To the untrained eye, Gliese 581 c is an inconspicuous body circling an inconspicuous star. The star, Gliese 581, is 20.5 light years away from Earth. It has an orbital eccentricity of 0.16 (the parameter of the conic section of the orbit), an orbital period of 12.932 sidereal days (that is, it takes roughly 13 days to complete a revolution about its star), and a mass of 0.0158 J (Jupiter masses). This is potentially interesting data. But without any means for comparison, it is difficult to know why.
Roughly speaking, a body circling a star is called a planet. Pluto was excluded as a planet because it did not “dominate” its surroundings or obtain its round shape due to the force of its own gravity. Before 1995, the thought of finding a planet in our universe outside of our solar system was a pipe dream. Thus, Gliese 581 c’s categorization as an “exoplanet” is already cause for celebration.
But there is more to the story. To interpret this data, one must examine similar values for Earth, which has an orbital eccentricity of 0.017, an orbital period of 365.26 sidereal days, and a mass of 0.00315 J. One could say this approximate similarity to Earth is interesting. But calling Gliese 581 c an “interesting” planet could be the astronomical understatement of the century.
Exoplanets (short for extrasolar planets) are planets outside of our solar system. Exoplanets have been the subject of intense scientific investigation since the 1990s, when several of them were flagged by astronomers as being potential bearers of extraterrestrial life.
The first exoplanet discovered was 51 Pegasi b, found in 1995 by a Swiss team led by Didier Queloz. In 2001, the discovery was authenticated by researchers Jeff Marcy and Paul Butler working with the Keck twin telescopes in Hawaii.
The discovery of 51 Pegasi b was exciting in part because it was thought to be a terrestrial planet. Terrestrial planets are comprised of silicate rock surrounding a central, metallic core. The Earth is a terrestrial planet, and any planet that exhibits Earth-like characteristics is immediately attractive as a life source. But further inspection revealed that 51 Pegasi b is most likely a gaseous giant, like Jupiter.
Finding planets is challenging, but finding Earth-like analogs is nearly impossible. The Earth is positioned in a myriad of peculiar and subtle angles that make it improbably well-suited to life-bearing properties, like photosynthesis. If Earth was thrown slightly off its axis or nudged slightly closer to the sun, the life-giving environment of Earth would be thrown into colossal disarray.
Earth thus resides in a near-perfect habitable zone (HZ). The habitable zone of a solar system is the area of space surrounding a star where conditions are maximally suitable to sustain life. These conditions include the planet’s temperature, density, pressure, radiation content, matter content, and distance from the star it orbits.
In the April 2008 issue of Scientific American, Nancy Kang wrote about using photosynthesis as an additional marker to characterize extrasolar planets. This is fairly easily accomplished by examining the spectrum of the star’s light.
Depending on the planet’s distance from a star, plants on the surface of exoplanets could appear blue, red, or black. How, then, do astronomers go about finding another planet that is as perfect as Earth? They hunt. Since the discovery of 51 Pegasi b, over 270 exoplanets have been discovered.
The exoplanet HD 209458b was discovered in 1999 orbiting the star HD 209458. Like 51 Pegasi b, HD 209458b is another extrasolar planet in the constellation Pegasus. The discovery of this planet marked several experimental milestones. Most notably, it was the first extrasolar planet that contained an atmosphere (carbon, hydrogen, and oxygen). The atmosphere also contained water vapor, another first for an extrasolar planet.
The exoplanet SWEEPS-10 has, to date, the fastest orbital period in the universe, zipping through one day in 10 hours. Another exoplanet, Upsilon Andromeda b, perpetually faces its star, incinerating one side of the planet while plunging the other side into frigid temperatures.
The planet HD 209458b is so close to its star that its atmosphere is being blasted away at the rate of 10,000 tons of material every second. Gliese 581 c is the first exoplanet found to lie within the HZ of its star. The planet is terrestrial, and large; Gliese 581 c is five times more massive than Earth. Scientists have further speculated that Gliese 581 c is either entirely terrestrial or covered with oceans.
With exoplanets being discovered at a now-rapid rate, it is a sprint to keep up to date with the wildly fascinating and potentially life-bearing, exoplanets that are coming into view.
In about a decade, the European Space Agency will launch Darwin, a planet-finding fleet of probes armed with three telescopes that will scope out, image, and analyze thousands of small, rocky Earth-sized exoplanets.
NASA’s Terrestrial Planet Finder will also use high-sensitivity telescopes to study the development, composition, and size of similar Earth-like exoplanets. When these projects take flight, our universe will come to life like never before by revealing that Earth, in all its splendor, may have some relatives.