On Earth, the presence of tectonic activity maintains the carbon cycle and acts as a thermostat, moderating the greenhouse effect. Earth-size planets in the habitable zone are more likely to be habitable if they are tectonically active. The habitable zone is that swath of space around a star where temperatures are neither too hot nor too cold for a rocky planet to potentially sustain liquid water on its surface. However, tectonic activity is driven by internal heat. Since planets cool as they age, they will eventually have insufficient internal heat to drive tectonic activity. The demise of tectonic activity on an old, cooling planet could adversely affect the planet’s habitability. It is likely that tectonic activity would cease for Earth once it reaches an age of ~10 billion years.
A study published in the July issue of the Monthly Notices of the Royal Astronomical Society (MNRAS) shows that the gravitational pull of an outer companion planet can generate enough tidal heating for an Earth-size planet in the habitable zone to arrest its cooling. In particular, the models focused on Earth-size planets in the habitable zone of low-mass stars that are less than 0.3 times the Sun’s mass. The presence of an outer companion planet can keep the orbit of the Earth-size planet around its host star non-circular. As a result, the gravitational pull on the planet from its host star is constantly changing, potentially generating enough tidal heating to sustain tectonic activity on the planet.
Artist’s impression of a tidally-locked Earth-size planet around a low-mass star. The presence of an outer companion planet can induce sufficient tidal heating to keep the Earth-size planet warm enough to sustain tectonic activity for tens of billions of years.
The reason for the focus on low-mass stars is because such stars are much fainter than the Sun. A planet would have to be much closer to the star to receive an equivalent amount of insolation Earth gets from the Sun. This places the planet in a much stronger gravitational field, making it more susceptible to tidal heating. Furthermore, low-mass stars have extremely long lives measured in hundreds of billions to several trillion years. For comparison, the Sun has a lifespan of only about 10 billion years. The extreme longevity of low-mass stars means planets around such stars can cool below what is required to drive tectonic activity long before the stars themselves reach even a fraction of their lifespans. Also, Earth-size planets are more easily detected around low-mass stars than around more massive stars like the Sun.
The presence of an outer companion planet to an Earth-size planet in the habitable zone of a low-mass star can induce sufficient tidal heating to drive tectonic activity on the Earth-size planet for tens of billions of years or more. A Neptune-size outer companion planet would easily fulfil such a role. In fact, a substantial range of masses and orbits for the outer companion planet can induce the appropriate amount of tidal heating on the inner Earth-size planet. The least massive stars, those with ~0.1 times the Sun’s mass, are expected to live for trillions of years. Earth-size planets in the habitable zone of such stars with outer companion planets could represent the longest-lived surface habitats in the universe.
C. Van Laerhoven, R. Barnes and R. Greenberg, “Tides, planetary companions, and habitability: habitability in the habitable zone of low-mass stars”, MNRAS (July 1, 2014) 441 (3): 2111-2123.