Friday, October 8, 2010

Resembling Earth

For many planet hunters, though, the ultimate goal is still greater (or actually, smaller) prey: terrestrial planets, like Earth, circling a star like the Sun. Astronomers already know that three such planets orbit at least one pulsar. But planet hunters will not rest until they are in sight of a small blue world, warm and wet, in whose azure skies and upon whose wind-whipped oceans shines a bright yellow star like our own.
- Ken Croswell

Located 20 light-years or 190 trillion kilometers away is a humdrum red dwarf star called Gliese 581. As of October 2010, the star Gliese 581 has a total of six known planets in orbit around it and one of which is the most Earth-like planet discovered so far. This planet is designated Gliese 581 g and it is the fourth planet from its parent star. Gliese 581 g orbits its parents star at a distance of 22 million kilometers, taking 36.6 days to complete one orbit.

The orbit of the planet Gliese 581 g is located well within the habitable zone where the distance from its parent star is just right to support Earth-like surface temperatures. Thus, Gliese 581 g is located neither too close nor too far from its parent star. Since the star Gliese 581 is much less luminous that our Sun, the planet Gliese 581 g is able to support Earth-like surface temperatures even though it is located much closer to its parent star than our Earth is from the Sun.

In addition to orbiting its parent star within the “Goldilocks Zone”, the mass of Gliese 581 g is estimated to be between 3.1 to 4.3 times the mass of the Earth. If Gliese 581 g is a dense rocky planet like the Earth, its diameter will be somewhere between 1.3 to 1.5 times of the Earth’s diameter. The surface gravity of Gliese 581 g is also expected to be between 1.1 to 1.7 times the surface gravity of the Earth, making it not too different from the Earth. In fact, it will not be much of a problem for a human being to walk on the surface of Gliese 581 g.

With an Earth-like greenhouse effect, the average surface temperature of Gliese 581 g is estimated to be between 236 to 261 degrees Kelvin. In comparison, the average surface temperature of the Earth is 288 degrees Kelvin or 15 degrees Centigrade. However, because Gliese 581 g is more massive than the Earth, it is possible that the planet will have a more massive atmosphere which can create a larger greenhouse effect than Earth’s atmosphere. This can increase the average surface temperature of Gliese 581 g closer to the average surface temperature of the Earth.

One key difference between Gliese 581 g and the Earth is that Gliese 581 g is probably tidally locked whereby the same hemisphere of the planet perpetually faces its parent star. This is somewhat like the Earth-Moon system where the same side of the Moon always faces the Earth. In such a scenario, one side of Gliese 581 g will be in eternal daylight while the other side will experience eternal night. On such a world, temperatures can range from blazing hot at the sub stellar point on the day side to freezing cold on the night side. The sub stellar point on the surface of Gliese 581 g is where its parent star is forever directly overhead and it is the spot with maximum insolation. Between the two extremes, Earth-like temperatures can exist where a world that is not too different from ours is both easily conceivable and highly probable.

The mere fact that a potentially habitable planet has been discovered so soon around such a nearby star, suggests that habitable planets are far more common than previously believed. This means that potential of having many billions of Earth-like planets in our Milky Way galaxy alone is extremely probable. The paper detailing this discovery is by Steven S. Vogt, at al. (2010) and it is entitled “The Lick-Carnegie Exoplanet Survey: A 3.1 M Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581”.

We live in a changing universe, and few things are changing faster than our conception of it.
- Timothy Ferris

Of the 500 or so known extrasolar planets, Gliese 581 g is probably the most interesting one discovered so far. Apart from the remarkable discovery of Gliese 581 g, a paper by Daniel Kubas, at al. (2010) entitled “A frozen super-Earth orbiting a star at the bottom of the Main Sequence” describes the discovery of a super-Earth which orbits a faint red dwarf star whose mass is close to the lower limit for a star.

This planet is designated MOA-2007-BLG-192Lb and its mass is 3.2 times the mass of the Earth. The planet orbits its parent star at a distance of about 100 million kilometers and this is about two-thirds the distance of the Earth from the Sun. However, because the parent star of MOA-2007-BLG-192Lb is a mere 8.4 percent the mass of the Sun, the planet receives over a thousand times less insolation that the Earth gets from the Sun even though it is located closer to its parent star than the Earth is from the Sun.

MOA-2007-BLG-192Lb was discovered using the gravitational microlensing technique as it provides a unique opportunity for the detection of low mass planets that are currently beyond the reach of most other methods. Gravitational microlensing occurs when a foreground star passes in front of a background star and the gravity of the foreground star acts as a lens and magnifies the apparent brightness of the background star. If the foreground star has a planet orbiting it, the gravity of the planet can induce a perturbation to the microlensing light curve. The duration of the perturbation depends on the mass of the planet, where a more massive planet will induce a perturbation with a longer duration. The discovery of the MOA-2007-BLG-192Lb shows that planet formation can occur down to the very low mass end of the stellar population.

The surface temperature of MOA-2007-BLG-192Lb is estimated to be around 55 degrees Kelvin and this is just below the melting temperature of pure nitrogen. However, internal heat generated from the decay of radioisotopes can raise the temperature on the surface of MOA-2007-BLG-192Lb to beyond the melting temperature of pure nitrogen. This can enable seas and oceans of liquid nitrogen to exist on the planet’s surface as long as the atmospheric pressure on the planet’s surface exceeds 0.1 bars.

The flow of internal heat onto the surface of a terrestrial planet can be strongly heterogeneous, making it highly probably that the surface temperatures on specific locations on MOA-2007-BLG-192Lb can exceed not just the melting point of nitrogen, but also the melting point of methane and even water. Therefore, lakes of liquid hydrocarbons like those on Saturn’s moon Titan can exist on MOA-2007-BLG-192Lb and open bodies of liquid water may even exist in volcanically active locales.