- Title: Superhabitable Worlds
- Authors: René Heller and John Armstrong
- First Author’s Institution: McMaster University
- Status: Published in Astrobiology
Note: This journal article covers two topics that we thought each deserved its own astrobite. Therefore, this astrobite is Part 1 of 2, about the effects of tidal heating on habitability. We’ll be posting Part 2 tomorrow, on the concept of superhabitability.
Humankind has only just begun our search for life beyond our Solar System. So far, we’ve focused on trying to find “habitable” worlds — planets or moons that could harbor life as we know it (once we have a good set of known habitable worlds, we can start trying to find out if they’re actually inhabited). But what qualifies a planet as “habitable”? Earth is clearly habitable, but life here exists in all kinds of different environments. If you polled all life on Earth, the one requirement for survival that they’d all agree on is the need for liquid water. So that’s where we start in our quest for habitable worlds: the ability to host liquid water.
Many previous authors have defined a stellar habitable zone (HZ) as a range of distances from a given star in which you could stick a planet, and the surface of the planet would be at the right temperature for liquid water (based on the energy received from the star). Scientists have spent a lot of time lately arguing about where the edges of the HZ are, because surface temperature can be affected by a huge range of planet characteristics, including the planet’s atmosphere, the presence of ice and snow, and the planet’s orbit. The authors of this paper discuss the effects of tidal heating on the habitable zones of planets and moons.
The gravitational force of a star on a planet can be strong enough to deform the planet, causing it to bulge out in the direction of the star (imagine a basketball being pulled into the shape of a rugby ball). The gravitational force that causes the deformation is called the tidal force, the same force responsible for Earth’s tides. If the planet is on an eccentric orbit, it becomes more deformed closer to the star and more spherical further away. The actual change in the planet’s shape is extremely small and would not be visible, but it causes friction inside the planet, and this friction creates heat called tidal heating (Fig 1). Jupiter’s moon Io is a great example of tidal heating in the Solar System. Check out this great tidal heating tutorial for more information.
Tidal heating has many implications for HZs. The heat can warm a planet that is otherwise too cold to have liquid water, effectively extending the outer edge of the stellar HZ. It can also overheat planets that are inside the stellar HZ, making them too hot for liquid water. Tidal heating also affects moons. An exomoon orbiting a planet outside its stellar HZ may be habitable if the planet’s tidal force heats the moon enough. Light reflected and emitted by the planet can also increase the moon’s surface temperature. This means there could be a circumplanetary HZ around the planet, in which a moon could be habitable even if the planet is not.
The authors explore this idea by calculating the circumplanetary HZs of two hypothetical Earth-mass exomoons with different eccentricities orbiting a Jupiter-mass planet. They used their own model to calculate the total energy from tidal heating, and this paper’s model for energy received from the star and planet. Fig 2 shows the habitable zones of the moons for different planet-star and moon-planet distances. Inside the stellar HZ, the exomoon is habitable as long as it’s not too close to the planet. Tidal heating and illumination from the planet can keep the moons habitable even when the planet is far outside its stellar HZ.
These authors are not the first to investigate the effect of tidal heating on the HZ, but they use this topic to start a larger conversation about our definition of habitability. They argue that rather than looking for Earth-sized worlds in stellar HZs, we should start thinking about what physical characteristics make a planet more suitable for life. If tidal heating can expand te habitable zone, could other mechanisms do the same? Tomorrow’s astrobite, “Better Than Earth: Superhabitable Worlds,” will cover this discussion in detail.