A newfound planet only 22 light-years away represents the best candidate yet for hosting liquid water on its surface, according to a team of astronomers announcing the find Thursday.
The planet has 4.5 times Earth's mass and orbits in the heart of its star's habitable zone – the region where water could remain stable on an orbiting planet's surface. Liquid water is a necessary ingredient for organic life.
For now, however, too little is known about the planet's composition and the makeup of any atmosphere – if it has one – to say with confidence that water is likely to be there, researchers say. Location alone does not make for a habitable planet. Some scientists suggest that the newfound planet could be caught with one hemisphere permanently facing its star, lessening any prospects for life.
Still, the researchers note the planet's position well within the habitable zone is an encouraging first sign.
This is not the first so-called super Earth astronomers have detected in a star's habitable zone. In December, scientists with NASA's Kepler mission announced the discovery of a super Earth orbiting within its host star's habitable zone. That planet was orbiting a sun-like star, marking an important step along the path toward meeting the mission's goal of uncovering Earth-size planets orbiting within the habitable zone of sun-like stars.
But the new planet, GJ 667Cc, orbits a red dwarf – a type of star that is much smaller, fainter, and far more common in the galaxy than sun-like stars. Given red dwarfs' prevalence, the finding suggests that the galaxy should be brimming with super Earths in habitable zones, the research team says.
The new planet's host star, GJ 667C, has about 30 percent of the sun's mass and gives off most of its radiation as infrared light. It's part of a triple-star system that also contains a pair of orange-dwarf stars. These don't come closer to GJ 667C than about 230 astronomical units, or 230 times the distance between Earth and the sun. They would be distant specks in the sky to a spacecraft orbiting the new planet.
Because the star is substantially fainter than the sun, its habitable zone is closer. GJ 677Cc orbits its red dwarf at about 0.12 astronomical units, zipping around the star once every 27.15 days.
Astrobiologists suggest red dwarfs have an important thing going for them as energy sources for life on any planets orbiting them: longevity.
Red dwarfs are dim because they are not burning their hydrogen fuel as furiously as more-massive stars. And unlike more massive stars, they have access to all of their hydrogen fuel. So they last longer than sun-like stars.
The sun boasts a respectable 10-billion year life span. For a star like GJ 677C, a life span is measured in trillions of years – plenty of time for life to emerge and evolve many times over if conditions permit. The team estimates that GJ 667C is only about 5 billion years old.
Half baked, half frozen world?
Yet others caution that the laws of physics may conspire against the presence of liquid water on planets in an orbit like GJ 667Cc's.
The planet may be close enough to the star that, if the planet's orbit is circular, the gravitational interaction between the two objects eventually will force the planet slow its rotation until it presents the same hemisphere to the star all the time, says University of Maine astronomer Neil Comins.
Astronomers call this synchronous rotation: The planet's rotation rate matches the time it takes the planet to orbits its star. One hemisphere of the planet is permanently roasting, the other is frozen.
Even with an atmosphere that circulates between the day side and night side, it's unclear if enough warmth will make it to the night side to keep water vapor transported there from freezing out as snow, rarely if ever to return to the day side. In effect, the night side could rob the day side of any water.
Achieving synchronous rotation likely would happen within a billion years or so for an Earth-size planet, Dr. Comins says, especially if it has a moon, which also acts to slow the planet's rotation.
For a super Earth, however, there might be hope. If the planet has no moon and is dense enough, the spin-down could take longer, allowing warmth to circulate and water to accumulate for a while longer before the big freeze and fry.
If a planet is in a elliptical orbit, which often is the case, the prospects are somewhat brighter, Comins suggests. The elliptical orbit would allow the planet to rotate and expose more of itself to light from its sun. Potentially habitable areas would fall in the twilight zone between hemispheres.
Some research suggests that the presence of the right kind of atmosphere could redistribute heat more readily than previously thought, “so it doesn't look that bad,” says Guillem Anglada-Escudé, a researcher at the University of Gottingen in Germany who led the international team.
Yet all of this remains speculation, he adds, until more is known about the planet.
How they found it
The discovery of GJ 667Cc emerged from a test Dr. Anglada-Escudé was making of new software, which was designed to analyze archived data to hunt for extrasolar planets. The team's planet-hunting approach measures the wobble that gravity from an orbiting planet imparts on its star.
The wobble shows up as back-and-forth shifts in the star's spectrum – a kind of bar code bearing information on the chemical elements present in the star's atmosphere. As the planet slips behind the star, the entire spectrum shifts toward red. When the planet tugs the star toward the observer, the star's spectrum shifts toward blue.
In 2009, researchers announced the discovery of a planet orbiting GJ 667C, with an orbital period of about 7 days. Last year, another team suggested a planet was orbiting with a 28-day period. But neither group provided any data or detailed analysis to back the claims. So Anglada-Escudé's team picked the system as the test dummy for its new detection software.
Not only did his team provide the first solid evidence of GJ 677Cb, a planet with 5.2 times Earth's mass orbiting the star every 7.2 days. It also found the super Earth in the habitable zone, as well as evidence for perhaps as many as two additional, more-distant planets.
The technique the team used, formally known as radial-velocity measurements, can provide only part of the picture, however. The team notes that there is a small chance that the orientation of the GJ 677C system could be just right, allowing scientists to watch the planets pass in front of their host star, an event called a transit.
The star is bright enough and close enough that even amateur-class telescopes could detect any transits, Anglada-Escudé says. Such measurements would allow astronomers to determine more precisely the shape of the planets' orbits, and importantly, the planet's densities. The density estimates would allow a first cut at estimating what the planets are made of and what is in their atmospheres, if they have any.
Within two or three years, he adds, “we may have the capability of looking at the spectral features of the atmospheres of these planets, if they really transit.”
Accomplishing that “would be a real breakthrough,” he says.
The results of the team's research have been accepted for publication in Astrophysical Journal Letters.