What does it take to build a gas giant? Building models of planet formation and studying exosolar systems have both provided us with some hints. But there's a small but growing list of cases where the two of these approaches disagree about what's possible. A new paper adds to that list by describing a gas giant planet that orbits a dwarf star, creating a situation where the planet is 25 percent the size of its host—the smallest difference between planet and star yet observed.

Gas giants, as their name implies, are mostly hydrogen and helium. But models of planet formation have suggested that they can only form in systems with a lot of heavier elements around. The idea is that a large core of rocky material has to form quickly, before the star fully ignites and drives off any nearby gas. If the rocky body gets big enough early enough, it can grab enough gas to start a runaway atmospheric accumulation, turning itself into a gas giant.

Studying exosolar systems provides some support for this idea. We can get a sense of how many heavier elements—generically termed metals—were around during planet formation by looking at their presence in the host star. If the star has a high metal content, then the planets probably had access to lots of heavier elements, too. For small, rocky planets, it doesn't seem to matter how many heavier elements were around, as they're found at stars with various degrees of metal content. The same is true for super-Earths and Neptune-sized planets.

But not gas giants. These are only found at planets with high metal content, supporting the idea that they require lots of heavy elements to form a big core quickly. This also implies that they should be rare near dwarf stars, since these tiny stars wouldn't be expected to have a lot of material nearby in the first place. Which brings us to the new discovery.

Next-gen planet hunting

It's the first planet found by a new project called the Next-Generation Transit Survey (NGTS). Based in Chile, the project is an array of a dozen small telescopes (20cm aperture) hooked up to red-sensitive CCD cameras. An automated system has the telescopes survey a population of about 20,000 stars, looking for periodic dimming caused as planets transit between the star and Earth. The red sensitivity of the cameras allows the system to work with dwarf stars, which produce much redder light than the Sun.

As the host of the first planet discovered with the new hardware, the star picked up the name NGTS-1, with the planet NGTS-1b. It's close to the host star, completing an orbit in only 2.65 days. Rather than passing directly between the host star and Earth, NGTS-1b only grazes across the edge of the star (envision a planet that, from Earth's perspective, transits across near one of the star's poles). Still, that's enough to provide some sense of its size, and it's a big one, 1.33 times the radius of Jupiter.

Once it was identified, the researchers imaged it with the HARPS instrument, which determines the planet's gravitational influence on the host star. This indicated that the planet is 0.8 times the mass of Jupiter. The differences with our local gas giant—larger radius but smaller mass—are probably a result of the small distance between NGTS-1b and its star, which heats and expands the gas of the planet.

By contrast, the star itself is quite small, at only a bit more than half the Sun's radius. That places it firmly in the M-dwarf category.

All of which makes for a rather unusual combination. NGTS-1b is only the third gas giant found orbiting an M-dwarf—and the most massive one found to date. It also means that, by radius, NGTS-1b is about 23 percent the size of its host star, more than twice the relative size difference between Jupiter and the Sun.

This is confusing

How does a tiny star end up with that much material? That's less clear. All indications are that NGTS-1 is an old star, which means it formed when heavier elements had even lower abundances than they do today. And, based on the size of the star, it formed under conditions where there wasn't a lot of material around in the first place. It's not at all clear how a gas giant formed under those conditions.

There have been some hints that giant planets can form much like stars do, from the direct collapse of a gas cloud. But these tend to be super-Jupiters, objects that are closer to a brown dwarf star than they are to Jupiter. So it's doubtful that they're relevant to this system.

It's worth remembering that there are two other Jupiter-class planets that also orbit M-dwarf stars. So any solution we arrive at to explain NGTS-1b should be general enough to account for these other cases, too.

Which is probably why the authors of the paper argue that it's best to understand the full extent of the problem first. To do that, we'd want a survey of dwarf stars to get a sense of how frequently they host gas giants. From there, we can start determining the conditions, like heavy element content, that are associated with gas giant formation. With that data in hand, it might be possible to update our models of planet formation to account for these unexpected systems.

The arXiv. Abstract number: 1710.11099 (About the arXiv). To be published in Monthly Notices of the Royal Astronomical Society.