Hot super-Earths could host life after all

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Rocky super-Earths may host life (Illustration: ESO)

• NewScientist.com news service
• Ker Than

Massive, rocky worlds called 'super-Earths' – even those orbiting searingly close to their stars – may provide the right conditions for life, new research suggests.

At up to 15 times the mass of Earth, the rocky bodies are bigger and easier to spot than Earth-sized worlds, which have yet to be detected. In fact, technological advances recently led to the discovery of up to 45 new super-Earths, and astronomers say a third of all Sun-like stars may host the brawny planets.

But could they host life? "There's no reason why the different chemical cycles that are important for life on our planet wouldn't work on super-Earths," says Lisa Kaltenegger of the Harvard-Smithsonian Center for Astrophysics in Massachusetts, US.

Kaltenegger helped organise a recent conference session on the topic, and she says the consensus of attendees was similarly positive – even for those planets once dismissed as being too harsh for life.

Fire and ice

Super-Earths orbiting close to their stars, for example, experience gravitational tugs that keep them 'tidally locked' to their hosts. That means one side of such a planet always faces its star, the way the Moon always shows the same side to Earth.

Astronomers previously assumed such planets would be two-faced worlds of fire and ice, with one half molten and the other frozen.

Early models suggested the atmospheres of such worlds would quickly vanish, as water vapour and other atmospheric molecules on the planet's dark side would turn to ice and plunge to the ground. "It was thought that after the atmosphere on the dark side was completely iced out, then it would suck atmosphere from the hot side, freezing that out as well," Kaltenegger told New Scientist.

But new models show that if a tidally locked super-Earth has an atmosphere at least as dense as Earth's, strong winds could transport heat from its hot side to its cold side. Similarly, if the planet has a global ocean, its currents could help spread the warmth.

This effect still wouldn't offset the intense heat the planets would experience at close distances to Sun-like stars. But it means super-Earths could potentially host life as close as 0.05 astronomical units away from dim stars known as red dwarfs, which make up about 85% of the stars in the galaxy (for comparison, Mercury lies 0.38 AU away from the Sun).

Shifting plates

And in some ways, super-Earths might even be more likely to support life than their Earth-sized cousins, scientists say.

Recent research suggests that super-Earths will experience more plate tectonic activity than smaller rocky worlds.

On Earth, plate tectonics – the shifting and colliding of continental plates – is necessary for life.

It plays a crucial role in the carbon-silicate cycle, which releases carbon dioxide into the atmosphere, warming the planet. Plate tectonics also locks the greenhouse gas in surface rocks and sequesters it in Earth's interior so that the planet doesn't heat up too much.

"The way we have experienced life on Earth is enabled by plate tectonics," says Diana Valencia, a graduate student at Harvard.

Super-Earths should have larger molten cores and should generate more heat than Earth-sized worlds, Valencia told New Scientist. This could cause more vigorous convection in the planets' mantles and create thinner plates that slip and slide more easily.

Eventually, missions such as NASA's upcoming Kepler space telescope could find an Earth-sized planet in our galaxy. "But there's going to be a lot of super-Earths discovered before that," Valencia says. "If we're concerned about finding life, those are the planets we should be investigating right now."

Astrobiology - Learn more in our out-of-this-world special report.

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Pluto Gets Respect: Dwarf Planets to Be Called 'Plutoids'

Who Needs Planets When You've Got Plutoids?

June 11, 2008 -- Pluto is finally getting its day in the sun, after being stripped of planetary status by astronomers two years ago.

From now on all similar distant bodies in the solar system will be called "plutoids." That's the decision by the International Astronomical Union, which met last week in Oslo, Norway, and announced the decision Wednesday.

The same group raised a cosmic fuss when it demoted the once-ninth planet to "dwarf" status in 2006. The new policy allows Pluto to be the standard for a whole new category of dwarf planets.

Pluto is one of only two plutoids, the other being Eris. Both are objects that circle the sun and are too small to be considered planets, but big enough to have a level of gravity that keeps them in a near spherical shape. Plutoids also must be farther from the sun than Neptune.

It was the 2003 discovery of Eris -- a body bigger and farther from the sun than Pluto -- that eventually led to Pluto's demotion. But the astronomers expect more plutoids to be discovered in the future.

When Pluto was demoted, the astronomical union always planned on naming the new category of objects after the former planet, but had to find the right name, said IAU president Catherine Cesarsky, a French astrophysicist. Their first choice, pluton, was already used by geologists.

The astronomers' action makes Pluto more important, Cesarsky said. Instead of being a "puny" outer planet, Pluto is now a "prototype of a new type of fascinating objects," she said.

"It doesn't really roll off the tongue very well," said Mike Brown, the California Institute of Technology astronomer who discovered Eris. "Maybe it'll make it."

It was not enough to satisfy leading Pluto-as-a-planet advocate Alan Stern, a former NASA space sciences chief and principal investigator on a mission to Pluto. Stern said a rival group could be formed to the IAU, which he said was too secretive in its decision-making.

"It's just some people in a smoke-filled room who dreamed it up," Stern said. "Plutoids or hemorrhoids, whatever they call it. This is irrelevant."

Another Pluto supporter was at least partially pleased.

"It's going in the right direction," laughed Ralph McNutt, a planetary scientist at Johns Hopkins University. "I'd still rather have it just be known as a planet."

"I grew up with nine planets, I'm sorry," McNutt said.

For those of you keeping score at home, the solar system now stands at: Planets 8, Plutoids 2.

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The "Great Attractor": What is the Milky Way Speeding Towards at 14 Million MPH?

Astronomers have known for years that something seems to be pulling our Milky Way and tens of thousands of other galaxies toward itself at a breakneck 22 million kilometers (14 million miles) per hour. But they couldn’t pinpoint exactly what or where it is.

A huge volume of space that includes the Milky Way and super-clusters of galaxies is flowing towards a mysterious, gigantic unseen mass named mass astronomers have dubbed "The Great Attractor," some 250 million light years from our Solar System.

The Milky Way and Andromeda galaxies are the dominant structures in a galaxy cluster called the Local Group which is, in turn, an outlying member of the Virgo supercluster. Andromeda--about 2.2 million light-years from the Milky Way--is speeding toward our galaxy at 200,000 miles per hour.

This motion can only be accounted for by gravitational attraction, even though the mass that we can observe is not nearly great enough to exert that kind of pull. The only thing that could explain the movement of Andromeda is the gravitational pull of a lot of unseen mass--perhaps the equivalent of 10 Milky Way-size galaxies--lying between the two galaxies.

Meanwhile, our entire Local Group is hurtling toward the center of the Virgo cluster at one million miles per hour.

The Milky Way and its neighboring Andromeda galaxy, along with some 30 smaller ones, form what is known as the Local Group, which lies on the outskirts of a “super cluster”—a grouping of thousands of galaxies—known as Virgo, which is also pulled toward the Great Attractor. Based on the velocities at these scales, the unseen mass inhabiting the voids between the galaxies and clusters of galaxies amounts to perhaps 10 times more than the visible matter.

Even so, adding this invisible material to luminous matter brings the average mass density of the universe still to within only 10-30 percent of the critical density needed to "close" the universe. This phenomena suggests that the universe be "open." Cosmologists continue to debate this question, just as they are also trying to figure out the nature of the missing mass, or "dark matter."

It is believed that this dark matter dictates the structure of the Universe on the grandest of scales. Dark matter gravitationally attracts normal matter, and it is this normal matter that astronomers see forming long thin walls of super-galactic clusters.

Recent measurements with telescopes and space probes of the distribution of mass in M31 -the largest galaxy in the neighborhood of the Milky Way- and other galaxies led to the recognition that galaxies are filled with dark matter and have shown that a mysterious force—a dark energy—fills the vacuum of empty space, accelerating the universe's expansion.

Astronomers now recognize that the eventual fate of the universe is inextricably tied to the presence of dark energy and dark matter.The current standard model for cosmology describes a universe that is 70 percent dark energy, 25 percent dark matter, and only 5 percent normal matter.

We don't know what dark energy is, or why it exists. On the other hand, particle theory tells us that, at the microscopic level, even a perfect vacuum bubbles with quantum particles that are a natural source of dark energy. But a naïve calculation of the dark energy generated from the vacuum yields a value 10120 times larger than the amount we observe. Some unknown physical process is required to eliminate most, but not all, of the vacuum energy, leaving enough left to drive the accelerating expansion of the universe.

A new theory of particle physics is required to explain this physical process.

The universe as we see it contains only the stable relics and leftovers of the big bang: unstable particles have decayed away with time, and the perfect symmetries have been broken as the universe has cooled, but the structure of space remembers all the particles and forces we can no longer see around us.

Discovering what it is that makes up the heart of the Great Attractor -- will surely rank as one of the greatest discoveries in the history of science.

Recent findings suggest these motions are the result of gravitational forces from not one, but two things: the Great Attractor, and a conglomerate of galaxies far beyond it.

The location of the Great Attractor was finally determined in 1986 and lies at a distance of 250 million light years from the Milky Way, in the direction of the Hydra and Centaurus constellations. That region of space is dominated by the Norma cluster, a massive cluster of galaxies, and contains a preponderance of large, old galaxies, many of which are colliding with their neighbors, and or radiating large amounts of radio waves.

Major concentration of galaxies lies beyond the Great Attractor, near the so-called Shapley Supercluster, 500 million light-years away—the most massive known super-cluster. Mapping X-ray luminous galaxy clusters in the Great Attractor region has shown that the pull our galaxy is experiencing is most likely due to both the nearby Great Attractor and these more distant structures.

In the 1987, a group of astronomers known as the "Seven Samurai," at Cal Tech uncovered this coordinated motion of the Milky Way and our several million nearest galactic neighbors. They found that galaxies are very unevenly distributed in space, with galactic super-clusters separated by incredibly huge voids of visible ordinary matter. The place towards which we all appear headed was originally called the New Supergalactic Center or the Very Massive Object until one of the discoverers, Alan Dressler, decided they needed a more evocative name and came up with "The Great Attractor."

The motion of local galaxies indicated there was something massive out there that are pulling the Milky Way, the Andromeda Galaxy, and other nearby galaxies towards it. For a while, nobody could see what it was, because it lies behind the plane of our Galaxy --- that means the gas and dust in our Galaxy obscures the light from the Great Attractor, and it is outshone by the stars and other objects in our Galaxy.

The Great Attractor is a diffuse concentration of matter some 400 million light-years in size located around 250 million light-years away within the so-called "Centaurus Wall" of galaxies , about seven degrees off the plane of the Milky Way. X-ray observations with the ROSAT satellite then revealed that Abell 3627 is at the center of the Great Attractor. It lies in the so-called Zone of Avoidance, where the dust and stars of the Milky Way's disk obscures as much as a quarter of the Earth's visible sky.

Posted by Casey Kazan. Image credit: Wally Pacholtz

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