This artist's conception shows the inner four planets of the Gliese 581 system and their host star, a red dwarf only 20 light-years from Earth. The large planet in the foreground is Gliese 581g, whose discovery was announced in September 2010. The planet is in the middle of the star's habitable zone and is only three to four times as massive as Earth.
Credit: Lynette Cook
A newfound Earth-sized planet discovered in the habitable zone of a nearby star looks very promising for the possibility of extraterrestrial life, but many unknowns remain.
Here is SPACE.com's look at what scientists know so far about the intriguing world, as well as a few questions that don't quite have answers yet. Consider it a new entry into Earth's own hitchhiker's guide to the galaxy:
How do I say the planet?s name?
Gliese 581g may look like it should rhyme with "Grease," but it is actually pronounced as two-syllables as (Glee-zuh). The name comes from the German astronomer Wilhelm Gliese, who catalogued the planet?s parent star Gleise 581 as part of a star survey first published in 1957.
Where is Gliese 581g?
The planet Gliese 581g orbits the red dwarf star Gliese 581, which sits 20 light-years from Earth in the constellation Libra. One light-year is about 6 trillion miles (10 trillion km).
How far is it from the parent star?
Early estimates suggest Gliese 581g is 0.15 astronomical units from its star. One astronomical unit is the average distance between the Earth and sun, which is approximately 93 million miles (150 million km). That distance means the planet is close enough to its star so that it can complete an orbit in a little less than 37 days.
One of its sibling planets was closer to the hot edge of the habitable zone around the star Gliese 581, and one was farther out the colder edge of the habitable zone. Gliese 581g is located just right between the two. [Graphic: Gliese 581 solar system orbits]
What is a habitable zone?
Think of a star's habitable zone as the swath of space surrounding a star where conditions for life as we know it are possible. Closer in, a planet roasts. Farther out, it freezes.
Planets within that habitable zone, also known as the Goldilocks zone, have a range of surface temperatures that allow for readily available liquid water and other conditions that may support the rise of life. This cosmic sweet spot can vary, because it depends upon the type of star and the point in time for any given star's lifespan.
For instance, our sun's current habitable zone is farther out than that of the star Gliese 581, a red dwarf about 50 times dimmer than our sun.
The cooler red dwarf allows the Gliese 581 planets to orbit much closer and still remain in the habitable zone.
A planet within the habitable zone does not have a guaranteed chance of originating life, because biology also depends upon the planet's size and a host of conditions, including chemical makeup. But what little researchers know about Gliese 581g makes it a highly promising candidate.
How big is Gliese 581g in relation to Earth?
The planet is lumped into the "nearly Earth-sized" category. It is between three and four times the mass of our Earth ? bigger, but small enough to be rocky rather than gaseous. Its radius is anywhere between 1.3 and two times the size of Earth.
How much would I weigh on Gliese 581g?
An Earth-sized planet with three times the mass of our planet would pull down on your body with three times the force of Earth's standard gravity. That means if you weighed 120 pounds on Earth, you would weigh about 120 x 3 pounds on an Earth-sized planet with three times the mass, or 360 pounds.
But Gliese 581g also has a somewhat larger radius, so that also factors into the equation. A 120 pound person would weigh about 213 pounds on Gliese 581g at the lower end of the size and mass estimates. This all remains theoretical until astronomers can pin down the actual size and mass.
What's it like on the surface?
There is no solid evidence at the moment that suggests what surface conditions might be like, or even if liquid water and an atmosphere are actually present.
What researchers know is that the planet exists at the right distance from its star to have liquid water. It's also at the right distance to have an atmosphere which can protect that water, if exists on the surface.
But one of the planet's discoverers, astronomer Steven Vogt of the University of California, Santa Cruz, pointed out that "it's pretty hard to imagine that water wouldn't be there."
He likened it to the examples of the Earth, its moon, Mars, and the moons of Jupiter and Saturn. He also noted that the Orion Nebula is making enough water every 24 seconds to fill all the oceans of the Earth.
Researchers also know that the planet is tidally locked to its star. That means one side experiences eternal daylight, and the other side experiences unending darkness. Such a locked configuration helps to stabilize the planet's surface climate, Vogt said.
3-D global circulation models have shown that the temperature differences on the day and night sides of the planet would not be enough for water to either freeze or boil off. They also suggest that the atmospheric circulation and wind patterns would be relatively benign.
Does it have moons?
There's one called Pandora ? just kidding! There's no info on any moons around Gliese 581g, or around any other planets in its solar system yet. But astronomers have long assumed that alien planets could have moons, and that some of the moons might harbor life.
How long would it take to get there?
This question depends upon how fast you travel. Given our current lack of Star Trek's warp drives, any interstellar expedition would have to travel far slower than the speed of light.
A spaceship traveling at a one-tenth of the speed of light would reach Gliese 581g within about 220 years, Vogt said. That would allow the spaceship to begin getting close-up pictures and a sense of the planet's atmosphere.
That time scale is not promising for existing human lifespans, but robotic explorers could more easily take up the challenge. However, the fastest spaceships built so far don?t come anywhere near even that one-tenth light-speed mark.
What kind of life would we expect to find?
Any discussion about alien life on Gliese 581g is purely speculative at this point, according to co-discoverer Paul Butler of the Carnegie Institution of Washington, in Washington, D.C.
Butler took a more cautionary approach as opposed to Vogt, who said his gut feeling told him "the chances of life on this planet are 100 percent."
Still, even Butler noted that anywhere you find water on Earth, you find life. He suggested that a similar condition should hold for almost anywhere in the universe, including Gliese 581g if it does hold water.
Why doesn?t the planet have a real name?
The planet is called Gliese 581g because its star, Gliese 581, is designated "a," and the four previously discovered planets in the system are called b, c, d and e.
But Vogt said that he has unofficially begun calling the planet "Zarmina's World," in honor of his wife.
What would aliens living on Gliese 581 see if they looked toward our sun?
You remember that we don't have evidence of alien life on the planet yet, right?
But assuming they exist, aliens could spot our own sun as star in their sky without requiring any telescopes or binoculars.
If the alien astronomers had our current level of technology, they would be also able to easily detect Neptune, and possibly Jupiter and Saturn.
- Alien World Tour: The Exoplanets Around Star Gliese 581
- Gallery: The Strangest Alien Planets
- Top 10 Extreme Planet Facts