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. Hi-Res Image
As
anybody who has started a campfire by rubbing sticks knows, friction
generates heat. Now, computer modeling by NASA scientists shows that
friction could be the key to survival for some distant Earth-sized
planets traveling in dangerous orbits.
The findings are consistent with observations that Earth-sized
planets appear to be very common in other star systems. Although heat
can be a destructive force for some planets, the right amount of
friction, and therefore heat, can be helpful and perhaps create
conditions for habitability.
“We found some unexpected good news for planets in vulnerable
orbits,” said Wade Henning, a University of Maryland scientist working
at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and lead
author of the new study. “It turns out these planets will often
experience just enough friction to move them out of harm’s way and into
safer, more-circular orbits more quickly than previously predicted.”
Simulations of young planetary systems indicate that giant planets
often upset the orbits of smaller inner worlds. Even if those
interactions aren’t immediately catastrophic, they can leave a planet in
a treacherous eccentric orbit – a very elliptical course that raises
the odds of crossing paths with another body, being absorbed by the host
star, or getting ejected from the system.
Another potential peril of a highly eccentric orbit is the amount of
tidal stress a planet may undergo as it draws very close to its star and
then retreats away. Near the star, the gravitational force is powerful
enough to deform the planet, while in more distant reaches of the orbit,
the planet can ease back into shape. This flexing action produces
friction, which generates heat. In extreme cases, tidal stress can
produce enough heat to liquefy the planet.
In this new study, available online in the July 1, 2014, issue of the
Astrophysical Journal, Henning and his colleague Terry Hurford, a
planetary scientist at Goddard, explored the effects of tidal stresses
on planets that have multiple layers, such as rocky crust, mantle or
iron core.
One conclusion of the study is that some planets could move into a
safer orbit about 10 to 100 times faster than previously expected – in
as a little as a few hundred thousand years, instead of the more typical
rate of several million years. Such planets would be driven close to
the point of melting or, at least, would have a nearly melted layer,
similar to the one right below Earth’s crust. Their interior
temperatures could range from moderately warmer than our planet is today
up to the point of having modest-sized magma oceans.
The transition to a circular orbit would be speedy because an
almost-melted layer would flex easily, generating a lot of
friction-induced heat. As the planet threw off that heat, it would lose
energy at a fast rate and relax quickly into a circular orbit. (Later,
tidal heating would turn off, and the planet's surface could become safe
to walk on.)
In contrast, a world that had completely melted would be so fluid
that it would produce little friction. Before this study, that is what
researchers expected to happen to planets undergoing strong tidal
stresses.
Cold, stiff planets tend to resist the tidal stress and release
energy very slowly. In fact, Henning and Hurford found that many of them
actually generate less friction than previously thought. This may be
especially true for planets farther from their stars. If these worlds
are not crowded by other bodies, they may be stable in their eccentric
orbits for a long time.
“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,” said Hurford. “This is great for dim stars
or ice worlds with subsurface oceans."
Surprisingly, another way for a terrestrial planet to achieve high
amounts of heating is to be covered in a very thick ice shell, similar
to an extreme “snowball Earth.” Although a sheet of ice is a slippery,
low-friction surface, an ice layer thousands of miles thick would be
very springy. A shell like this would have just the right properties to
respond strongly to tidal stress, generating a lot of heat. (The high
pressures inside these planets could prevent all but the topmost layers
from turning into liquid water.)
The researchers found that the very responsive layers of ice or
almost-melted material could be relatively thin, just a few hundred
miles deep in some cases, yet still dominate the global behavior.
The team modeled planets that are the size of Earth and up to
two-and-a-half times larger. Henning added that superEarths – planets at
the high end of this size range – likely would experience stronger
tidal stresses and potentially could benefit more from the resulting
friction and heating.
Now that the researchers have shown the importance of the
contributions of different layers of a planet, the next step is to
investigate how layers of melted material flow and change over time.
Elizabeth Zubritsky
NASA's Goddard Space Flight Center
NASA's Goddard Space Flight Center
