October 4, 2010

New Exoplanet Could Be First Hospitable to Life

An artist’s rendition of Gliese 581g.
Image: Copyright Lynette Cook
The first Earth-like exoplanet (a planet outside our Solar System has been discovered orbiting in a star’s habitable zone, a range of distances around a planet in which an Earth-like planet can keep and maintain liquid water. Essentially, this could mean that the planet may be hospitable to Earth-like life. The range is often referred to as the “Goldilocks zone” in that it is neither too hot nor too cold for life.

The existence of the planet, Gliese 581g, was recently announced by a team of astronomers from the University of California, Santa Cruz and the Carnegie Institute of Washington. The discovery used date from observations having been collected for over a decade at the Keck Observatory in Hawaii.

The Planet and Consequences for Life

Gliese 581g is one of two planets that was recently discovered orbiting Gliese 58, bringing the the total known planets orbiting the star to six. The star itself is located about 20 light years away from the Earth, and some of its other planets, lying on the edge of the habitable zone, have also been debated to be capable of harboring life. However, life on these planets would likely only be possible if these other planets had certain specific conditions, such as a very thick atmosphere in one case to result in a greenhouse effect that would sufficiently warm up the planet.

On the other hand, Gliese 581g lies very comfortably in the habitable zone. The planet itself is about three to four times the mass of the Earth. This mass likely means that the planet has a definite and rocky surface, and with enough gravity to hold on to an atmosphere. If its density is close to that of the Earth’s, the planet’s radius would be 1.2 to 1.4 times the size of the Earth’s. The gravity on the surface, therefore, would be similar to or slightly higher than on Earth.

Astronomers are estimating that the distance of Gliese 581g from its star is about 0.15 AU (1 AU is the distance from the Earth to the Sun), meaning that it can orbit around its star in a little less than 37 days. In our Solar System, this orbit would be even smaller than Mercury’s, and would make the planet severely hot. However, its star, Gliese 581, is classified as a red dwarf star, making it much cooler than our star, and at this radius, the average surface temperature on the planet is estimated to be between -31 to -12 °C.

This may be surprising in that this temperature is well below freezing, and likely not conducive to life as we know it. However, astronomers have been able to deduce that the planet is also tidally locked to the star. This means that there is one side continually facing the star while the opposing side is continually facing away from the star, just like how the Moon orbits the Earth (we only see one side of the Moon from the Earth). The consequence of this is that one side of Gliese 581g is in perpetual daylight and likely with a very high surface temperature. Meanwhile, the opposing side experiences the opposite treatment, receiving permanent nighttime and a low surface temperature. Life, could like likely only exist in a band between the light and the dark sides of the planet. This has some interesting consequences.

It is possible that lifeforms that prefer warmer temperatures could develop and exist on the brighter, warmer side of the planet, while those preferring a more colder and darker environment could live more towards the darker side. Plus differing longitudes on the planet could result in a wide range of hospitable temperatures. Therefore, despite the potential existence of just a narrow band of life, there could be very diverse ecosystems existing on the planet.

Method of Discovery

The discovery of Gliese 581g comes from 11 years of observations of the star Gliese 581. The team of astronomers working on this project used the HIRES spectrometer installed on the Keck I Telescope at the Keck Observatory in Hawaii. Using this tool, the astronomers were able to make very precise measurements of Gliese 581’s radial velocity. The radial velocity is the velocity of any object, including a star like Gliese 581, in the line of sight from Earth. As a planet orbits around its star, the star pulls on the planet gravitationally to keep it in orbit. The planet also pulls on the star, and this result in the star “wobbling” a little bit. The wobble can be detected from Earth by measuring the radial velocity of a star. Astronomers on Earth usually cannot actually see an exoplanet, but can detect changes in the radial velocity of the star. Studying this wobble allows astronomers to calculate the mass and distance from the star of the planet. Looking at the wobble over time allows astronomers to actually calculate the orbit of the planet.

If there are multiple planets, like in the case of Gliese 581, this technique gets a little more complicated. There is no longer just one pull from a planet, but multiple pulls. These gravitational pulls result in a combined complex wobble of the star. Performing analyses on this complex wobble, allows astronomers to detect the planets, and also calculate their orbits and masses.

This work however, requires many observations and measurements of radial velocity, spaced out over time. Detecting this particular planet took 238 observations, each of which lasted about an entire evening on the telescope.

Additional steps are helpful to verify that the wobble is in fact caused by orbiting planets. In some cases, stars may wobble due to processes within the star itself. In this study, a separate group of astronomers working with a robotic telescope at Tennessee State University made careful and precise measurements of the brightness of the star. This step gives important evidence that the radial velocity changes are likely caused by this orbiting planet, and not something else.

What does it mean?

The planet’s position in the habitable zone itself is a great discovery. Although it does not confirm the existence of life, this planet is the most likely to harbor life out of all exoplanets that have yet been discovered. Plus, the circumstances surrounding its discovery also yield stunning realizations.

The discovery has been made relatively quickly. There are only a small number of stars that have been studied by astronomers for the existence of exoplanets. This particular exoplanet has been discovered at a time when only a few Earth-like exoplanets have been discovered. Plus this discovery is very nearby, only about 20 light years away. Both these suggest that the existence of habitable Earth-like planets is in no way rare, as some believe. In our galaxy alone, the astronomers working on the project hypothesize, there could be tens of billions of star systems containing habitable Earth-like exoplanets, which is definitely a mind-blowing realization.

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