When a planet passes in front of a star, it is like having the planet in a huge laboratory setting with spectrometers picking up energy from that star as it passes around that planet and through its atmosphere, if it has an atmosphere. It is possible to use this information to estimate the size of the planet and to say something about its composition.
There are two ways in which this super natural laboratory is foiled. One, the star itself is noisy, spitting out a wide range of unruly energy types unpredictably. The other is if the instruments on our end, at Earth, are messed up by things like the atmosphere, or the fact that the Earth is spinning so we can only see the star and planet for a few hours a day.
For these two reasons, astronomers seem to like to do two things. One is to find “quiet” stars, stars that have a steadier output of energy, so they make better backlights, as it were, for the planets. The other is to put instruments on rocket ships and fly them out of the Earth’s atmosphere.
Both things are happening, and it is working.
A new paper in Nature Astronomy reports on a very interesting observation addressing planet types not previously seen much, if ever, and that are missing in our own solar system. They use the term “Missing Link” to refer to planets that are larger and less rocky, or smaller and less gassy, then the Sun’s solar system style plants are.
The abstract says, in part:
One of the primary goals of exoplanetary science is to detect small, temperate planets passing (transiting) in front of bright and quiet host stars. This enables the characterization of planetary sizes, orbits, bulk compositions, atmospheres and formation histories. These studies are facilitated by small and cool M dwarf host stars. Here we report the Transiting Exoplanet Survey Satellite (TESS)1 discovery of three small planets transiting one of the nearest and brightest M dwarf hosts observed to date, TOI-270 (TIC 259377017, with K-magnitude 8.3, and 22.5 parsecs away from Earth). The M3V-type star is transited by the super-Earth-sized planet TOI-270 b (1.247+0.089?0.083 R?) and the sub-Neptune-sized planets TOI-270 c (2.42?±?0.13 R?) and TOI-270 d (2.13?±?0.12 R?). The planets orbit close to a mean-motion resonant chain, with periods (3.36 days, 5.66 days and 11.38 days, respectively) near ratios of small integers (5:3 and 2:1). TOI-270 is a prime target for future studies because (1) its near-resonance allows the detection of transit timing variations, enabling precise mass measurements and dynamical studies; (2) its brightness enables independent radial-velocity mass measurements; (3) the outer planets are ideal for atmospheric characterization via transmission spectroscopy; and (4) the quietness of the star enables future searches for habitable zone planets. Altogether, very few systems with small, temperate exoplanets are as suitable for such complementary and detailed characterization as TOI-270.
An extensive discussion is to be found here at US Riverside’s news agency.