- #1
|Glitch|
- 429
- 117
Earth-sized planets covered in water may be abundant around red dwarfs, the most common type of star in the Universe.
Yann Alibert and Willy Benz at the University of Bern used computer simulations to predict the properties of planets that could form around red dwarfs and host liquid water. They found that the radius of the planets would be 0.5–1.5 times that of Earth, with most being around the same size as Earth. More than 90% of the simulated planets were at least 10% water by mass, suggesting that they were completely surrounded by deep oceans.
Source: Small stars host water worlds - Nature 539, 8 (November 3, 2016), doi:10.1038/539008d
It has always been my impression than spectral type M stars were notorious for being flare stars. As a result of their small radius and relatively low effective surface temperature, the Habitable Zone has to be relatively close to the surface of the star and small in size. As a result, it would seem to preclude exoplanets with a viable atmosphere in the Habitable Zone of such stars. However, I find that my initial impression may not have been correct.
A sample of 23,253 stars in Walkowicz et al. (2011), with effective temperature Teff less than 5,150°K (which includes spectral type K and M main sequence stars) and a surface gravity log g > 4.2 was examined for flares over a time period of 33.5 days. Of those 23,253 stars, only 373 stars were identified as having obvious flares. Some stars had only one flare, while others showed as many as fifteen. The strongest events increased the brightness of the star by 7% to 8%. That is a population of only 1.6%. Granted, 33.5 days is not a very long period, but that is still not the percentage of flare stars I originally assumed would be associated with spectral type M stars.
Althougth, the Walkowicz et al. (2011) study does add a caveat that younger stars tend to be more active than older stars. What that means in the case of spectral type M main sequence stars, which could potentially live for trillions of years, I am not exactly sure. In a universe that is only ~13.78 billion years old, is there such a thing as an "old" spectral type M main sequence star?
Furthermore, Walkowicz et al. (2011) explains that observations made of solar flares from spectral type M main sequence stars tend to be 10 to 1,000 times as energetic as solar flares. For the 1.6% of flare stars this pertains to, that would seem to rule out any possibility of an exoplanet within the Habitable Zone from having an atmosphere, much less liquid water on its surface. However, that still leaves 98.4% of spectral type M main sequence stars that are not flare stars.
Since the above study, Alibert et al. (2016), used 0.1 M☉, 1.004 R☉, and 2,935°K Teff for its initial conditions, I was able to estimate the "conservative" Habitable Zone, using Kopparapu et al.(2014), as being between 0.14 AU and 0.38 AU, with the frost-line/snow-line (160°K) at 0.79 AU. Interestingly, they found through their simulation that planets with low mass (specifically < 0.4 M⊕) tend to be totally devoid of water.
Alibert et al. (2016) also assumes that the mean surface temperature of the simulated exoplanet is no colder than 273.16°K (0.01°C; 32.02°F) with a minimum atmospheric pressure of 611.657 pascals (6.11657 mbar; 0.00603659 atm; 0.088713417 psi) at "sea level."
Sources:
Formation and composition of planets around very low mass stars - arXiv 1610.03460
A terrestrial planet candidate in a temperate orbit around Proxima Centauri - Nature 536, 437-440 (August 25, 2016), doi:10.1038/nature19106 (arXiv free preprint)
White-light flares on cool stars in the Kepler Quarter 1 Data - The Astronomical Journal, Volume 141, Number 2, January 13, 2011 (free article)
Evolution of protoplanetary disks: constraints from DM Tauri and GM Aurigae - Astronomy & Astrophysics, Volume 442, Number 2 (November 1, 2005) (arXiv free preprint)
Flares on active M-type stars observed with XMM-Newton and Chandra - Thesis by Urmila Mitra Kraev, University College London [PDF]
Habitable Zones Around Main-Sequence Stars: Dependence on Planetary Mass - The Astrophysical Journal Letters, Volume 787, Number 2 , May 15, 2014 (free article)
Last edited: