Wild planetary orbits could affect habitability

New findings suggest that the ability of life to thrive on an alien world might depend on the wild orbits of its planetary neighbors.

Courtesy of European Organisation for Astronomical Research in the Southern Hemisphere
The Gliese 581 planetary system, a low-mass-exoplanet recently discovered. New findings indicate that the habitability of some planets outside of our solar system could vary based on the orbits of their giant planetary neighbors.

The ability of life to thrive on alien worlds may be impacted by the wild and weird orbits of giant neighboring planets, new studies suggest.

The heftier worlds in other planetary systems could exert large forces on smaller worlds, pushing and pulling them into changing orbits. In some cases, these weird orbits could cause some extrasolar planets to fluctuate between being habitable and being inhospitable to life.

These changing conditions of habitability could impact the search for life on other worlds and astronomers' theories on the formation of planetary systems like our own.


"There is this crazy zoo of planets out there that probably are habitable," research team member Rory Barnes of the University of Washington said, "but their properties are very different from Earth and they're different from Earth because of their eccentric neighbors."

The results of the two studies that looked at these wild orbits were presented here today at the 216th meeting of the American Astronomical Society. [The Strangest Alien Planets.]

Weird orbits

When astronomers hunt for exoplanets, they look for signs of rocky, Earth-like planets in zones where conditions, such as temperature and liquid water, remain stable enough to potentially support life.

But, new findings from computer modeling indicate that the habitability of some exoplanets could vary, based on the orbits of giant planetary neighbors.

The discovery of these so-called weird orbits will have important implications for existing theories of how multi-planet systems evolve. The findings also show that some violent events can happen to disrupt planets' orbits after a planetary system forms.

"The findings mean that future studies of exoplanetary systems will be more complicated," said Barbara McArthur of The University of Texas at Austin, who was the lead researcher for one of the studies. "Astronomers can no longer assume all planets orbit their parent star in a single plane."

McArthur and her team at the McDonald Observatory in Austin, Texas, discovered a planetary system in which the orbits of two planets are at a steep angle to each other.

Tilted planets

The researchers used data from the Hubble Space Telescope, the Hobby-Eberly Telescope, and other ground-based telescopes combined with extensive modeling to unearth a slew of information about the planetary system surrounding the nearby star Upsilon Andromedae.

Upsilon Andromedae, a yellow-white dwarf star, is located about 44 light-years away, and is similar to our sun. It is, however, a bit younger, slightly more massive, and shines a bit brighter than the sun.

For over a decade, astronomers have known that three Jupiter-type planets orbit Upsilon Andromedae. McArthur's team was able to measure the exact mass of two of the three known planets. But, the surprising result came when the researchers observed that the two planets are inclined and orbit at a steep angle to one another. The team's results will also be detailed in the June 1 edition of the Astrophysical Journal.

"Most probably Upsilon Andromedae had the same formation process as our own solar system, although there could have been differences in the late formation that seeded this divergent evolution," McArthur said. "The premise of planetary evolution so far has been that planetary systems form in the disk and remain relatively co-planar, like our own system, but now we have measured a significant angle between these planets that indicates this isn't always the case."

Traditionally, it was understood that a big cloud of gas collapses down to form a star, and planets are a natural byproduct. Leftover material forms a disk. In our solar system, there is a natural fossil of that creation event, because all of the eight major planets orbit in nearly the same plane.

Several different gravitational scenarios could potentially be responsible for the unexpected inclined orbits of planets in Upsilon Andromedae.

"Possibilities include interactions occurring from the inward migration of planets, the ejection of other planets from the system through planet-planet scattering, or disruption from the parent star's binary companion star, Upsilon Andromedae," McArthur said.

"Our dynamical analysis shows that the inclined orbits probably resulted from the ejection of an original member of the planetary system," Barnes said. "However, we don't know if the distant stellar companion forced that ejection, or if the planetary system itself formed such that some original planets were ejected."

He added that the revised configuration still lies on the precipice of stability, because "the planets pull on each other so strongly that they are almost able to throw each other out of the system."

Habitable cycles

Barnes' own research emphasizes the important role that planetary systems architecture has on the potential habitability of exoplanets.

A lone terrestrial, or Earth-like, planet with a mostly circular orbit toward the inner edge of its sun's habitable zone would be expected to maintain that orbit and remain within that zone, said Barnes.

But, adding a planet to the system that is comparable in size to Jupiter, and giving it a highly elliptical orbit – similar to most exoplanets discovered so far – could cause strange things to happen to the smaller planet, possibly causing it to cycle between habitable and uninhabitable conditions.

In such cases, the smaller planet's orbit will elongate, taking it further beyond the sun's habitable zone, before then trying to readjust to become more circular again. These events occur all within a time period of only 1,000 years, and could even happen repeatedly.

This could mean that an exoplanet's average yearly temperature could change significantly during each millennium, altering conditions on the alien planet's surface in the process.

"For part of the time liquid water could exist on the surface, but at others it would boil off," said Barnes, who also presented his findings at today's meeting.

The effect would be similar for an Earth-like planet at the outer edge of its habitable zone, except that the altered orbit would likely take it too far from its sun, resulting in planetary glaciation for part of its orbital period.

"The bigger issue here is that the habitable zone is very complicated," Barnes said. "Earth's climate is affected slightly over tens of thousands of years by the orbits of other planets in the solar system, but it is possible that in many exoplanetary systems the layout of the planets is very important to habitability."

A planet's habitable zone becomes even more complex for exoplanets that orbit low-mass stars, such as stars that are one-third the mass of the sun. In such systems, the habitable zone is much closer to the smaller star, and any forces from the star's gravity have critical effects in determining whether the planet is habitable.

The addition of a Jupiter-like planet with an eccentric orbit could change the orbit of the smaller planet, thereby also altering the conditions on the alien planet.

"There could be planets out there that have their geological properties change over very long timescales," Barnes said. "You can imagine planets that cycle in and out of intense volcanism and earthquake stages."

Forces within a planetary system also fix the planet's rotation period. If a planet's orbit becomes more elongated, the length of that planet's day could change also change significantly.

"The length of the day changes almost day to day," Barnes said. "It's fascinating to think about how evolution occurs on such a world."


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