Habitability: Friction in Orbit

Planets in eccentric orbits can experience powerful tidal forces. A planet covered by a very thick ice shell (left) is springy enough to flex a great deal, generating a lot of internal friction and heat. Some terrestrial planets (right) also will flex, especially with partially molten inner layers. Image Credit: NASA's Goddard Space Flight Center

Planets in eccentric orbits can experience powerful tidal forces. A planet covered by a very thick ice shell (left) is springy enough to flex a great deal, generating a lot of internal friction and heat. Some terrestrial planets (right) also will flex, especially with partially molten inner layers.
Image Credit: NASA’s Goddard Space Flight Center

NASA researchers have shown that friction could be the key to habitability on Earth-sized planets in treacherous, elliptical orbits. The study might have implications in defining the habitable zone around distant stars.

Eccentric Heat

A highly elliptical, non-circular orbit can be a dangerous route of travel for a small planet. It increases the odds that a celestial body will cross paths with another large object, or be pulled into and consumed by its host star. Planets in highly elliptical orbits also stand a chance of being ejected from the system entirely.

However, the new study shows that, despite the added dangers, elliptical orbits could also provide a source of heat for small worlds and aid in their ability to support life. It all comes down to tidal heating.

Tidal heating refers to the way in which an object is deformed as it passes close to a larger body and then moves farther away. It’s the same process that allows for a subsurface ocean on moons of giant planets – like Jupiter’s moon Europa. This ‘flexing’ of the world produces heat.


Europa is seen in a cutaway view through two cycles of its 3.5 day orbit about the giant planet Jupiter. Europa’s orbit is eccentric, which means as it travels around Jupiter, large tides, raised by Jupiter, rise and fall. Jupiter’s position relative to Europa is also seen to librate, or wobble, with the same period. This tidal kneading causes frictional heating within Europa, much in the same way a paper clip bent back and forth can get hot to the touch, as illustrated by the red glow in the interior of Europa’s rocky mantle and in the lower, warmer part of its ice shell. This tidal heating is what keeps Europa’s ocean liquid and could prove critical to the survival of simple organisms within the ocean, if they exist. The giant planet Jupiter is now shown to be rotating from west to east, though more slowly than its actual rate. Credit: NASA/JPL-Caltech

Life on the Ellipse

In some cases, tidal heating is enough to melt a planet completely. However, the new study shows that cold, stiff planets resist tidal heating more than scientists had expected. If these planets are located far enough away from their host star, their elliptical orbits may be stable for quite some time.

The traditional habitable zone is found at a distant from a star where there is enough heat for liquid water to persist on a planet’s surface – but not so much heat that the water quickly boils away. For a planet in an elliptical orbit, tidal heating could provide the necessary energy for liquid water (one of the major requirements for life as we know it), even if the planet’s orbit carries it outside of the habitable zone.

In a press release from NASA, Terry Hurford, co-author and planetary scientist at Goddard, explained:

“In this case, the longer, non-circular orbits could increase the ‘habitable zone,’ because the tidal stress will remain an energy source for longer periods of time. This is great for dim stars or ice worlds with subsurface oceans.”

The study, “Tidal Heating in Multilayered Terrestrial Exoplanets,” was published in The Astrophysical Journal.

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