When is exoplanet news “juicy”? Yesterday at a Kepler press conference held at NASA Ames, Roger Hunter, Kepler project manager, introduced the proceedings as juicy. And as three scientists presented the findings, it turned out to be a good adjective. The researchers believe they have discovered the first water worlds (besides Earth) in our galaxy.

Two systems are providing new evidence of rocky Earth-like planets in the habitable zone—the range of distance from a star where the surface temperature of an orbiting planet might be suitable for liquid water. Kepler 62 has five planets total, but two of those, 62e and 62f, orbit inside the habitable zone. Kepler 69 has two planets but only one in the habitable zone, 69c.

For exoplanets and their stars, size matters when it comes to habitability. At 1,200 light years away, the star Kepler 62 is two-thirds the size of our Sun. That brings the habitable zone in a bit closer to the star. The two planets of interest, 62e and 62f, are 1.6 and 1.4 times the diameter of Earth, respectively. This also puts them in the “just-right” size for habitability.

At the press conference, William Borucki, Kepler science principal investigator at NASA Ames, said that 62e and 62f “are the best candidates to be habitable, not just within the habitable zone.”

Computer models suggest that the largest rocky planets will have a diameter no greater than 1.5 times that of Earth’s, explained Lisa Kaltenegger of the Max Planck Institute for Astronomy and Harvard-Smithsonian Center for Astrophysics. And a planet’s mass, between 1.2-2.5 times Earth’s mass, can be an indicator for liquid water. While Kepler 62e and 62f are too small to measure their mass, Kaltenegger and her team’s modeling makes these planets very wet, indeed.

Kepler 69c, on the other hand, is 2,700 light years away and 1.5 times Earth’s diameter. It orbits near the inner, hotter edge of its star’s habitable zone. Thomas Barclay, Kepler scientist from the Bay Area Environmental Research Institute, likens it to a super Venus, rather than a super Earth. “We don’t have anything like it in our solar system,” he said.

“The Kepler spacecraft has certainly turned out to be a rock star of science,” said John Grunsfeld, at NASA Headquarters in Washington. “The discovery of these rocky planets in the habitable zone brings us a bit closer to finding a place like home. It is only a matter of time before we know if the galaxy is home to a multitude of planets like Earth, or if we are a rarity.”

The findings are published this week in Science (Kepler 62) and the Astrophysical Journal (Kepler 69).

For an interactive on Kepler’s planetary discoveries and their orbits, click here.

Image: NASA Ames/JPL-Caltech

Two researchers hypothesize that an asteroid belt, just the right size and distance from its star, might be necessary for a star system to support a life-bearing planet.

This might sound surprising, since asteroids can occasionally impact Earth and trigger mass extinctions. Ouch! But an emerging view proposes that asteroid collisions with planets may provide a boost to the birth and evolution of complex life.

For one thing, asteroids delivered water and organic compounds to the early Earth. Consistent with the theory of punctuated equilibrium, occasional asteroid impacts might also accelerate the rate of biological evolution by disrupting a planet’s environment to the point that species must evolve new adaptation strategies.

Rebecca Martin, of the University of Colorado, and Mario Livio, of the Space Telescope Science Institute, looked at our solar system and used theoretical models and actual observations of other star systems and exoplanets to study the theory that life needs an asteroid belt.

They suggest that the location of an asteroid belt relative to a Jupiter-like planet is particularly favorable to life. The asteroid belt in our solar system, located between Mars and Jupiter, is a region of millions of space rocks sitting near the “snow line,” beyond which volatile materials such as water ice are far enough from the Sun’s heat to remain intact.

Our solar system’s formation was just right for life, Livio says. “To have such ideal conditions you need a giant planet like Jupiter that is just outside the asteroid belt [and] that migrated a little bit, but not through the belt,” he explains. “If a large planet like Jupiter migrates through the belt, it would scatter the material. If, on the other hand, a large planet did not migrate at all, that, too, is not good because the asteroid belt would be too massive. There would be so much bombardment from asteroids that life may never evolve.”

Using our solar system as a model, Martin and Livio proposed that asteroid belts in other solar systems would always be located approximately at the snow line. They created models of protoplanetary disks, the dense gas and dust around a newly formed star; then they looked at observations from NASA’s Spitzer Space Telescope of 90 such regions that have warm dust, which could indicate the presence of an asteroid belt-like structure. The warm dust fell right near the snow line.

But for life to exist, the system also needs a large gas giant, like Jupiter, to “manage” the size of the asteroid belt. So Martin and Livio looked at data for 520 giant planets found outside our solar system. Only 19 of them reside outside the snow line, suggesting that most of the giant planets that formed outside the snow line have migrated too far inward to preserve the right-sized asteroid belt needed to foster life on an Earth-like planet near the belt. The team calculated that less than four percent of the observed systems may actually harbor such a compact asteroid belt.

“Our study shows that only a tiny fraction of planetary systems observed to date seem to have giant planets in the right location to produce an asteroid belt of the appropriate size, offering the potential for life on a nearby rocky planet,” says Martin. “Our study suggests that our solar system may be rather special.”

Their findings are published in the Monthly Notices of the Royal Astronomical Society: Letters.

Image: NASA, ESA, and A. Feild (STScI)

This is exciting as it gives us more evidence that the dry, red planet used to be wet with water. There is earlier evidence for the presence of water on Mars, but this evidence—images of rocks (like the one to the right) containing ancient streambed gravels—is the first of its kind. The sizes and shapes of stones in these images offer clues to the speed and distance of a long-ago stream’s flow.

“From the size of gravels it carried, we can interpret the water was moving about three feet per second, with a depth somewhere between ankle and hip deep,” says Curiosity science co-investigator William Dietrich of UC Berkeley. “Plenty of papers have been written about channels on Mars with many different hypotheses about the flows in them. This is the first time we’re actually seeing water-transported gravel on Mars. This is a transition from speculation about the size of streambed material to direct observation of it.”

“The shapes tell you they were transported and the sizes tell you they couldn’t be transported by wind. They were transported by water flow,” concurs Curiosity science co-investigator Rebecca Williams of the Planetary Science Institute.

The science team may use Curiosity to learn the elemental composition of the material, revealing more characteristics of the wet environment that formed these deposits.

The slope of Mount Sharp in Gale Crater remains the rover’s main destination. Clay and sulfate minerals detected there from orbit can be good preservers of carbon-based organic chemicals that are potential ingredients for life.

“A long-flowing stream can be a habitable environment,” says Mars Science Laboratory project scientist John Grotzinger of Caltech. “It is not our top choice as an environment for preservation of organics, though. We’re still going to Mount Sharp, but this is insurance that we have already found our first potentially habitable environment.”

Image: NASA/JPL-Caltech/MSSS

This morning, NASA announced the confirmation of an exoplanet in its star’s habitable zone—the region around a star where liquid water, and life, could potentially exist on an Earth-sized planet.

The newly confirmed planet, Kepler-22b, is the smallest yet found to orbit in the middle of the habitable zone of a star similar to our sun. The planet has a radius about 2.4 times that of Earth. Scientists don’t yet know if Kepler-22b has a predominantly rocky, gaseous, or liquid composition, but its discovery brings us one step closer to finding Earth-like planets.

Previous research hinted at the existence of near-Earth-size planets in habitable zones, but clear confirmation proved elusive. Two other small planets orbiting stars smaller and cooler than our sun recently were confirmed on the very edges of their habitable zones, with orbits more closely resembling those of Venus and Mars.

Kepler discovers planets and planet candidates by measuring dips in the brightness of more than 150,000 stars to search for planets that cross in front, or “transit,” the stars. Kepler requires at least three transits to verify a signal as a planet.

“Fortune smiled upon us with the detection of this planet,” says Bill Borucki, Kepler principal investigator at NASA Ames Research Center, who led the team that discovered Kepler-22b. “The first transit was captured just three days after we declared the spacecraft operationally ready. We witnessed the defining third transit over the 2010 holiday season.”

In addition, the Kepler science team uses ground-based telescopes and the Spitzer Space Telescope to confirm planet candidates the spacecraft finds. In this case, Spitzer’s infrared instruments confirmed the new exoplanet’s existence.

Kepler-22b lies 600 light years away from Earth. While the planet is larger than Earth, its orbit of 290 days around a sun-like star resembles that of our world. The planet’s host star belongs to the same class as our sun, called G-type, albeit slightly smaller and cooler.

Of the 54 habitable zone planet candidates reported in February 2011, Kepler-22b is the first to be confirmed. This milestone will be published in the Astrophysical Journal.

The confirmation also provided our friends at SETI with very good news today. As of 6:18 a.m., SETI received renewed funding for its Allan Telescope Array, specifically to target Kepler’s discoveries, like Kepler-22b. From a SETI press release:

“This is a superb opportunity for SETI observations,” said Jill Tarter, the Director of the Center for SETI Research at the SETI Institute. “For the first time, we can point our telescopes at stars, and know that those stars actually host planetary systems—including at least one that begins to approximate an Earth analog in the habitable zone around its host star. That’s the type of world that might be home to a civilization capable of building radio transmitters.”

To astronomers, a “potentially habitable” planet is one that could sustain life, not necessarily one where humans would thrive. Habitability depends on many factors, but having liquid water and an atmosphere are among the most important.

The new findings are based on 11 years of observations of the nearby red dwarf star Gliese 581 using the HIRES spectrometer on the Keck I Telescope in Hawaii. The spectrometer allows precise measurements of a star’s radial velocity (its motion along the line of sight from Earth), which can reveal the presence of planets. The gravitational tug of an orbiting planet causes periodic changes in the radial velocity of the host star. Multiple planets induce complex wobbles in the star’s motion, and astronomers use sophisticated analyses to detect planets and determine their orbits and masses.

The new planet, designated Gliese 581g, has a mass three to four times that of Earth and orbits its star in just under 37 days. Its mass indicates that it is probably a rocky planet with a definite surface and enough gravity to hold on to an atmosphere. Gliese 581, located 20 light years away from Earth in the constellation Libra, has two previously detected planets that lie at the edges of the habitable zone, one on the hot side (planet c) and one on the cold side (planet d). While some astronomers still think planet d may be habitable if it has a thick atmosphere with a strong greenhouse effect to warm it up, others are skeptical.

(Those familiar with the Academy’s originally produced planetarium show Fragile Planet—which opened in 2008—will be familiar with Gliese 581d, visualized as a potentially habitable planet.)

Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz, who led the Exoplanet Survey, had this to say about the discovery:

Our findings offer a very compelling case for a potentially habitable planet. The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common.

One very interesting feature of the planet is that it’s tidally locked to its star, meaning that one side is always facing the star and basking in perpetual daylight, while the side facing away from the star lies in perpetual darkness. This could stabilize the planet’s surface climates, according to Vogt. The most habitable zone on the planet’s surface would be the boundary between shadow and light.

Universe Today gives us more details:

The researchers estimate that the average surface temperature of the planet is between -24 and 10 degrees Fahrenheit (-31 to -12 degrees Celsius). Actual temperatures would range from blazing hot on the side facing the star to freezing cold on the dark side…

The surface gravity would be about the same or slightly higher than Earth’s, so that a person could easily walk upright on the planet, Vogt said.

Vogt goes on to say in Universe Today that:

If these are rare, we shouldn’t have found one so quickly and so nearby… There could be tens of billions of these systems in our galaxy.

Cool!