Red dwarf stars have fascinated me for decades, ever since I learned that a potentially habitable planet around one might well be tidally locked. Trying to imagine a living world with a sun that didn’t move in the sky was the kind of exercise that I love about science fiction, where playing with ideas always includes a vivid visual element. What kind of landscapes would a place like this offer to the view? What kind of weather would tidal lock conjure? Stephen Baxter’s novel Proxima (Ace, 2014) is a wonderful exercise in such world-building.
Thus my continuing interest in the splendid work being done by RedDots, which takes as its charter the detection of terrestrial planets orbiting red dwarfs near the Sun. You’ll recall that this is the team that discovered Proxima Centauri b, a star under increased scrutiny of late as other potential planetary signals are examined. RedDots also gave us Barnard’s Star b and has found three planets around the red dwarf GJ 1061.
Now we learn about a system of super-Earths orbiting nearby Gliese 887, which is the brightest red dwarf in our sky (as per RECONS, the Research Consortium On Nearby Stars). More massive than the Earth but smaller than the ice giants in our system, the two worlds may or may not be rocky — this is radial velocity work, so we have only minimum mass figures to work with. The minimum masses reported in the paper in Science are 4.2 ± 0.6 and 7.6 ± 1.2 Earth masses (M⊕). My guess is that these planets are more like Neptune than Earth, but we’ll see.
The RedDots team found the two planets using the HARPS spectrograph at the European Southern Observatory in Chile. There is also an unconfirmed signal with a period of roughly 50 days, possibly a third planet of a similar mass, but note this: “We regard the third signal at ~50 days as dubious and likely related to stellar activity.”
Lead author of the paper is Sandra Jeffers (University of Göttingen), who notes the opportunity this system provides astronomers for follow-up work. The two planets have orbital periods of 9.3 and 21.8 days respectively and circle a star that is about 11 light years away. A space-based observatory might be able to tease out their reflected light. From the paper:
GJ 887 has the brightest apparent magnitude of any known M dwarf planet host. This brightness, combined with the high photometric stability of GJ 887, exhibited in the TESS data, and the high planet-star brightness and radius ratios, make these planets potential targets for phased-resolved photometric studies, especially in emission. Spectrally resolved phase photometry has been shown to be sensitive to the presence of an atmosphere and molecules such as CO2.
Image: Dr Sandra Jeffers. Credit: University of Göttingen.
The team believes that the star has few starspots:
The TESS variability can be explained by one starspot, or a group of starspots, with a total diameter of 0.3% of the stellar surface, indicating that GJ 887 is slowly rotating with very few surface brightness inhomogeneities.
And that’s interesting because it implies a lower value for the star’s stellar wind, which could cause planetary atmospheres to erode and, if strong enough, conceivably strip them altogether. Thus the likelihood that there may be atmospheres on these planets, which renders them interesting potential targets for the who-knows-when-it-will-fly James Webb Space Telescope.
Image: Artist’s impression of the multiplanetary system of newly discovered super-Earths orbiting nearby red dwarf Gliese 887. Credit: Mark Garlick.
The paper is Jeffers et al., “A multiplanet system of super-Earths orbiting the brightest red dwarf star GJ 887,” Science Vol. 368, Issue 6498 (26 June 2020), pp. 1477-1481 (abstract).