In the ongoing search for life-friendly exoplanets, an interesting observation from the Tokyo Institute of Technology and Tsinghua University, from Nanowerk:
Simulations by researchers at Tokyo Institute of Technology and Tsinghua University indicate that Earth-like planets are more likely to be found orbiting Sun-like stars rather than lower-mass stars that are currently targeted, in terms of water contents of planets.
The search for habitable planets currently focuses on so-called M dwarfs – stars with less than half the mass of the Sun. These stars are thought to have more habitable orbiting planets that are easier to find compared with G dwarfs – stars that have a similar mass to the Sun. However, according to recent simulations by a collaborative research team composed of Shigeru Ida at Tokyo Tech and Feng Tian at Tsinghua University, M dwarf systems may not be the best places to look.
For planets to be habitable, they must orbit stars within the ‘habitable zone’ where it is not too hot or too cold. In addition, recent studies on habitability of planets suggest that the water-land ratio must be similar to the Earth. That is, the water mass fraction should not be far from that of the Earth’s (~0.01wt%): planets with too much water (> 1 wt%)—“ocean planets”—lead to an unstable climate and lack of nutrient supply; and water-poor planets like Venus “dune planets”—become too arid for inhabiting.
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Ida and Tian simulated planet distributions around stars with 0.3, 0.5 and 1.0 times the mass of the Sun. They then applied a model for water loss and accounted for the change in luminosity. They found that Earth-mass planets with Earth-like water contents occur 10–100 times less frequently around M dwarfs than around G dwarfs. They conclude, “We suggest that stars close to the size of the Sun should be the primary targets for detecting Earth-like planets.”
Here’s the abstract of the Nature Geoscience paper:
Efforts to identify habitable extrasolar planets have focused on systems around M dwarfs, faint stars with less than half the solar mass. Habitable planets around M dwarfs are thought to be more plentiful and easier to detect than those orbiting Sun-like G dwarfs1, 2, 3, 4. However, unlike G dwarfs, M dwarfs experience a prolonged decline in luminosity early in their history, leading to an inward migration of the habitable zone to where planets may have lost their water through dissociation and hydrodynamic escape. Water-poor planets, such as Venus, are considered uninhabitable. In contrast, planets with too much water (>1 wt%) would lack continents5, leading to climate instability6 and nutrient limitation problems7. Here we combine a numerical planet population synthesis model with a model for water loss to show that the evolution of stellar luminosity leads to two types of planets of Earth-like mass (0.1 to 10 Earth masses) in the habitable zones around M dwarfs: ocean planets without continents, and desert planets, on which there are orders of magnitude less surface water than on Earth. According to our simulations, Earth-mass planets with Earth-like water contents are rare around M dwarfs and occur 10–100 times less frequently than around G dwarfs. We suggest that stars close to the size of the Sun should be the primary targets for detecting Earth-like planets. (paywall)
See also: Don’t let Mars fool you. Those exoplanets teem with life!
and
How do we grapple with the idea that ET might not be out there?
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