I hope everyone is having a good break. I have a bit of free time, so thought I would mention a paper I’ve just finished. It’s about the the three super-Earths that orbit a star known as GJ 9827. The system was already known to host 3 planets with radii slightly above that of the Earth, one of which was presented as one of the most massive, and densest, known super-Earths (it’s not).
Our paper was based on stellar radial velocity measurements from the HARPS-N spectrograph. If you measure radial velocities of the host star, then you can use these to infer the masses of the orbiting planets. We also had an accurate parallax measurement from Gaia, which allowed us to determine quite a precise mass and radius for the host star. In addition, after we’d submitted the paper, another paper appeared with a whole lot of extra radial velocity measurements, so we redid our analysis with all their data included too.
The basic result is illustrated on the right, which shows the radial velocity curves for planets GJ 9827 b and GJ 9827 d. I haven’t shown the result for GJ 9827 c because it’s essentially a non-detection (we do know that there must be a third planet, but we can’t really detect how it influences the radial velocity of the star).
We find that GJ 9827 b (with a radius of 1.6 Earth radii) has a mass of 4.9 ± 0.5 Earth masses, and GJ 9827 d (with a radius of 2 Earth radii) has a mass of 4 ± 0.8 Earth masses. The mass of GJ 9827 b is quite a bit lower than the first estimate, which suggested that it was one of the most massive, and densest, super-Earths. On the other hand, we find that GJ 9827 d is probably slightly more massive, and denser, than an earlier estimate.
If you have an estimate of the mass and the radius, then you can say something about a planet’s internal composition. The figure on the left shows the GJ 9827 planets on a mass radius diagram. What it shows is that GJ 9827 b has an internal composition similar to that of the Earth and Venus. It does, however, have an orbital period of only 3.6 days, so it is far too hot to be potentially habitable. GJ 9827 d has a much lower density than GJ 9827 b and clearly retains some kind of volatile atmosphere (maybe water vapour, or hydrogen/helium).
As you can almost see in the figure above, there’s a suggestion of a radius gap within the exoplanet population; small exoplanets tend to hve radii less than about 1.5 Earth radii and are rocky, or have radii above 2 Earth radii and retain a volatile envelope/atmosphere. What makes this system particularly interesting is that it hosts a planet on either side of this radius gap. What’s more, the smaller one is more heavily irradiated than the larger one. This is consistent with stellar irradiation stripping the atmospheres from very close-in, small exoplanets.
Systems like GJ 9827 will, therefore, play an important role in understanding the origin, and evolution, of these super-Earth exoplanets. Do they form in situ, or do they form further out and then migrate inwards? Do the really small ones form rocky, or did they once retain substantial atmospheres that were then stripped by irradiation from the central star? Understanding the latter is important if we want say something about the frequency of actual Earth-like exoplanets. If the known small, rocky exoplanets originally retained substantial atmospheres, that have since been stripped, then true Earth analogues are probably rarer than if these planets are primordially rocky.
There was something I wanted to add to this, but that doesn’t really fit within the post. It seems that many of the truly contrarian positions with respect to climate science are often promoted by scientists who are now retired (e.g., here). It always strikes me as quite arrogant to imagine that you can see something obvious that is being missed by the next generation of scientists/researchers; do these people think that the next generation are really this incompetent, or biased?
The work I discussed above is part of a programme of research involving a team of scientists from around the world. My contribution has mostly been theoretical (I run some simulations and help to write parts of some of the papers). I led the above because we were short of people to champion some of the targets. The work was, however, well outside my immediate area of expertise and it required a lot of help from others in the science team (and a steep learning curve from myself).
Most of those who were providing direct help were people at an earlier stage of their careers than I am. What struck me was just how good they are, both in terms of their technical expertise and their thinking about how to present our results. I’ve often said that I would hate to be trying to build a career in science now, because early career researchers today seem so much better than I was at their stage. The work on this paper only re-inforced that view.
What I’m getting at is that the idea that a small number of retired scientists can see major, obvious, problems in climate science that are somehow being missed by active researchers today is just utterly bizarre. Even more so, when you consider some of the ridiculous ideas that they promote.