Can Planets In Roller Coaster Orbits Be Habitable?

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The great exoplanet gold rush is on to find habitable Earth-sized planets. By definition these are planets in the “Goldilocks” zone around sun-like stars where temperatures are just right for liquid water to exist on the surface.

But how do you assess the habitability of a planet that spends only part of its time in a Goldilocks zone because its orbit is so elliptical? Like a roller coaster it speeds up as it falls toward the lowest point of its orbit and then slows down as it climbs to its farthest point from its parent star.

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A team of astronomers announced this is the case for the planet 55 Cancri f, the fourth planet in the five-planet 55 Cancri system.

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Kaspar von Braun of Caltech reports on new observations that show that at its farthest point from the star the planet is 1.3 times farther out than Earth is from the sun. At its closest point, the planet is about one-half the distance between Earth and the sun. This means that the Neptune-mass world is in the habitable zone for 192 days out of its 260-day orbit.

The measurements were made with the Center for High Angular Resolution Astronomy (CHARA) array of telescopes operated by of the Georgia State University. Using optical interferometrty to combine the light from a series of small 1-meter telescopes the CHARA is the world’s sharpest telescope at near-infrared wavelengths.

This so-called “super-Earth” could simply be a big ball of water without continents or volcanism.

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A Real Pandora?

But it’s conceivable the planet could have a large moon, like Saturn’s giant moon Titan. Such a moon could possess an atmosphere and bodies of water. It would be something like the blue-skin alien inhabited moon Pandora from the 2009 sci-fi blockbuster “Avatar.”

Computer simulations show that moons can form within disks of dust and gas produced during the final stages of a large planet’s growth. These disks could form a string of roughly equal-sized moons like the four Galilean satellites around Jupiter, or one giant world like Titan, which retains a dense atmosphere.

The consequences for such a moon orbiting 55 Cancri f would be short sweltering summers and longer winters; average surface temperatures would rise to 302 Kelvin (29C/84F) during the summer, and plummet to 221 Kelvin (-52C/-62F) during the winter.

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These temperatures are based on simple modeling and ignore the fact the moon might have a thick atmosphere containing the greenhouse gas carbon dioxide that would moderate temperatures.

Assuming the adaptability of life on Earth is the norm for biological evolution, then it’s not hard to imagine life evolving on a world where extreme seasons are driven by orbital extremes.

Anybody living on the moon would note three brilliant morning or evening stars that are in very close proximity to 55 Cancri’s golden yellow disk. These planets orbit far inside 55 Cancri f’s orbit. The closest, having the mass of Uranus, completes an orbit in just 3 days. The second, the size of Jupiter, orbits in about 15 days. The third, a Saturn-mass world, orbits just every 44 days. For some of the year the night sky would be dominated by the glow of a companion red dwarf star nearly 100 billion miles farther out.

55 Cancri is such a fascinating system I have no doubt it will be scrutinized with ever-larger space telescopes of the future, and the eventual target for an interstellar probe. The system is older that the sun, and so there would be time for life to evolve to an advanced stage.

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Because the star is only 41 light-years away it’s close enough that any technological civilization living there could be receiving our radar, radio and television signals that have leaked off Earth.

Right now — in the unlikely chance that signals could be decoded — the aliens would be watching our televised 1971 Apollo 15 moon landing, which would just be arriving.

Image credit: NASA, K. von Braun

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