Editor’s Note: As a series at ENV, we are pleased to present "Exoplanets." Daniel Bakken is an engineer who teaches astronomy at the college level, and an entrepreneur in compound semiconductor crystal growth. In a series of articles he critically examines recent claims about exoplanets beyond our solar system, asking whether our own planet Earth is a rarity, or common, in the cosmos. For previous articles in the series, see here.
For a planet to be habitable, its star must also meet certain standards. Many stars are in multiple star systems, yet for the stability of a habitable planet, single stars are likely required, contrary to science fiction’s frequent double sunsets. Simulations show that only if a double star is in a very tight arrangement, so that the two stars are essentially like a single star, or if the second star is in an extremely wide orbit, would the necessary dynamic stability be met.1
Notwithstanding what Bill Nye says, the Sun is not an "unremarkable" star. It is larger than over 95% of stars in the galaxy.2 The stellar classification system used by astronomers ranks stars in surface temperature, and assigns letters to them, in the sequence O, B, A, F, G, K and M, O being the hottest, largest stars and M being the lowest in temperature, and smallest in mass, when dealing with stars on the main sequence. Stars on the main sequence are burning hydrogen into helium, and are relatively stable.3 It is likely that only F and G dwarf stars are homes for potentially habitable planets as they are stable on the main sequence for billions of years, yet are large enough so their habitable zones are outside the tidal locking limit.4 The Sun is a G-type dwarf star, and F-type dwarf stars are slightly larger. Stars between O and A are large and evolve too quickly, providing only a short-lived habitable zone.5 The K and M dwarf stars are smaller red dwarfs.
Stars with masses of 0.1-0.5 solar mass make up 75 percent of the stars in our Milky Way galaxy.6 These represent the red dwarfs, the M class. But these stars have low effective temperatures, and thus emit their peak radiation at longer wavelengths (red and near-infrared).7 They can have stable continuously habitable zones over long time scales, up to 10 billion years, barring other disruptions. It is also easier to detect terrestrial sized planets around them.8 But a serious problem with red dwarf stars in the K and M classes is their energetic flares and coronal mass ejection events. Potentially habitable planets need to orbit these stars closer, to be in these stars’ habitable zones. Yet the exposure to their stellar winds and more frequent and energetic flares becomes a serious issue for habitability. Because of these stars’ smaller mass, ejections get released with more violence.9 Any planet’s atmosphere would be subject to this ionizing radiation, and likely expose any surface life to much more damaging radiation.10 The loss of atmospheres in these conditions is likely, but the timescales are dependent on several factors including the planet’s mass, the extent of its atmosphere, the distance from the parent star, and the strength of the planet’s magnetic field.11 To protect its atmosphere for a long period, like billions of years, a planet with more mass and thus higher gravity could hold on to the gases better. But this larger planet would then hold on to lighter gases, like hydrogen and helium, and prevent an atmosphere similar to Earth’s from forming.12 Another consequence is that the increased surface pressure would prevent water from being in the liquid phase.13
Another stellar parameter for advanced life has to do with UV (ultra-violet) radiation. The life-support star must provide just enough UV radiation, but not too much. UV radiation’s negative effects on DNA are well known, and any life support body must be able to sustain an atmosphere to shield them. Yet the energy from UV radiation is also needed for biochemical reactions. So life needs enough UV radiant to allow chemical reactions, but not so much as to destroy complex carbon molecules like DNA. Just this flux requirement alone requires the host star have a minimum stellar mass of 0.6 solar masses, and a maximum mass of 1.9 solar masses.14
Another requirement for habitable planets is a strong magnetic field to prevent their atmosphere from being lost to the solar winds. Planets orbiting a red dwarf star are also more affected by the star’s tidal effects, slowing the planet’s rotation rate. It is thought that strong magnetic fields are generated in part by the planet’s rotation.15 If the planet is tidally braked, then any potential for a significant magnetic field is likely to be seriously degraded. This will lead to loss of water and other gases from the planet’s atmosphere to the stellar winds.16 We see this in our solar system, where both Mercury and Venus, which orbit closer to the Sun than Earth, have very slow rotation rates, and very modest magnetic fields. Mercury has very little water, and surprisingly, neither does Venus. Even though Venus has a very dense atmosphere, it is very dry. This is due to UV radiation splitting the water molecules when they get high in the atmosphere, and then the hydrogen is lost to space, primarily, again, by solar wind.17
Along with tidal braking, which can happen in relatively short timescales compared to the appearance of intelligent life, a planet’s star can also cause a reduction in its tilt axis, effectively removing the effect of seasons on the planet. After tidal braking slows the rotation further, it will likely become tidally locked with one side perpetually lit, and the other forever in night. The planet’s water will likely become lost to an ice trap on the cold side. A very thick atmosphere may allow some reasonable temperatures at the twilight zones of a potential planet. Some models show that a liquid water environment could exist under or at the edge of a large ice cap.18 Yet this would not likely lead to a widespread habitable climate, or a technological civilization either. Such planets have been called "Tidal Venuses," because Venus rotates extremely slowly compared to Earth, due to the tidal braking of the Sun. 19 Observations of some super-Earths tend to support this. In the cases where we can compute the planets’ densities, they tend to be significantly lower than the Earth’s, likely indicating a large, non-Earthlike atmosphere. They are like "mini-Neptunes" rather than super-Earths.20
Because many of the exoplanets discovered thus far have been "hot Jupiters," very large planets orbiting very close to their parent stars, moons of these hot Jupiters have been suggested as possibly able to support life. For Class I habitable bodies, there are several difficulties with this proposal. Modelling shows that moons that form in-situ around these massive gas giants may have an upper limit of about the size of Ganymede, Jupiter’s largest moon. Moons this size don’t have enough mass to keep an atmosphere within the habitable zone of a star.21
Theoretically, Earth-mass moons may be possible with a parent planet of about 13 times the mass of Jupiter. This is in the size range of a brown dwarf star, not a planet, but a few exoplanets at this extreme size have been found.22 The most immediate problem, again, is tidal braking. The potentially habitable moon would experience even greater tidal disruption of its rotation, because of the proximity to a large planet, and may even become tidally locked to its host planet very quickly, shortly after its formation. This would effectively destroy any possible magnetic field and climate. The moon would also likely be within the giant planet’s magnetic field, and experience high levels of particulate radiation. These giant planets do not form within their stars’ habitable zones, but would have formed much further away, outside what is known as the "frost line," where volatiles like hydrogen, helium and ices would be available to form a Jovian planet, these large planets then migrate inwards. This would mean that the potentially habitable moon would also be formed from these lighter materials, much like the composition of the outer planet moons in our solar system. This would likely rule out a Class I environment on a moon of one of these giant planets, since it would have an icy crust.
One other factor is the dynamics of these giant planet’s migration towards the habitable zone of the parent star. For these large planets to migrate that far, planet-planet scattering is the likely cause, and this would be disruptive to any large moons orbiting these planets during this process.23 The more distant the moon from the host planet, the more vulnerable it is to this process, and it may become ejected from the system.24 The gravity well formed by the giant planet would also attract many more asteroids and comets to their environments, increasing the amount of impactors on the moons of these planets.25
Next up: The Carbonate-Silicate Cycle and the Moon.
(1) Gribbin, Alone in the Universe, 89, and Gonzalez, Life, 6.
(2) Brownlee and Ward, Rare Earth, 23.
(3) Michael Seeds and Dana Backman, Horizons: Exploring the Universe, 12th ed. (Boston: Brooks/Cole, 2012), 148-149.
(4) Gribbin, Alone in the Universe, 86.
(5) Forget, "On the Probability of Habitable Planets," 184.
(9) Gonzalez, "Setting the Stage for Habitable Planets," 39.
(10) Ibid.," 39-40.
(11) Ibid., 40.
(12) Forget, "On the Probability of Habitable Planets," 185.
(13) Ibid., 185.
(14) Gonzalez, "Setting the Stage for Habitable Planets," 39.
(15) Ibid., 40.
(17) Gonzalez, Habitable Zones, 23.
(18) Forget, "On the Probability of Habitable Planets," 185.
(19) Gonzalez, "Setting the Stage for Habitable Planets," 40.
(20) Forget, "On the Probability of Habitable Planets," 185.
(21) Gonzalez, "Setting the Stage for Habitable Planets," 40.
(23) Ibid., 41.
(25) Gonzalez, "Setting the Stage for Habitable Planets," 42.