Moons Can Help Planets Remain Stable Long Enough for Life to Form
A new study suggests that large moons can form and remain stable for long times around distant planets as well, potentially helping alien life evolve.
Researchers also suggest that if the recently discovered rocky alien planet Kepler-62f has a moon, the moon could last more than 5 billion years, perhaps long enough to help foster the evolution of complex life. The investigators detailed their findings in the International Journal of Astrobiology.
In the past two decades or so, astronomers have confirmed the existence of more than 1,700 planets beyond the Solar System, and they may soon prove the existence of thousands more of such exoplanets. Of special interest are distant planets in habitable zones, the regions around stars just warm enough for worlds to possess liquid water on their surfaces, as there is life virtually wherever liquid water is found on Earth.
To support complex forms of life, a world needs more than just an orbit within its star’s habitable zone. It probably also needs a climate that remains stable over long time spans as well. One major factor controlling a world’s climate is its obliquity, also known as axial tilt, which has to do with the amount its axis of rotation is tilted in relation to the path it takes around its star.
Earth’s seasons, for example, depend on the axial tilt, as the amount of light hitting the northern and southern hemispheres varies with the way the northern and southern hemispheres point toward or away from the Sun.
Earth’s axial tilt was stabilized with the help of the gravitational pull of its large moon, which is roughly one-quarter the diameter of the Earth.
“If the Earth did not have the Moon, the Earth’s axial tilt would have changed rapidly and the climate of the Earth would have changed often,” said lead study author Takashi Sasaki, a planetary scientist at the University of Idaho.
In contrast, Mars has relatively small moons, and its axial tilt has changed substantially over long periods of time, fluctuating chaotically on a 100,000-year time scale. These wobbles in Mars’ axial tilt might help explain why vast underground pockets of ice have been discovered far from the Red Planet’s poles. In the distant past, Mars’ axis might have been tilted at a significantly more extreme angle than it is now, and ice caps were able to reach across the planet. Even after Mars’ axial tilt became less extreme, this ice far from the poles survived, protected by subsequent layers of dust.
A planet whose axial tilt fluctuates wildly like Mars may not maintain a favorable climate for a long enough time for complex forms of life to evolve. For example, it took about 3.8 billion years for life on the 4.6-billion-year-old Earth to evolve from single-celled organisms to multicellular life such as plants, animals and fungi.
“Because the Earth has had a long-term stable climate, life on the Earth has had time to evolve from single cells to complex life forms,” Sasaki said.
Since the Moon is a key reason why Earth has had a relatively stable climate for a long time, the Moon is one of the key factors in Earth’s evolution of complex life forms, he said.
Sasaki and his colleague Jason Barnes sought to understand how long moons might last around rocky planets in habitable zones, given varying masses and compositions of moons, planets and stars. They focused on systems where moons could last 5 billion years, assuming that such a duration is long enough for complex life to evolve.
Their model accounted for how a planet or moon’s gravitational pull increases in relation to increasing mass. In addition, their calculations factored in how gravitational tidal forces are greater the closer two bodies are to one another. The gravitational pull of a planet’s star can also influence the relationship between that world and its moon.
Three potential scenarios were possible. First, a moon could get closer and closer to its planet until it breaks apart or collides with its host, as Mars’ moon Phobos is predicted to do about 50 million years from now. Next, a moon could get farther and farther away until it escapes the planet. Last, a moon can reach a stable distance from its planet, as is the case for the dwarf planet Pluto’s moon Charon.
The rate at which a moon gets closer to or farther away from its planet depends on the extent to which the tidal forces they exert on each other dissipates and slows their rates of spin. For instance, as the Moon’s orbit has taken it farther away from Earth over time, the Moon’s rate of spin has slowed to the point that it now always shows just one side to Earth. Eventually, the Earth will also slow its rate of spin enough to always show just one side to the Moon.
The degree to which a moon and its planet dissipate the tidal forces they exert on each other relies greatly on the mass, compositions and structures of those bodies. For instance, the way tides slosh water around in the shallow seas of Earth dissipates large amounts of tidal energy. Planets with no oceans or with deep oceans would dissipate less tidal energy than Earth.
The researchers examined four typical planet compositions: Earth-like planets composed of 67 percent mostly silicon-based rock and 33 percent iron; planets with 50 percent rock and 50 percent ice; planets with 100 percent rock; and planets with 100 percent iron. These planets were one-tenth to ten times Earth’s mass and orbited the habitable zones of stars that ranged from 40 percent to equal the mass of the Sun.
The scientists found that stars with less than 42 percent of the Sun’s mass may not be good places to look for complex life because moons cannot survive for more than 5 billion years in these systems. This is because the habitable zones are closer to stars that have dimmer and lower masses than in brighter, higher-mass star systems. For instance, in solar systems with stars 40 to 50 percent of the Sun’s mass, the habitable distance is approximately one-quarter of the distance between the Sun and Earth. Since these planet-moon systems are so close to their host stars, their stars gravitational pull perturbs the planet-moon systems too much for the moons to remain around their planets, Sasaki said.
This finding runs counter to the belief that lower-mass stars are good for habitable planets because they live longer than higher-mass stars, potentially giving them more time for life to evolve. For example, while the lifetime of a planet with a mass twice the Sun’s mass is about 1.2 billion to 1.3 billion years, the lifetime of a star with half the Sun’s mass is about 80 billion years, Sasaki said. However, he noted “our results show that small-mass stars may not be good parent stars for habitable planets.”
For stars more than 42 percent the Sun’s mass, whether a moon survives depends on factors such as the planet’s composition and how well the planet dissipates tidal energy. A moon has a longer lifetime the higher the density of its host planet.
The researchers also investigated the prospect for moons in the Kepler-62 system, which at a distance of 1,200 light years from Earth has a star that is a bit cooler and smaller than the Sun, as well as two planets in the habitable zone: Kepler-62e and Kepler-62f. The planets are 1.4 and 1.6 times Earth’s diameter, respectively.
The scientists found that Kepler-62e would have to be composed almost entirely of a high-density material, such as iron, for a moon orbiting it to exist for more than 5 billion years. They also discovered that Kepler-62f could have a moon for more than 5 billion years if it had a variety of different compositions, particularly if it had an absence of oceans or only deep oceans, either of which would cause the planet to dissipate less tidal energy.
In the future, instead of looking at moons around Earth-sized planets in habitable zones, Sasaki said they would like to investigate moons around giant planets in habitable zones.
“If a giant planet at habitable zone has a big enough moon, there may be life on the moon,” Sasaki said. “Finding the conditions favorable for habitable moons is a direction we might go.”