Here’s GJ 367b, an iron planet smaller and denser than Earth

This is an article I wrote for The Conversation about a new exoplanet, for which I was a co-author on the discovery paper. One reason for reproducing it here is that I can reverse any edit that I didn’t like!

As our Solar System formed, 4.6 billion years ago, small grains of dust and ice swirled around, left over from the formation of our Sun. Through time they collided and stuck to each other. As they grew in size, gravity helped them clump together. One such rock grew into the Earth on which we live. We now think that most of the stars in the night sky are also orbited by their own rocky planets. And teams of astronomers worldwide are trying to find them.

The latest discovery, given the catalogue designation GJ 367b, has just been announced in the journal Science by a team led by Dr Kristine Lam of the Institute of Planetary Research at the German Aerospace Center.

The first signs of it were seen in data from NASA’s Transiting Exoplanet Survey Satellite (TESS). Among the millions of stars being monitored by TESS, one showed a tiny but recurrent dip in its brightness. This is the tell-tale signature of a planet passing in front of its star every orbit (called a “transit”), blocking some of the light. The dip is only 0.03 percent deep, so shallow that it is near the limit of detection. That means that the planet must be small, comparable to Earth.

But Dr Lam also wanted to know the planet’s mass. To do that her team set about observing the host star at every opportunity with HARPS, an instrument attached to a 3.6-metre telescope at the European Southern Observatory in Chile, that was specially designed to find planets. It does this by detecting a slight shift in the wavelength of the host star’s light, caused by the gravitational pull of the planet. It took over 100 observations to detect that shift, meaning that the planet, in addition to being small, must also have a low mass.

Artist’s impression of GJ 367b (Credit: Patricia Klein)

Eventually, as observations accumulated, the numbers were tied down: GJ 367b has a radius of 72 percent of Earth’s radius (to a precision of 7 percent), and a mass of 55 percent of Earth’s mass (to a precision of 14 percent). That demonstrates that astronomers can both find Earth-sized planets around other stars, and then measure their properties. The measurements tell us that this planet is denser than Earth. Whereas Earth has a core of iron surrounded by a rocky mantle, this planet must be nearly all iron, making it similar to our Solar System’s Mercury.

Mercury orbits our Sun every 88 days. Blasted by fierce sunlight, the “daytime” side is bare rock heated to 430 degrees Celsius. GJ 367b is even more extreme. The recurrent transits tell is that it orbits its star in only 8 hours. Being so close, the daytime side will be a furnace heated to 1400 Celsius, such that even rock would be molten. Perhaps GJ 367b was once a giant planet with a vast gaseous envelope, like Neptune. Over time, that gaseous envelope would have boiled off, leaving only the bare core that we see today. Or perhaps, as it formed, collisions with other proto-planets stripped off a mantle of rock, leaving only the iron core.

GJ 367b is, of course, way too hot to be habitable. But it shows that we can find and characterise rocky, Earth-sized planets. The task now is to find them further from their star, in the “habitable zone”, where the surface temperature would allow water to exist as a liquid. That is harder. The further a planet is from its star, the less likely it is to transit, and the longer the recurrence time between transits, making them harder to detect. Further, orbiting further out, the gravitational tug on the host star is reduced, making the signal harder to detect.

But GJ 367b’s host star is a red dwarf, a star much dimmer than our Sun. And, with less heating from starlight, the habitable zone around red dwarfs is much closer in. NASA’s Kepler spacecraft has already found planets in the habitable zone of red-dwarf stars, and the TESS survey promises to find many more.

The next step is to ask whether such planets have atmospheres, what those atmospheres are made of, and whether they contain water vapour. Even there, answers may soon be forthcoming, given the imminent launch of the James Webb Space Telescope. If JWST is pointed at a star when a planet is in transit, it can detect the starlight shining through the thin smear of atmosphere surrounding the planet, and that might show subtle spectral features caused by molecules in the planetary atmosphere. We’ve already found water vapour in the atmospheres of gas-giant exoplanets. As planet discoveries continue apace, it is becoming feasible that we could, before long, prove the existence of a planet that has an atmosphere and a rocky surface, on which water is running freely.

A rocky, Mercury-like exoplanet.