Habitable Planets

Exoplanets where life as we know it can exist will likely be of the terrestrial type. Habitable exoplanets will orbit within a star's habitable zone, the region of space around a star where a planet would receive roughly the same energy as the Earth.

According to NASA, at the end of the year 2007 four planets defined as terrestrial have been discovered. Of the four, at most two are within a star's habitable zone. Both of these planets orbit the dwarf star Gliese 581.

In contrast, scientists have discovered 250 gas giants, with 57 of those defined as hot Jupiters orbiting extremely close to their parent star.

That the overwhelming number of known exoplanets are gas giants relates to the detection capability of our current instrument technology. These instruments are most able to detect a large planet that noticeably perturbs its parent star.

The graphic to the left illustrates the habitable zone and possible habitable planetary bodies orbiting different star types.

Outside our Solar System, two examples are given, Gliese 581c orbiting a red dwarf star and discussed below and a hypothetical gas giant orbiting an orange subgiant star. In the latter case, habitability would be limited to an unknown moon of the terrestrial type. Given the short life span of giant stars, the time of habitability will be noticeably truncated.

The graphic shows how both the distance of the habitable zone from the star and its width are directly dependent on the star's mass.

Exoplanet discoveries to date do not include habitability as a common feature. Future discoveries achieved with more sophisticated instruments may tell a different story. Awaited with great expectation is the discovery of a planet resembling the first image above.

Gliese 581

Gliese 581 is a red dwarf star located 20.4 light years from Earth. Three planets (5, 7.7 and 17 times the mass of earth) have been detected in orbit around this star. All are of the terrestrial type.

This red dwarf star has a total energy output (bolometric luminosity) 1.3% of our Sun. The habitable zone would have to be extremely closer to this star in order to receive an amount of energy comparable to the Earth. One, and perhaps two, of the discovered exoplanets are orbiting within this zone.

Gliese 581 c

Gliese 581 c is the smallest planet found up to now (5 times the mass of Earth). Gliese 581 c orbits the Red Dwarf star at a considerably closer distance (seven million miles) than Earth orbits our own Sun (ninety-three million miles). The Red Dwarf would appear some twenty times larger than our moon in the sky.

Gliese 581 c appears to measure some 12,000 miles across, compared to Earth at 8,000 miles. Gravity would probably be 1.6 times as strong as Earth's, so a 150-pound object on Earth would weigh 240 pounds. Gliese 581 c completes a full orbit of the star in 13 days, making for a very short year. Water would be liquid, with a mean temperature of between 0 and 40 degrees Celsius.

Tidal or Gravitational Lock

This phenomenon, also known as captured rotation, involves the tying of the orbital period of a planet or moon to its axial period through a tidal effect caused by the gravitational pull of the primary. The tidal lock experienced by the Moon, for example, explains why it always keeps the same face directed toward the Earth. Likewise, the five inner satellites of Jupiter complete one orbit for every axial rotation. The opportunity for life to evolve on planets around red dwarf stars may be compromised by this phenomenon. So small is the habitable zone around a red dwarf that any planets within this zone may be forced into a 1:1 lock thus preventing the day-night cycle and climatic variations that are conducive to biological development as we know it on Earth.

But hold the enthusiasm, things may not be so rosy for this super Earth. At its distance from the parent star, it is almost certainly subject to a tidal lock, with one side always facing the star. The other side would be facing perpetual darkness and cold, warmed only by winds that reach it from the lit side.

A second bit of bad news is that Gliese 581 c, due to its strong gravity and proximity to the hotter edge of the habitable zone, will almost certainly have a runaway greenhouse effect, and would be too hot on the sunlit side to be habitable.

Gliese 581 d

Gliese 581 d, although larger (7.7 earth's mass), is near the outer edge of the habitable zone. Scientists surmise that conditions on the planet may be conducive to supporting life. Scientists originally believed that Gliese 581 d would be too cold for liquid water to exist, and therefore could not support life as we understand it. However, due to a theorized greenhouse effect, research now suggests that atmospheric conditions on the planet create temperatures at which water can exist, and therefore the planet may be capable of supporting life. However, at 7.7 Earth's mass, humans would not wish to live there.

The only hope in this dwarf star solar system would be an extremely unlikely moon in orbit around Gliese 581 d. Such a moon, if near earth size, could be habitable.