I want to mention the recent confirmation of K2-415b because this world falls into an interesting category: Planets with major implications for studying their atmospheres. Orbiting an M5V M-dwarf every 4.018 days at a distance of 0.027 AU, this is not a planet with any likelihood for life. Far from it, given an equilibrium temperature expected to be in the range of 400 K (the equivalent figure for Earth is 255 K). And although it’s roughly Earth-sized, K2-415b turns out to be at least three times more massive.

What this planet has going for it, though, is that it transits a low mass star, and at 70 light years, it’s close. Consider: If we want to take advantage of transmission spectroscopy to study light being filtered through the planetary atmosphere during ingress and egress from the transit, nearby M-dwarf systems make ideal targets. Their habitable zones are close in, so we get frequent transits around small stars. But the number of Earth-sized transiting worlds around nearby examples of such stars is small, totalling 14 in eight 8 different systems (limiting the range to 30 pc, or roughly 100 light years). That includes the high-priority seven around TRAPPIST-1.

Image: This is Figure 3 from the paper. Caption: Sensitivity plot (5 ? contrast curve) for K2-415 in the K0 band based on the combined IRCS image. The inset shows the zoomed image of the target with a FoV of 4” × 4”. Credit: Hirano et al.

As we move into ever deepening searches for atmospheric biomarkers, worlds like these are going to be in the vanguard, fodder for the emerging class of 30-meter telescopes and, of course, space-based observatories like the James Webb Space Telescope. We’re going to get to know this small group of worlds well as this decade continues, which will also greatly assist us in understanding how M-dwarf planets evolve. When it comes to atmospheres around small red stars, there are no guarantees.

After all, these stars especially in their youth are known to be intense emitters of extreme ultraviolet and X-ray radiation, with striking levels of flare activity. Can an atmosphere survive this bombardment, or an intense stellar wind? Will these planets evolve secondary atmospheres through surface outgassing from volcanoes and interactions with magma? Doubtless we’ll find examples of various scenarios, which should deepen our knowledge of how the atmospheres of habitable worlds emerge.

It will also be interesting to see if the apparent and recently noted (in TESS data) lack of detected planets around the lowest mass stars will be supported by later datasets. K2-415b forces that question, in that it orbits one of the lowest mass stars hosting an Earth-sized planet. Counting planets of any size, only ten transiting planet-hosting stars are cooler than K2-415. One of these is TRAPPIST-1.

K2, the extended Kepler mission, observed K2-415b, an unusually close system at 70 light years given that of the original Kepler target stars, fewer than one percent were closer than 600 light years (we have much to thank K2 for as it moved out of the original Kepler field, a classic case of making a virtue out of necessity). The follow-up campaign described in today’s paper validates the planet. And the authors note that future radial velocity studies will be on the lookout for additional planets in this system.

This too could get interesting. Lead author Teruyuki Hirano (Graduate University for Advanced Studies, Japan) and colleagues explain::

K2-415b is located slightly inner of the classical habitable zone (Kopparapu et al. 2016) based on the insolation flux onto the planet, but an outer planet, if any, in the system with a slightly longer period (e.g., 10 ? 15 days) could sit inside the habitable zone. Little is known on the properties of multi-planet systems around the lowest mass stars (< 0.3 M), but assuming that their properties are similar to those in the “Kepler-multi” systems, the planets could have a typical spacing of ? 20 mutual Hill radii (Weiss et al. 2018). Recently, Hoshino & Kokubo (2023) also showed that the typical orbital spacing of planets formed by giant impacts is ? 20 mutual Hill radii independently of stellar masses through N-body simulations. Thus, it is quite possible that a secondary planet having ? 1 M_? has an orbital period of 10 ? 15 days.

The paper is Hirano et al., “An Earth-sized Planet around an M5 Dwarf Star at 22 pc,” accepted for publication at the Astrophysical Journal and available as a preprint.