TOI-561 b: the first rocky planet with an atmosphere detected by the James Webb Telescope

TOI-561 b is the innermost of at least three planets orbiting a rather old star, the 10 billion year old, which is about 280 light years from Earth. It is a strange planet for us since it is only 1.5 times the size of the Earth, stands only 1.5 million kilometres from its star, compared to our 150, and completes a full orbit in less than 11 hours. In essence it is roughly the same size as us, but has a year that is less than half a day of ours, and because it is so close to its star, the temperatures are estimated to be such that it could melt rock. Not exactly a cosy place.

Now, under such conditions, scientists think that planets generally lose any atmosphere, due to heat and bombardment from the intense radiation coming from the nearby star. Previous observations, made from 2020 with other satellites, have shown us that TOI-561 b has a very low density for a rocky world, so a new explanation must be found, from the mechanism by which it was formed. The fact that we see an outer atmosphere could account for the low density.

To check whether or not the planet has an atmosphere, the team used the JWST telescope to measure the temperature of the diurnal side of TOI-561 b in May 2024.

The space telescope, with its sophisticated instruments, measured a temperature of only 1,700 degrees Celsius, 1,000 less than the planet was expected to have.

After many justified computer simulations, the scientists realised that the atmosphere could be there, indeed, given the temperature, the strong winds generated by the difference with the surface of the planet itself allowed for the cooling found.

In essence, in this somewhat hellish world, a balance seems to have been created between the molten lava surface and the atmosphere, allowing for balance between the two components, and the density we observe is that of planet plus atmosphere

It is interesting to witness the development of this brand new part of astrophysics today. That there were other worlds around the stars that populate both our Galaxy and others was, until 1995, an established belief, at least in the last century, but unproven. Certainly discovering the first planet revolving around a star, a discovery made in Geneva with a relatively small telescope but with a good deal of ingenuity, realised one of the great dreams of mankind, clearly linked to the possibility of discovering other life in the Universe.

Today we are convinced that practically every star out of the billions that populate our galaxy, may have planets around it, we are not in essence a special case, quite the contrary. We are discovering, as in this case, things we never thought could exist and this is also telling us a lot about our origins.