Can There be Life on Planets in Orbit around a Red Dwarf?

Figure - This artist's impression shows two Earth-sized worlds passing in front of their parent red dwarf star, which is much smaller and cooler than our Sun. The star and its orbiting planets TRAPPIST-1b and TRAPPIST-1c reside 40 light-years away. The planets are between 20 and 100 times closer to their star than Earth is to the Sun. Researchers think that at least one of the planets, and possibly both, may be within the star's habitable zone, where moderate temperatures could allow for liquid water on the surface. Hubble looked for evidence of extended atmospheres around both planets and didn't find anything. Credit:NASA, ESA, and G. Bacon (STScI)

A red dwarf is a small and relatively cool star on the main sequence, of either K or M spectral type. Red dwarfs range in mass from a low of 0.075 solar masses (M☉) to about 0.50 M☉ and have a surface temperature of less than 4,000 K.

Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun, but because of their low luminosity, individual red dwarfs cannot be easily observed. From Earth, not one is visible to the naked eye. Proxima Centauri, the nearest star to the Sun, is a red dwarf (Type M5, apparent magnitude 11.05), as are twenty of the next thirty nearest stars. According to some estimates, red dwarfs make up three-quarters of the stars in the Milky Way. [1]

The prospects for the habitability of M-dwarf planets have long been debated, due to key differences between the unique stellar and planetary environments around these low-mass stars, as compared to hotter, more luminous Sun-like stars.

Over the past decade, significant progress has been made by both space- and ground-based observatories to measure the likelihood of small planets to orbit in the habitable zones of M-dwarf stars. We now know that most M dwarfs are hosts to closely-packed planetary systems characterized by a paucity of Jupiter-mass planets and the presence of multiple rocky planets, with roughly a third of these rocky M-dwarf planets orbiting within the habitable zone, where they have the potential to support liquid water on their surfaces.

Theoretical studies have also quantified the effect on climate and habitability of the interaction between the spectral energy distribution of M-dwarf stars and the atmospheres and surfaces of their planets. These and other recent results fill in knowledge gaps that existed at the time of the previous overview papers published nearly a decade ago by Tarter et al. (2007) and Scalo et al. (2007).

In a recent review (Shields et al. 2016) the authors provide a comprehensive picture of the current knowledge of M-dwarf planet occurrence and habitability based on work done in this area over the past decade, and summarize future directions planned in this quickly evolving field.

They conclude that M-dwarf planets are more resistant to global-scale glaciation, and may have more stable climates over long timescales, which could be advantageous for biological evolution. And photosynthesis, a process that is essential for a large fraction of life on the Earth, is no longer held to be an implausible mechanism to occur on M-dwarf planets. While perhaps less productive than on planets orbiting stars with more higher-energy photons available, M-dwarf planets could be capable of anoxygenic photosynthesis at the same level as on Earth if available photons extend to long-enough wavelengths. There are examples of life on Earth with peak absorbance at near-IR wavelengths, and carrying out photosynthesis in conditions of low light intensity. [2]

1. Red dwarfs - (Wikipedia)
2. Shields et al. 2016 (accepted in Physics Reports) - The Habitability of Planets Orbiting M-dwarf Stars (arXiv)