3 smallest planets outside solar system found

Washington: A team of astronomers has discovered the three smallest confirmed planets ever detected outside our solar system.

The three planets, discovered by scientists led by Philip Muirhead from the University of California, all orbit a single star, are smaller than Earth and appear to be rocky with a solid surface.

Until now, astronomers have found at most only four other rocky planets, also called terrestrial planets, around other stars.

The trio of new planets is too close to the central star to be in its habitable zone, the ring-shaped region around a star where the temperature is mild enough for liquid water, and possibly life, to exist.

However, the planets are the first rocky ones to be found orbiting a type of dim, small star called a red dwarf, the most common kind in the Milky Way. Their existence suggests that the galaxy could be teeming with similarly rocky planets and that there’s a good chance that many are in the habitable zone.

The red dwarf, called KOI-961, was first flagged as a potential planetary system by the Kepler mission, a space telescope that looks for planets around sunlike stars by scanning the sky for stars that periodically dip in brightness—the result of one or more planets passing in front of them.

Although Kepler reported 900 potential planetary systems in February, only about 85 of those were red-dwarf systems. The fact that a relatively small sample of red dwarfs produced three terrestrial planets means that either the Caltech-led team was really lucky or, more likely, that these planets are commonly found around red dwarfs.

This report comes just a few weeks after the Kepler team announced it had detected two rocky planets around a sunlike star—Kepler-20e and Kepler-20f—the first Earth-sized planets ever found and the smallest known at the time.

In January 2011, the Kepler team reported the discovery of the first unequivocally rocky planet around another star, Kepler-10b. Another planet—Corot-7b, which was found in 2009—could also be a rocky planet.

With the exception of Kepler-20e, which is about the size of Venus, the other previously discovered planets are all bigger than Earth. All three of the ones found by the Caltech-led team are smaller—the outermost one is about half the size of Earth and the other two are three-fourths the size of Earth.

In fact, the entire KOI-961 planetary system is remarkably tiny. KOI-961 has a diameter one-sixth that of the sun’s, making it just 70 percent bigger than Jupiter.

Each of the three planets needs less than two days to zip around their star, and all three are about one hundred times closer to that star than Earth is to the sun.

Since they’re so close to their star, they’re hot—the outermost planet is estimated to be about 200 degrees Celsius while the innermost planet is a scorching 500 degrees Celsius.

“The really amazing thing about this system is that the closest size comparison is to Jupiter and its moons,” John Johnson, one of the paper’s co-authors, said.

“This is causing me to have to fully recalibrate my notion of planetary and stellar systems,” he said.

Kepler’s initial measurements, which are automated to help it sift through roughly 150,000 stars, underestimated the size of KOI-961 and any planets it might have had.

No one realized this until amateur astronomer and paper coauthor Kevin Apps alerted Muirhead and his team to the idea that KOI-961 bore a remarkable resemblance to another red dwarf called Barnard''s Star, a nearby star that’s one of the most well-studied.

When the astronomers used telescopes at the Palomar and Keck Observatories to take a closer look at both stars, they found that the two are practically twins.

The characteristics of Barnard’s Star allowed the team to infer the properties of KOI-961, which is needed to deduce the nature of the planetary system from the star’s light curve, a plot of how the star dims over time due to transiting planets. In particular, the depth of the light curve, that is, how much the curve dips reveals the planets’ sizes.

Because the planets are so small, the only way they could have enough gravity to hold themselves together is if they are balls of rock, like Mercury, Venus, Earth, and Mars.

“Just three years ago, just talking about a rocky planet would have been pure speculation,” Johnson said.

“But these are unambiguously rocky,” he said.

Still, before they could make any conclusions, the researchers had to confirm that the dips in light detected by Kepler really were due to planets—and not something else, such as a pair of background stars in orbit around each other.

Saturn's Moon Enceladus Spreads its Influence

Chalk up one more feat for Saturn's intriguing moon Enceladus. The small, dynamic moon spews out dramatic plumes of water vapor and ice -- first seen by NASA's Cassini spacecraft in 2005. It possesses simple organic particles and may house liquid water beneath its surface. Its geyser-like jets create a gigantic halo of ice, dust and gas around Enceladus that helps feed Saturn's E ring. Now, thanks again to those icy jets, Enceladus is the only moon in our solar system known to influence substantially the chemical composition of its parent planet.

In June, the European Space Agency announced that its Herschel Space Observatory, which has important NASA contributions, had found a huge donut-shaped cloud, or torus, of water vapor created by Enceladus encircling Saturn. The torus is more than 373,000 miles (600,000 kilometers) across and about 37,000 miles (60,000 kilometers) thick. It appears to be the source of water in Saturn's upper atmosphere.

Though it is enormous, the cloud had not been seen before because water vapor is transparent at most visible wavelengths of light. But Herschel could see the cloud with its infrared detectors. "Herschel is providing dramatic new information about everything from planets in our own solar system to galaxies billions of light-years away," said Paul Goldsmith, the NASA Herschel project scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

The discovery of the torus around Saturn did not come as a complete surprise. NASA's Voyager and Hubble missions had given scientists hints of the existence of water-bearing clouds around Saturn. Then in 1997, the European Space Agency's Infrared Space Observatory confirmed the presence of water in Saturn's upper atmosphere. NASA's Submillimeter Wave Astronomy Satellite also observed water emission from Saturn at far-infrared wavelengths in 1999.

While a small amount of gaseous water is locked in the warm, lower layers of Saturn's atmosphere, it can't rise to the colder, higher levels. To get to the upper atmosphere, water molecules must be entering Saturn's atmosphere from somewhere in space. But from where and how? Those were mysteries until now.

Build the model and the data will come.

The answer came by combining Herschel's observations of the giant cloud of water vapor created by Enceladus' plumes with computer models that researchers had already been developing to describe the behavior of water molecules in clouds around Saturn.

One of these researchers is Tim Cassidy, a recent post-doctoral researcher at JPL who is now at the University of Colorado's Laboratory for Atmospheric and Space Physics, Boulder. "What's amazing is that the model," said Cassidy, "which is one iteration in a long line of cloud models, was built without knowledge of the observation. Those of us in this small modeling community were using data from Cassini, Voyager and the Hubble telescope, along with established physics. We weren't expecting such detailed 'images' of the torus, and the match between model and data was a wonderful surprise."

The results show that, though most of the water in the torus is lost to space, some of the water molecules fall and freeze on Saturn's rings, while a small amount -- about 3 to 5 percent -- gets through the rings to Saturn's atmosphere. This is just enough to account for the water that has been observed there.

Herschel's measurements combined with the cloud models also provided new information about the rate at which water vapor is erupting out of the dark fractures, known as "tiger stripes," on Enceladus' southern polar region. Previous measurements by the Ultraviolet Imaging Spectrograph (UVIS) instrument aboard the Cassini spacecraft showed that every second the moon is ejecting about 440 pounds (200 kilograms) of water vapor.

"With the Herschel measurements of the torus from 2009 and 2010 and our cloud model, we were able to calculate a source rate for water vapor coming from Enceladus," said Cassidy. "It agrees very closely with the UVIS finding, which used a completely different method."

"We can see the water leaving Enceladus and we can detect the end product -- atomic oxygen -- in the Saturn system," said Cassini UVIS science team member Candy Hansen, of the Planetary Science Institute, Tucson, Ariz. "It's very nice with Herschel to track where it goes in the meantime."

Neptune is *really* far away

Mike Brown is an astronomer, specifically one who studies Kuiper Belt Objects, those giant frozen iceballs that haunt the solar system out past Neptune.

In fact, Neptune’s biggest moon Triton has a lot of characteristics similar KBOs — it may be one captured by Neptune — so observing it gives an interesting opportunity for a compare-and-contrast study. So this past weekend Mike was using the Keck telescope in Hawaii to observe Triton along with its (adoptive?) parent planet, and took this fantastic image of the pair:

This false-color image shows the two worlds in the infrared, specifically at a wavelength of about 1.5 microns, twice what the human eye can see. Methane strongly absorbs this color of light, so where Neptune (in the upper left) looks dark you’re seeing lots of methane clouds, and where it’s bright there are clouds higher up, above the methane. Triton is in the lower right, and is bright because it’s covered in ice which is highly reflective.

Now this is all very pretty and interesting and sciencey, but if you know me at all you know there’s more to this story.

Mike tweeted about the image, and I oohed and ahhhed at it, of course. But then he tweeted again, saying he was also observing Jupiter’s moon Europa, but it was too bright to get good images using the monster 10-meter Keck telescope. It "saturated the detector" which is astronomer-speak for "overexposed".

Heart and Soul

The Heart and Soul nebulae are seen in this infrared mosaic from NASA's Wide-field Infrared Survey Explorer, or WISE. The image covers an area of the sky over ten times as wide as the full moon and eight times as high (5.5 x 3.9 degrees) in the constellation Cassiopeia.

Located about 6,000 light-years from Earth, the Heart and Soul nebulae form a vast star-forming complex that makes up part of the Perseus spiral arm of our Milky Way galaxy. The nebula to the left is the Heart, designated IC 1805 and named after its resemblance to a human heart. To the right is the Soul nebula, also known as the Embryo nebula, IC 1848 or W5. The Perseus arm lies further from the center of the Milky Way than the arm that contains our sun. The Heart and Soul nebulae stretch out nearly 580 light-years across, covering a small portion of the diameter of the Milky Way, which is roughly 100,000 light-years across.

The two nebulae are both massive star-making factories, marked by giant bubbles that were blown into surrounding dust by radiation and winds from the stars. WISE's infrared vision allows it to see into the cooler and dustier crevices of clouds like these, where gas and dust are just beginning to collect into new stars. These stars are less than a few million of years old -- youngsters in comparison to stars like the sun, which is nearly 5 billion years old.

Also visible near the bottom of this image are two galaxies, Maffei 1 and Maffei 2. Both galaxies contain billions of stars and, at about 10 million light-years away, are well outside our Milky Way yet relatively close compared to most galaxies. Maffei 1 is the bluish elliptical object and Maffei 2 is the spiral galaxy.

NASA's first mission capable of finding Earth-size and smaller planets around other stars

The centuries-old quest for other worlds like our Earth has been rejuvenated by the intense excitement and popular interest surrounding the discovery of hundreds of planets orbiting other stars. There is now clear evidence for substantial numbers of three types of exoplanets; gas giants, hot-super-Earths in short period orbits, and ice giants. The challenge now is to find terrestrial planets (i.e., those one half to twice the size of the Earth), especially those in the habitable zone→ of their stars where liquid water might exist on the surface of the planet.

The Kepler Mission, NASA Discovery mission #10, is specifically designed to survey our region of the Milky Way galaxy to discover hundreds of Earth-size and smaller planets in or near the habitable zone→ and determine the fraction of the hundreds of billions of stars in our galaxy that might have such planets.


The Transit Method of Detecting Extrasolar Planets

When a planet passes in front of a star as viewed from Earth, the event is called a “transit”. On Earth, we can observe an occasional Venus or Mercury transit. These events are seen as a small black dot creeping across the Sun—Venus or Mercury blocks sunlight as the planet moves between the Sun and us. Kepler finds planets by looking for tiny dips in the brightness of a star when a planet crosses in front of it—we say the planet transits the star.

Once detected, the planet's orbital size can be calculated from the period (how long it takes the planet to orbit once around the star) and the mass of the star using Kepler's Third Law of planetary motion. The size of the planet is found from the depth of the transit (how much the brightness of the star drops) and the size of the star. From the orbital size and the temperature of the star, the planet's characteristic temperature can be calculated. From this the question of whether or not the planet is habitable (not necessarily inhabited) can be answered.

Kepler instrument

The Kepler instrument is a specially designed 0.95-meter diameter telescope called a photometer or light meter. It has a very large field of view for an astronomical telescope — 105 square degrees, which is comparable to the area of your hand held at arm's length. It needs that large a field in order to observe the necessary large number of stars. It stares at the same star field for the entire mission and continuously and simultaneously monitors the brightnesses of more than 100,000 stars for the life of the mission—3.5 or more years.

The photometer must be spacebased to obtain the photometric precision needed to reliably see an Earth-like transit and to avoid interruptions caused by day-night cycles, seasonal cycles and atmospheric perturbations, such as, extinction associated with ground-based observing.

Results from the Kepler mission will allow us to place our solar system within the context of planetary systems in the Galaxy.