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Extrasolar Planets with Earth-like Orbits

Leslie Mullen, NASA Astrobiology Institute, August 8, 2001

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Summary: Scientists are discovering extrasolar planets with Earth-like orbits. Could it be possible for Earth-like orbiting planets to habor life just like Earth?

Scientists are discovering extrasolar planets with Earth-like orbits. Could it be possible for Earth-like orbiting planets to habor life just like Earth?

Most of the planets discovered outside our solar system don’t have orbits like Earth’s. Either the planets are closer to their stars, with orbital periods of only a few days, or they have highly elliptical orbits – some of which better resemble the paths of comets. Recently, however, a team of astronomers from the Geneva Observatory in Switzerland announced they had discovered a planet with an orbital path very similar to Earth’s.

Dubbed HD 28185 b, this planet has a nearly circular orbit and is about the same distance away from its star as the Earth is from the Sun. HD 28185 b is 150.6 million kilometers from its star; the Earth is 149.6 million km from the Sun – a difference of only 1 million kilometers. HD 28185 b takes 385 days to orbit its star, twenty days longer than the orbital period of Earth.

An Earth-like orbit would tend to give a planet relatively stable temperatures. This would increase the chances that any water on the planet would be able to remain in liquid form. Finding planets with liquid water is one of the key goals of astrobiology, because water is believed to be essential for life.

Because of this very Earth-like orbit, is it possible that HD 28185 b could harbor life? At present, there’s no way to tell for sure. There is no evidence of water existing on HD28185 b. In fact, the planet is massive enough to be a gas giant resembling Jupiter, and if water was present on the planet it would most likely not resemble the bodies of water found on Earth.

The extrasolar planet does share another life-friendly condition with Earth: Just as the orbit of HD 28185 b is similar to the Earth’s, the star it orbits is very similar to our Sun. Like our Sun, the star HD 28185 is a G-class main sequence yellow star. The "G" refers to the temperature of the star (other temperature classes are O,B,A,F,K and M). A "main sequence" star is a star in the middle of its life cycle. The Sun and HD 28185 are not exactly the same temperature however – HD 28185 is a G5 star, whereas our Sun is a G2 (which means our Sun is hotter).

Only one other extrasolar planet that we know of - iota Hor b, discovered in 1999 – has orbital conditions similar to the Earth’s. That planet has an orbital period of 320 days, and orbits about 145 million kilometers away from its star, a G5 yellow star named iota Horologii.

Like HD 28185 b, iota Hor b is probably a gas giant planet similar to Jupiter. Iota Hor b has a mass of at least 2.26 times that of Jupiter, or 720 times the mass of Earth. HD28185 b has a minimum mass of 3.5 times that of Jupiter, or about 1000 times that of the Earth. Many scientists agree that life is unlikely to appear on giant gaseous planets.

"The very large mass of this planet would almost certainly preclude the development of multicellular life," says Peter Ward, planetary geologist with the University of Washington and member of the NASA Astrobiology Institute. "While there might be some possibility of microbial life, even that seems a bit unlikely, because of the high pressures."

For life as we know it to survive in these distant solar systems, extrasolar worlds would need to be terrestrial rather than gaseous, says Ward.

"The planet should be stony like ours," says Ward. He also says an extrasolar planet able to sustain complex life should, "have water, and have the phenomena known as plate tectonics - which on Earth acts as our planetary thermostat."

Such conditions could potentially be met on any moons that may orbit these extrasolar planets. Moons tend to be rocky, and water and tectonics on moons are possible. It is thought that Jupiter’s moon Europa, for instance, may have a liquid water ocean and possibly even some sort of tectonic activity.

Extrasolar moons of HD 28185 b and iota Hor b, if they exist, would have the additional advantage of getting enough solar radiation to support Earth-like temperatures. This would help keep water liquid, although it may not be necessary for life to appear.

According to Chris Chyba, director of the Center for the Study of Life in the Universe at the SETI Institute, while it is important to have temperatures above the freezing point of water at least some of the time, stable temperatures are not needed for life to gain a foothold.

"I don’t see any reason to think that widely varying temperatures are a problem for the origin of life, and they may be an advantage," says Chyba. "Freeze-thaw cycles might help provide higher concentrations of prebiotic organic molecules needed for the origin of life."

However, says Chyba, "it is difficult to know to what extent wide temperature fluctuations would inhibit multi-cellular life from flourishing."

So, although any moons orbiting HD 28185 b in the Earth-zone would not necessarily have an edge in the emergence of life, their position would help keep water liquid and would seem to be more amenable to sustaining any complex multicellular life that might evolve there.

But according to Ward, extrasolar moons have other qualities that would make sustaining complex life a tricky proposition.

"The problem is that any such moon would probably be tidally locked, where the same side is always facing the larger planet," says Ward. This would mean that some parts of the moon would always be exposed to sunlight, while other parts would be perpetually cold and dark.

But Chyba disagrees. "I see no reason whatsoever why it would matter to the prospects of either simple or complex life on a Moon orbiting a giant planet whether or not that Moon were tidally locked," he says.

Ward says another limitation is that "large Jupiters tend to get bombarded more than smaller planets. Because of this, any moons would likely be subject to repeated bombardment, which is not good for complex life."

But Chyba says that moons orbiting Jupiter-like planets are not necessarily more prone to get hit by asteroids and comets.

"The impact rate for kilometer-sized bodies to strike Europa is perhaps once every couple of million years — this is a bit less than the impact frequency of kilometer-sized objects hitting Earth," says Chyba.

The discoverers of HD 28185 b are Michel Mayor, Dominique Naef, Francesco Pepe, Didier Queloz, Nuno C. Santos, Stephane Udry, and Michel Burnet. The astronomers used the radial velocity technique to find the planet. This technique looks for a wobble in a star caused by the gravity of the orbiting planet. The wobble shows up as a periodic change in the spectrum of the star’s light. This change can be measured by high-resolution spectrographs mounted on telescopes.

The scientists measured the shift of HD 28185 using the CORALIE spectrometer on the Swiss 47-inch (1.2-meter) Leonard Euler telescope at the European Space Observatory's La Silla Observatory. Instruments on telescopes at the Haute-Provence Observatory in France and on the twin Keck telescopes on Mauna Kea, Hawaii were also used to verify the findings.

A puzzle astronomers need to solve is how these two gas giant planets ended up in such Earth-like orbits. Many of the extrasolar planets with circular orbits found thus far have been extremely close to their stars, with orbits of only a few days. The current model for these short-orbit "hot Jupiters" is that they formed in the colder regions of their solar system and then migrated in, because the regions where they are now located are too warm for gas giant planet formation. Perhaps the two Jupiter-mass planets in Earth-like orbits will help scientists better understand the evolution of such extrasolar planets.

What’s Next
A fundamental limitation of the radial velocity method is that it doesn’t allow astronomers to determine the inclination of the planetary orbit relative to Earth. As a result, only lower limits can be set for the masses of planets found using this technique. The astronomers at the Geneva Observatory plan to use new interferometer systems on telescopes to better determine the masses of the extrasolar planets.

According to Laurance Doyle of the SETI Institute, it may also be possible to read the spectrum of light reflecting off HD 28185 b. This could tell us whether or not the planet’s atmosphere is favorable to any forms of life.

"The reflected light spectral lines from the planet will be Doppler-shifted away from those of the star," says Doyle. Thus, they will be visible as a distinct signature.

Doyle also says that although detecting any moons orbiting HD 28185 b would be difficult, it could be done indirectly. The moons would have the effect of causing an apparent delay in the orbit of the planet.

"I've calculated that the moon Callisto by itself would delay Jupiter's transit by about 8 seconds, so perhaps this also is a possible way to detect moons around giant [extrasolar] planets," says Doyle.