Leaky mini-Neptune planets are losing their air and becoming super-Earths

Earth is the largest of the planets in our solar system. Neptune is four times the size of Earth.

We started to find planets in the mass gap. Over time, enough was found to get some statistics, and these planets fall into two distinct mass ranges: the "super-Earths" that are up to 1.6 times Earth's diameter, and the "mini-Neptunes" which are about the same size.

That leaves a mass gap. Where are the planets between 1.6 and 2 times the size of Earth?

One idea is that when a forming planet gets to about 1.6 times Earth's size its gravity is strong enough to really start drawing in gas around it and it jumps to being a mini-Neptune. It may work for planets farther out from a newborn star where it is cold and gas plentiful, but what about those closer in? An idea to explain those is that if a planet is too close to the star, it will be too hot to hold on to the atmosphere. It loses mass and size until it becomes a super-Earth.

The idea of mini-Neptunes losing their atmospheres and becoming super-Earths has only been an idea until now, but two different newly discovered exoplanets around different stars both appear to be mini-Neptunes. This is the first time it has been seen.

The planets transit their host stars, creating a mini-eclipse. We can't see the planets directly, but we can see the atmospheres of the planets. H2O is absorbed in the ultraviolet and the IR. It is possible to see a dip in the brightness of the starlight if you look closely at those colors. As the gas moves away from the planet, we can see a shift in the color of the light, which can show how much gas is leaking and how quickly.

HD 63433c is a planet that is in the vicinity of a young Sun-like star. Astronomers used the Hubble Telescope to observe it. It takes just 20 Earth days to go around the star, because it has 2.67 times Earth's diameter. It is just 22 million kilometers from the star. It should be losing air to space despite being 7 times the mass of Earth.

They can see hydrogen absorbing light from the star, and from the Doppler shift it is moving away from the planet at 180,000 km/hr. The planet is losing 10% of its mass worth of gas every billion years. It will take a while to lose enough gas to become a super-Earth, but one thing some planets have is time. There is another planet in that system, a mini-Neptune closer in to the star, and no atmospheric hydrogen loss was detected from it. It is most likely so hot that it has lost as much gas as it can. Some planets have run out of time.

HD 73583b is an exoplanet that is nearly three times the diameter of Earth and is 103 light years away from us. The planet is just 9 million kilometers away from the star and very hot. They used the NIRSPEC spectrograph on the Keck telescope to detect the loss of helium on the planet. Over time, it will likely become a super-Earth because it is losing gas at a rate that is twice that of HD 63433c.

Light pressure and the stellar wind pushed the gas away from the star, much like they pushed back on the tail of a comet. It is difficult to explain this behavior. Either it is an observational error, which is unlikely given the care they took to process the data, or there is an odd physical mechanism at play. The team should be able to understand what the planet is doing with more observations.

The atmospheres of both planets are larger than you would think because they absorb so much energy from their stars. We have seen puffy planets before, but they have all been Jupiter-sized with hundreds of times Earth's mass. This is the first time that this has been seen in this size planet. It looks like mini-Neptunes do become super-Earths over time. It is not clear how much atmosphere will be left once the process is over.

There are no super-Earths or mini-Neptunes in our solar system, or else we would have a different name for them. These are the most common kind of planet we see around other stars. It is odd that we do not have one, but it is not clear why. We use our own planetary system as a template for understanding all the others. It depends on your point of view.