What does it take to build a gas giant? Building models of planet formation and studying exosolar systems have both given us some hints. But there is a small but growing list of cases where the two approaches disagree on what is possible. A new paper adds to that list by describing a gas giant planet orbiting 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 much heavier elements around them. The idea is that a large core of rocky material must form quickly before the star fully ignites and expels any nearby gas. If the rocky body gets big enough early on, it could grab enough gas to start a runaway atmospheric accumulation and turn itself into a gas giant.
Studying exosolar systems provides some support for this idea. We can get an idea of how much heavier elements – generally called metals – were present during planet formation by looking at their presence in the parent star. If the star has a high metal content, then the planets probably also had access to much heavier elements. For small, rocky planets, it doesn’t seem to matter how much heavier elements were around, as they are found around stars with varying degrees of metal content. The same goes for super-Earths and Neptune-sized planets.
But no gas giants. These are only found on planets with a high metal content, supporting the idea that they need a lot of heavy elements to quickly form a large core. This also implies that they should be rare near dwarf stars, since these small stars would not be expected to have much material around in the first place. Which brings us to the new discovery.
Next generation planet hunting
It is the first planet found by a new project called the Next-Generation Transit Survey (NGTS). Based in Chile, the project consists of a series of a dozen small telescopes (20 cm aperture) connected to red-sensitive CCD cameras. An automated system lets the telescopes examine a population of about 20,000 stars, looking for periodic eclipses caused when planets move between the star and Earth. The cameras’ red sensitivity allows the system to work with dwarf stars, which produce much redder light than the sun.
When the host discovered the first planet with the new hardware, the star was named NGTS-1, with the planet NGTS-1b. It is close to the guest star and completes an orbit in just 2.65 days. Instead of passing directly between the host star and Earth, NGTS-1b only grazes the edge of the star (imagine a planet passing, from Earth’s perspective, one of the star’s poles). Still, that’s enough to give some idea of its size, and it’s a large 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 gravity on the host star. This indicated that the planet is 0.8 times the mass of Jupiter. The differences from our local gas giant — larger radius but smaller mass — are likely a result of the small distance between NGTS-1b and its star, which heats and expands the planet’s gas.
The star itself, on the other hand, is quite small, with only a little more than half the radius of the Sun. That places it firmly in the M-dwarf category.
All this makes for a rather unusual combination. NGTS-1b is only the third gas giant to orbit an M dwarf — and the most massive yet found. It also means that NGTS-1b is about 23 percent the size of its host star in radius, more than twice the relative size difference between Jupiter and the sun.
This is confusing
How does a small star get so much material? That is less clear. Everything indicates that NGTS-1 is an old star, meaning it formed when heavier elements had even lower abundances than they do now. And, based on the size of the star, it formed under conditions where there wasn’t much material in the first place. It is not at all clear how a gas giant formed under those conditions.
There is some evidence that giant planets can form just like stars from the direct collapse of a cloud of gas. But these tend to be super-Jupiters, objects closer to a brown dwarf star than to Jupiter. It is therefore doubtful whether they are relevant to this system.
It is worth remembering that there are two other Jupiter-class planets also orbiting M dwarf stars. So any solution we find to explain NGTS-1b should be general enough to account for these other cases as well.
That’s probably why the authors of the article argue that it’s best to understand the full scope of the problem first. To do that, we want an overview of dwarf stars to get an idea of how often they host gas giants. From there, we can begin to determine the conditions, such as heavy element content, associated with the formation of gas giants. With that data in hand, it may be possible to update our models of planet formation to account for these unexpected systems.
The arXiv. Abstract number: 1710.11099 (About the arXiv). Will be published in Monthly communications from the Royal Astronomical Society.