Tread lightly: `Eggshell planets' possible around other stars

Date:
November 10, 2021
Source:
Washington University in St. Louis
Summary:
Strange 'eggshell planets' are among the rich variety of exoplanets
possible, according to a study. These rocky worlds have an
ultra-thin outer brittle layer and little to no topography. Such
worlds are unlikely to have plate tectonics, raising questions as
to their habitability.

Planetary geologists have said at least three such worlds found
during previous astronomical surveys may already be known.



FULL STORY ========================================================================== Strange 'eggshell planets' are among the rich variety of exoplanets
possible, according to a study from Washington University in
St. Louis. These rocky worlds have an ultra-thin outer brittle layer and
little to no topography. Such worlds are unlikely to have plate tectonics, raising questions as to their habitability.


==========================================================================
Only a small subset of extrasolar planets are likely eggshell planets.

Planetary geologist Paul Byrne, first author of the new modeling study
in the Journal of Geophysical Research: Planets, said at least three
such worlds found during previous astronomical surveys may already
be known. Scientists could use planned and future space telescopes to
examine these exoplanets in greater detail and confirm their geological characteristics.

"Understanding whether you've got the possibility of plate tectonics is
a really important thing to know about a world, because plate tectonics
may be required for a large rocky planet to be habitable," said Byrne, associate professor in the Department of Earth and Planetary Sciences
in Arts & Sciences and a faculty fellow of the university's McDonnell
Center for the Space Sciences. "It's therefore especially important
when we're talking about looking for Earth-like worlds around other
stars and when we're characterizing planetary habitability generally."
"What we've laid out here is essentially a how-to guide, or handy manual,"
he said. "If you have a planet of a given size, at a given distance from
its star and of a given mass, then with our results you can make some
estimates for a variety of other features -- including whether it may
have plate tectonics." A new way to think about exoplanets To date,
exoplanets have largely been the domain of astronomers, because space scientists rely on astronomical techniques and instruments to detect exoplanets. More than 4,000 exoplanets have been discovered and are
considered "confirmed." Byrne's study offers new and concrete ways that
other scientists could identify eggshell planets, as well as other types
of exoplanets that could be interesting because of their particular combinations of size, age and distance to their host star.



==========================================================================
"We have imaged a few exoplanets, but they are splotches of light orbiting
a star. We have no technical ability to actually see the surface of
exoplanets yet," Byrne said. "This paper is one of a small but growing
number of studies taking a geological or geophysical perspective to
try and understand the worlds that we cannot directly measure right
now." Planets have certain qualities that are inherent to the planets themselves, like their size, interior temperature and the materials that
they are made of.

Other properties are more of a function of the planet's environment,
like how far it is from the sun. The planets that humans know best are
those in our own solar system -- but these truths are not necessarily
universal for planets that orbit other stars.

"We know from published work that there are exoplanets that experience conditions in a more extreme way than what we see in our solar system,"
Byrne said. "They might be closer to their star, or they might be much
larger, or have hotter surfaces, than the planets we see in our own
system." Byrne and his collaborators wanted to see which planetary
and stellar parameters play the most important role in determining
the thickness of a planet's outer brittle layer, which is known as
the lithosphere.

This thickness helps determine whether, for example, a planet can support
high topography such as mountains, or has the right balance between
rigidity and flexibility for one part of the surface to dive down,
or subduct, beneath another -- the hallmark of plate tectonics. It is
this process that helps Earth regulate its temperature over geological timescales, and the reason why plate tectonics is thought to be an
important component of planetary habitability.



==========================================================================
For their modeling effort, the scientists chose a generic rocky world
as a starting point. ("It was kind of Earth sized -- although we did
consider size in there, too," he said).

"And then we spun the dials," Byrne said. "We literally ran thousands
of models." Perhaps similar to parts of Venus They discovered that
surface temperature is the primary control on the thickness of brittle exoplanet lithospheres, although planetary mass, distance to its star
and even age all play a role. The new models predict that worlds that
are small, old or far from their star likely have thick, rigid layers,
but, in some circumstances, planets might have an outer brittle layer
only a few kilometers thick -- these so-called eggshell planets.

Although we are a long way from directly imaging the surfaces of these
eggshell planets, they might resemble the lowlands on Venus, Byrne
noted. Those lowlands contain vast expanses of lavas but have little high-standing terrain, because the lithosphere there is thin as a result
of searing surface temperatures.

"Our overall goal is more than just understanding the vagaries of
exoplanets," Byrne said. "Ultimately we want to help contribute
to identifying the properties that make a world habitable. And not
just temporarily, but habitable for a long time, because we think
life probably needs a while to get going and become sustainable."
The fundamental question behind this research is, of course, are
we alone? "That is the big reach," Byrne said. "Ultimately most of
this work is tied into this final destination, which is 'how unique,
or not, is Earth?' One of the many things we are going to need to know
is what kinds of properties influence a world like Earth. And this study
helps address some of that question by showing the kinds of ways these parameters interact, what other outcomes might be possible and which
worlds we should prioritize for study with new-generation telescopes." ========================================================================== Story Source: Materials provided by
Washington_University_in_St._Louis. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Paul K. Byrne, Bradford J. Foley, Marie E. S. Violay, Michael
J. Heap,
Sami Mikhail. The Effects of Planetary and Stellar Parameters on
Brittle Lithospheric Thickness. Journal of Geophysical Research:
Planets, 2021; DOI: 10.1029/2021JE006952 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211110104608.htm

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