Orbital variations can trigger ‘snowball’ states in habitable zones around sunlike stars

A NASA artist’s impression of Earth as a frigid “‘snowball”
planet. New research from the University of Washington
indicates that aspects of an otherwise habitable-seeming
exoplanet planet’s axial tilt or orbit could trigger such a
snowball state, where oceans freeze and surface life is
impossible. Credit: NASA

Aspects of an otherwise Earthlike planet’s tilt and orbital
dynamics can severely affect its potential habitability—even
triggering abrupt “snowball states” where oceans freeze and
surface life is impossible, according to new research from
astronomers at the University of Washington.


The research indicates that locating a planet in its host
star’s “habitable zone”—that swath of space just right to allow
liquid water on an orbiting rocky planet’s surface—isn’t always
enough evidence to judge potential habitability. 

Russell Deitrick, lead author of a paper to be published in the
Astronomical Journal, said he and co-authors set out to
learn, through computer modeling, how two features—a planet’s
obliquity or its orbital eccentricity—might affect its
potential for life. They limited their study to orbiting in the habitable zones of “G
dwarf” stars, or those like the sun.

A planet’s obliquity is its tilt relative to the orbital axis,
which controls a planet’s seasons; orbital eccentricity is the
shape, and how circular or elliptical—oval—the orbit is. With
elliptical orbits, the distance to the host star changes as the
planet comes closer to, then travels away from, its .

Deitrick, who did the work while with the UW, is now a
post-doctoral researcher at the University of Bern. His UW
co-authors are atmospheric sciences professor Cecilia Bitz,
astronomy professors Rory Barnes, Victoria Meadows and Thomas
Quinn and graduate student David Fleming, with help from
undergraduate researcher Caitlyn Wilhelm.

The Earth hosts life successfully enough as it circles the sun
at an of about 23.5 degrees, wiggling
only a very little over the millennia. But, Deitrick and
co-authors asked in their modeling, what if those wiggles were
greater on an Earthlike planet orbiting a similar star?

Previous research indicated that a more severe axial tilt, or a
tilting orbit, for a planet in a sunlike star’s habitable
zone—given the same distance from its star—would make a world
warmer. So Deitrick and team were surprised to find, through
their modeling, that the opposite reaction appears true.

“We found that planets in the habitable zone could abruptly
enter ‘snowball’ states if the eccentricity or the semi-major
axis variations—changes in the distance between a planet and
star over an orbit—were large or if the planet’s obliquity
increased beyond 35 degrees,” Deitrick said.

The new study helps sort out conflicting ideas proposed in the
past. It used a sophisticated treatment of ice sheet growth and
retreat in the planetary modeling, which is a significant
improvement over several previous studies, co-author Barnes
said.

“While past investigations found that high obliquity and
obliquity variations tended to warm planets, using this new
approach, the team finds that large obliquity variations are
more likely to freeze the planetary surface,” he said. “Only a
fraction of the time can the obliquity cycles increase
habitable planet temperatures.”

Barnes said Deitrick “has essentially shown that ice ages on
exoplanets can be much more severe than on Earth, that orbital
dynamics can be a major driver of habitability and that the
habitable zone is insufficient to characterize a planet’s
habitability.” The research also indicates, he added, “that the
Earth may be a relatively calm planet, climate-wise.”

This kind of modeling can help astronomers decide which planets
are worthy of precious telescope time, Deitrick said: “If we
have a planet that looks like it might be Earth-like, for
example, but modeling shows that its orbit and obliquity
oscillate like crazy, another planet might be better for
follow-up” with telescopes of the future.”

The main takeaway of the research, he added, is that “We
shouldn’t neglect orbital dynamics in habitability studies.”

Explore further:

Giant clue in the search for Earth 2.0

More information: Exo-Milankovitch Cycles II: Climates
of G-dwarf Planets in Dynamically Hot Systems, arXiv:1805.00283
[astro-ph.EP] arxiv.org/abs/1805.00283

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