The Search for Extraterrestrial Civilizations

star field


Number of Civilizations

Search for Exoplanets      

Search for Intelligence

Exoplanet Religion 

Science and Religion         

 Home                               News 

 

Small Planets Around Small Red Stars

Posted July 26, 2013

There are likely to be many more Earth-size planets in the habitable zones around cool M dwarf stars than around hot G dwarf stars like the sun). Although the average number of habitable-zone planets per M dwarf is lower, there are thought to be many more M dwarfs than sun-like stars in our local region of the Galaxy. These planets, which are cooler and 45 per cent smaller than the Sun, also have observational advantages over G stars in the search for planets in habitable zones. These population numbers and advantages are important in selecting planetary atmospheres for future ground and space telescope searches for evidence of life.

A recent estimate puts planets between 0.5 and 1.4 Earth radii in size occurring in habitable zones at a rate of 0.15 planets per cool type M dwarf star, within an error range of +0.13 and – 0.06. These figures come from Courtney D. Dressing and David Charbonneau of the Harvard Smithsonian Center for Astrophysics. They used data in the Kepler Input Catalog to provide improved estimates for M dwarfs with temperatures below 4,000K.They found 3897 such stars, including 64 associated with 95 candidate planets.

In comparison, sun-like stars with a temperature range of 5,000K to 6,500K, are expected to have habitable zone planets between 0.5 and 2.0 Earth radii in size occur at the rate of 0.34± 0.14, according to an estimate by Wesley A. Traub of the NASA Jet propulsion Laboratory, Pasadena. Traub used a data on 1235 planets obtained during the first 136 days of Kepler operation. The two sets of estimates show that among 100 G stars there may be 34 planets in habitable zones compared with 15 in habitable zones around 100 M stars. However, there appears to be a trend towards increasing number of planets around lower temperature stars. 75 per cent of the stars within 33 light years of Earth are thought to be M dwarfs. That changes the ratio to 34:45.

Dressing and Charbonneau estimate that there may be a habitable planet not detectable by the transit method within 16 light years of the Earth. A habitable planet that transits its dwarf star may be within 68 light years. About 9 dwarf stars with earth-sized planets in habitable zones should be discoverable by future missions to characterize the atmospheres of habitable planets.

Red dwarfs, as cool M stars are sometimes called, have not been popular targets for finding planetary systems harboring life, and even less as for planetary civilizations. The problem is that their planets orbit much closer to their star. This makes them vulnerable to radiation from stellar flares, to increased stellar ultraviolet radiation, and to having their spin period become locked to their orbital period. This last is the effect seen with our Moon. It causes one face of a locked planet to be extremely cold as it gazes out on deep space. The other face may be extremely hot as its star hovers constantly overhead.

However, Dressing and Charbonneau point out that tidal lock may not necessarily occur, because the planet could enter into a spin-orbit resonance like Mercury. In addition, they point to recent studies showing the atmosphere may not freeze if sufficient carbon dioxide is present. For an atmosphere of dense clouds, a liquid water ocean could exist on a tidally locked planet at the sub-stellar region (a condition referred to as “Eyeball Earth”).

There are also mitigating conditions with respect to strong stellar flares and high ultraviolet emission. For a planet without a strong magnetic field, such a flare can destroy the ozone layer that protects the planet from ultraviolet radiation. However, Dressing and Charbonneu consider that most of the radiation would not reach the surface of the planet. Furthermore, the role of ultraviolet radiation in the evolution of early life on Earth is far from clear. A certain level may be necessary for creating the initial living biological systems, even though ultraviolet radiation is capable of destroying many biological molecules.

[Note: Evolution occurs when a balance is struck between the variation induced in biomolecules by the environment (necessary for them to evolve) and the damage the environment inflicts (necessary to select those variants with greatest fitness). Ultraviolet radiation is just one environmental variable that affects variation and selection over the relevant reproductive time span.]

Having established to their satisfaction that a truly habitable zone exists around M dwarfs, in spite of tidal lock and stellar flares, Dressing and Charboneau put forward three reasons for targeting these stars in future studies.

First, the Kepler mission requires several more years of observations to detect Earth-size planets in the habitable zone of G dwarfs, due to higher than expected photometric noise resulting from stellar variability. However, the same mission is already sensitive to the presence of Earth-size planet in the habitable zone of an M dwarf. In addition, a transiting planet in the habitable zone of an M dwarf transits five times a year compared with once a year for a G dwarf. And because an M dwarf is 45 per cent smaller than the Sun, the transit signal of a planet orbiting an M dwarf is 3.3 times larger than that of one orbiting a G dwarf. The reduced difficulty in detecting planets around M dwarfs has already made them targets for many ground telescope surveys.

[Note: Measurements are under way to find out whether the Kepler spacecraft's reaction wheels can be restored to operation. This is essential if the Kepler mission is to resume.]

Second, there are many more M dwarfs in the Sun's neighborhood than G dwarfs. Estimates suggests they may be up to 12 times as abundant.

Third, confirming the planetary nature and measuring the mass of an Earth-sized planet orbiting within the habitable zone of an M dwarf is easier that comparable confirmation and measurement of a G dwarf planet in a habitable zone. The radial velocity signal used to confirm the presence of a planet with a ground-based telescope is 23 cm/s for a planet around a 3800 K dwarf, whereas that for a G dwarf is 9 cm/s. These measurements are important in distinguishing between a high-density Earth-type planet and a low density Neptune, an important first step before further investigation of a planet's atmosphere.

Future facilities such as the James Web Space Telescope and the Giant Magellan Ground Telescope will be able to take spectra of Earth-sized planets in the habitable zone of M dwarfs, say Dressing and Charbonneu, but not of such planets in the habitable zones of more massive stars. Examining the atmosphere of a potentially habitable planet like Earth means selecting a planet around a small, cool dwarf.

Sources

The Occurrence Rate of Small Planets around Small Stars, by Courtney D. Dressing and David Charbonneau, Astrophysical Journal, 767, 95, 2013.

Terrestrial, Habitable-Zone Exoplanet Frequency from Kepler, by Wesley A. Taub, Astrophysical Journal, 745, 20, 2012.