Exoplanets where life as we know it can exist will likely be of the terrestrial type. A terrestrial planet is a planet that is primarily composed of silicate rocks. By September 2010, NASA counted 16 terrestrial planets having been discovered.

Of the 16, 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 305 gas giants, with 113 of those defined as hot Jupiters orbiting extremely close to their parent star.

The overwhelming number of known exoplanets being gas giants relates to the detection capability of our instrument technology. Up till now these instruments are most able to detect a large planet that noticeably perturbs its parent star.

This gas giant bias is about to change. According to Harvard astronomer Dimitar Sasselov with the Keppler project, more than 1100 candidate planets have been discovered and await orbital confirmation. As shown in the graph on the left, the largest category comprises candidates "like Earth" in size, with radii less than two times Earth's radius (<2Re).

Even planets substantially larger than earth within the habitable zone could harber life. In fact, according to one article, some scientists theorize such super earths - from about 2 to 10 Earth masses - may be superior at fostering life.

Habitable Zone

Habitable exoplanets are more than just terrestral of type. They 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.

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

The graphic shows two 'habitable' examples outside our Solar System; Gliese 581 g 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 for this hypothetical example will be noticeably truncated.

In the year 2010, only one exoplanet discovery appears clearly conducive to life. This is the Gliese 581 g mentioned above and discussed further down this web page. Awaited with great expectation is the discovery of a planet resembling the image at the top of this page. Here is a video discussing the formation of habitable planets.

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.

Gliese 581 g

An artist's conception (Credit: Lynette Cook via NSF) shows the inner four planets of the Gliese 581 system and their host star, a red dwarf star only 20 light-years away from Earth. The large planet portrayed in the foreground is the newly discovered Gliese 581g.

In 2010 another planet was confirmed in orbit around Gliese 581. Astronomers participating in the Lick-Carnegie Exoplanet Survey detected the planet by tracking the faint gravitational wobbles it produced in its parent star. A video of the scintists discussing their discovery may be found here.

[The discovery team found the planet after searching through many years of data from two telescopes. However, another group of scientists looking for the planet could not confirm its existence. So it is still not certain that this planet exists.]

The newfound planet, known as Gliese 581 g, is estimated to be 3.1 to 4.3 times as massive as Earth, and makes a complete circuit around its sun in just under 37 days. Assuming a rocky composition like Earth's, it would be 1.2 to 1.4 times as wide as our own planet, qualifying it as a "super-Earth."

The red dwarf star's dimness and the planet's orbital distance (0.146 AU, less than half the distance between Mercury and our sun) suggests that the planet's average surface temperature is just below water's freezing point, somewhere between -31 and -12 degrees Celsius (-24 and 10 degrees Fahrenheit).

Gliese 581 g appears to be tidally locked to its star, with one side perpetually in the sun and the other side perpetually dark. That means the highs on the day side would be hellishly hot. The lows on the night side would be unendurably cold. But there would be a livable zone along the line between shadow and light.

The astronomers said, "Any emerging life forms would have a wide range of stable climates to choose from and to evolve around, depending on their longitude." Gliese 581g is in a planetary zone that is, "not too hot and not too cold, but just right" for water to exist somewhere in liquid form.

Gliese 581 g is the sixth planet to be detected around the parent star. Two other planets in the system are on the edges of the Goldilocks zone: Gliese 581 c (potentially "too hot") and Gliese 581 d (potentially "too cold"). Gliese 581 g is in the middle.