply isn’t happening fast enough. “Because the orbital period is longer and there is usually less solid material available at such a great distance than, for example, at the distance between our Jupiter and the sun. To put it very simply, a growing planetary core at a very great distance from the parent star has less to eat and also eats more slowly.” The fact that an oversized gas giant can still be found around AB Aurigae – only 2 million years after the formation of this star and at a great distance from this star, therefore strongly suggests that it can develop in a different, much faster way. has been established. “This new discovery is strong evidence that some gas giants can be formed because the protoplanetary disk is unstable,” said researcher Alan Boss.

Archival and modern images reveal the motion of the protoplanet orbiting the parent star counterclockwise. Images: Thayne Currie (Subaru Telescope, Eureka Scientific Inc.), Alyssa Pagan (STScI).

Incidentally, the great distance between the parent star and the protoplanet is not the only indication that a planet has been formed in this protoplanetary disk in an unconventional way. “The disk that contains the planet also has several spiral arms,” Currie said. And that’s something researchers expect to see when planets emerge due to instability in the protoplanetary disk. In fact, the AB Aurigae system is very similar to what we see in computer simulation when we simulate planet formation due to protoplanetary disk instability. The similarities are striking.”

Observations

The researchers base their conclusions – published in the magazine Nature Astronomy – not just on Hubble observations. They also used observations from the Subaru telescope for the study. “Interpreting this system is extremely challenging,” Currie points out. This has of course everything to do with the fact that AB Aurigae b is still hidden in the protoplanetary disk. “That’s one of the reasons we needed Hubble for this project,” said Currie. “(We wanted, ed.) to be able to separate the light from the disk from the light from the planet.” Hubble also came in handy for determining the orbit of the protoplanet; the telescope has been around for a while and so there were quite a few archival images showing the planet (in hindsight) showing off. Those images — combined with archival footage from Subaru — eventually allowed the researchers to see the planet orbit the star and confirm its existence.

“Nature is smart,” Currie concludes. “And this one can form planets in different ways.” Now that hard evidence has been found for this, the question naturally arises whether there are not many more gas giants formed in this unconventional way waiting to be discovered and whether this unconventional planet formation is really so unconventional. We asked Currie, and he certainly expects nuclear accretion to be the primary method by which gas giants emerge, so collapsing protoplanetary disks will remain rather unconventional. “To obtain such an unstable disk, it must firstly be very heavy and secondly cool very efficiently. However, few protoplanetary disks are as massive as AB Aurigae’s. In addition, the criterion of rapid cooling implies that instability in the protoplanetary disk occurs more quickly at a great distance from the star (50 to 100 times the distance between the Earth and the Sun). We now know that a large proportion of the stars possess gas giants, but only a few percent of those stars possess gas giants at 50 to 100 AU.” That said, Currie does expect there to be unconventionally formed gas giants still waiting to be discovered. However, compared to gas giants created by nuclear accretion, they are expected to remain heavily outnumbered.