The James Webb Space Telescope Just Uncovered A Possible Explanation for Why Giant Gaseous Planets Often Fail

The problem is the neighbors and their pesky ultraviolet radiation.

Colorful cosmic clouds and bright stars in a deep space nebula.

It’s hard to build a family these days — and that’s especially true when the neighbors’ stellar winds keep eroding all the gas out of your protoplanetary disk.

A team of astronomers recently watched stellar winds from nearby massive stars blowing gas away from the disk around a tiny newborn star — and this could explain why small stars rarely form big planets.

If we’re ever going to find life on other worlds, our best chances are around small, dim stars called red dwarfs. They’re also the most numerous stars in the galaxy, so it’s no surprise that astronomers are very interested in understanding these little star systems better. And one puzzle is that they rarely, if ever, have giant gas planets. However, it's not impossible. French National Center for Scientific Research (CNRS) astrophysicist Olivier Berne and his colleagues may have figured out why, and they recently published their work in the journal Science.

This JWST image of the Orion Nebula shows a region where dense clumps of gas are collapsing to form new stars, from red dwarfs to blue giants.


What happens when planets grow up in rough neighborhoods

Berne and his colleagues used the James Webb Space Telescope (JWST) and ALMA radio telescope to study the disk of gas and dust swirling around a newborn small star in the Orion Nebula — the slightly blurry spot of light near the tip of the constellation Orion’s sword. The disk, with the uninspiring moniker d203-506, contains the raw ingredients for planets, which should slowly coalesce out of the disk over the next few million years.

Most small stars don’t have gas giants in their orbits, and Berne and his colleagues’ observations could help explain why.

Like many dwarf stars, this little solar system is trying to grow up in a crowded cosmic neighborhood, close to several massive stars, each about 10 times the mass of our Sun and 100 times brighter. Those stellar behemoths are not quiet neighbors; they’re blasting the whole area of the Orion Nebula with powerful ultraviolet radiation, wreaking havoc on d203-506.

When such intense X-ray and ultraviolet radiation hits the disk of gas and dust around a young star (which astronomers call a protoplanetary disk), it heats up the gas. That means the molecules that make up the gas start moving faster — enough to break free of the young dwarf star’s gravity and go flying off into space. Astronomers call that process photoevaporation. Although Berne and his colleagues couldn’t see the powerful stellar winds that were causing it (at least not with JWST and ALMA, which see the universe in the much longer wavelengths of infrared light), they could see how the disk was heating up — and how the heated gas was evaporating into space.

Based on the data from JWST and ALMA, Berne and his colleagues built computer models of what was happening in d203-506, and it turns out that the young star system is losing gas to its neighbors’ stellar winds at an alarming rate. All the gas in the disk could be gone in less than a million years, perhaps as little as 130,000 years.

“This is faster than even very early planet formation,” writes Berne and his colleagues in their recent paper. In other words, all the gas will blow away long before d203-506’s planets really start forming. The system will never be able to build a stormy gas giant like Jupiter or Saturn, just a collection of small, rocky worlds like our Earth or the seven worlds of the TRAPPIST-1 system.

How it works

This process is probably because most dwarf stars have small, rocky planets but not gas giants in their orbits.

Most of the galaxy’s smallest stars — red dwarfs like TRAPPIST-1, Proxima Centauri, and SPECULOOS-2 — form in crowded stellar nurseries like the Orion Nebula, so they grow up surrounded by larger, brighter siblings like the massive stars near d203-506. The problem, for systems like d203-506, is that their host stars don’t have strong enough gravity to hold onto their gas once it’s heated up, so it’s easy for them to lose it all, leaving behind the heavier grains of dust.

Larger stars like our Sun, on the other hand, have more mass and stronger gravity, which means that gas molecules have to move a lot faster to escape. Instead of evaporating into space, most of the heated gas just bounces around the star system and collides with more things, speeding up the planet formation process.

Astrophysics isn’t always fair.

Physicists’ simulations of how star systems form have predicted that the stellar winds from those massive stars should blow away most of the gas orbiting small dwarf stars, but it’s been hard to catch them in the act until recently.

What about us?

Earth may also have formed in a cluster, surrounded by more massive stars that eventually drifted apart. That’s based on what astronomers have observed about how the Sun moves in relation to nearby stars, and the chemical composition of parts of the Solar System. And since that’s the case, the protoplanetary disk in which Earth and the rest of our Solar System formed probably got bombarded by high-energy ultraviolet radiation from those massive stars when the Sun was young.

Our Sun is between two and ten times more massive than most red dwarf stars, and about 10 times less massive than the giant stars surrounding d203-506. That means its gravity was probably strong enough to hold onto some of the protoplanetary disk’s gas despite all the radiation from its stellar siblings, but it wasn’t quite strong enough to keep all the gas from evaporating away.

If that hadn’t been the case, our Solar System might have more than two gas giants, or Jupiter might be even larger than it is. Our Solar System might be a very different place, and life on Earth might be very different, if it had happened at all.

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