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Five new rocky planets discovered

MORE THAN THREE-QUARTERS of the planet candidates discovered by NASA’s Kepler spacecraft have sizes ranging from that of Earth to that of Neptune, which is nearly four times as big as Earth. Such planets dominate the galactic census but are not represented in our own Solar System. Astronomers don’t know how they form or if they are made of rock, water or gas.

The Kepler team has today reported on four years of ground-based telescope follow-up observations targeting Kepler’s exoplanet systems (ie. planets beyond our Solar System) at the American Astronomical Society meeting in Washington. These observations confirm the numerous Kepler discoveries are indeed planets and yield mass measurements of these enigmatic worlds that vary between Earth and Neptune in size.

Included in the findings are five new rocky planets ranging in size from ten to eighty percent larger than Earth. Two of the new rocky worlds, dubbed Kepler-99b and Kepler-406b, are both forty percent larger in size than Earth and have a density similar to lead. The planets orbit their host stars in less than five and three days respectively, making these worlds too hot for life as we know it.

Artist's impression of an exoplanet

Astronomers have used ground-based telescopes to do follow-up observations of exoplanets detected by NASA’s Kepler space observatory. They’ve confirmed that many are between the Earth and Neptune in size. (Artist’s impression courtesy of NASA Ames / JPL-Caltech / Tim Pyle.)

Wobbly measurements

A major component of the follow-up observations were Doppler measurements of the planets’ host stars. The team measured the wobble of the host star, caused by the gravitational tug on the star exerted by the orbiting planet. That measured wobble reveals the mass of the planet: the higher the mass of the planet, the greater the gravitational tug on the star and hence the greater the wobble.

“This marvellous avalanche of information about the mini-Neptune planets is telling us about their core-envelope structure, not unlike a peach with its pit and fruit,” said Geoff Marcy, professor of astronomy at University of California, Berkeley who led the summary analysis of the high-precision Doppler study. “We now face daunting questions about how these enigmas formed and why our Solar System is devoid of the most populous residents in the galaxy.”

Artist's impression of exoplanets orbiting a red star

Artist’s impression of several exoplanets orbiting a red star. Courtesy ESO.

Using one of the world’s largest ground-based telescopes at the W. M. Keck Observatory in Hawaii, scientists confirmed 41 of the exoplanets discovered by Kepler and determined the masses of 16. With the mass and diameter in-hand, scientists could immediately determine the density of the planets, characterising them as rocky or gaseous, or mixtures of the two.

These density measurements dictate the possible chemical composition of these strange, but ubiquitous planets. The density measurements suggest that those planets smaller than Neptune – or mini-Neptunes – have a rocky core, but the proportions of hydrogen, helium and hydrogen-rich molecules in the envelope surrounding that core vary dramatically, with some having no envelope at all.

One step closer

A complementary technique used to determine mass, and in turn density of a planet, is by measuring the transit timing variations (TTV). Much like the gravitational force of a planet on its star, neighbouring planets can tug on one another causing one planet to accelerate and another planet to decelerate along its orbit.

Ji-Wei Xie of the University of Toronto, used TTV to validate 15 pairs of Kepler planets ranging from Earth-sized to a little larger than Neptune. Xie measured masses of 30 planets thereby adding to the compendium of planetary characteristics for this new class of planets.

“Kepler’s primary objective is to determine the prevalence of planets of varying sizes and orbits. Of particular interest to the search for life is the prevalence of Earth-sized planets in the habitable zone,” said Natalie Batalha, Kepler mission scientist at NASA’s Ames Research Centre. “But the question in the back of our minds is: are all planets the size of Earth rocky? Might some be scaled-down versions of icy Neptunes or steamy water worlds? What fraction are recognisable as kin of our rocky, terrestrial globe?”

Artist's impression of the Kepler space observatory

Artist’s impression of the Kepler space observatory. Courtesy NASA / Wendy Stenzel.

The mass measurements produced by Doppler and TTV hint that a large fraction of planets smaller than 1.5 times the radius of Earth may be comprised of the rocky silicates, iron, nickel and magnesium that are found in the terrestrial planets (Mercury, Venus, Earth and Mars) here in the Solar System.

Armed with this type of information, scientists will be able to turn the fraction of stars harbouring Earth-sizes planets into the fraction of stars harbouring bona-fide rocky planets. And that’s a step closer to finding a habitable environment beyond the Solar System.

Adapted from information issued by NASA.

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Earth-sized planets common in the Milky Way

Artist's impression of an Earth-like exoplanet

There are billions of Sun-like stars in our galaxy, 17 percent of which have close-orbiting planets not much bigger than Earth. Image: ESO/M. Kornmesser.

  • NASA’s Kepler space mission aims to detect Earth-like planets
  • New analysis shows that early Kepler analyses missed over 30 planets
  • It’s now thought 17% of Sun-like stars have planets not much bigger than Earth

AN ANALYSIS OF THE FIRST three years of data from NASA’s Kepler mission, which already has detected thousands of potential exoplanets, contains good news for those searching for habitable worlds outside our Solar System.

It shows that 17 percent of all Sun-like stars have planets one to two times the diameter of Earth orbiting close to their host stars, according to a team of astronomers from the University of California, Berkeley, and the University of Hawaii at Manoa.

This estimate includes only planets that circle their stars within a distance of about one-quarter of Earth’s orbital radius – which would be well within the orbit of Mercury if it were in our Solar System. This is the current limit of Kepler’s detection capability.

Further evidence suggests that the fraction of stars having planets the size of Earth or slightly bigger orbiting within Earth-like orbits may amount to 50 percent.

The team – UC Berkeley graduate student Erik Petigura, former UC Berkeley post-doctoral fellow Andrew Howard, now on the faculty of the Institute for Astronomy at the University of Hawaii, and UC Berkeley professor of astronomy Geoff Marcy – reported its findings on Wednesday (Australian time) at a session on the Kepler mission during the American Astronomical Society meeting in Long Beach, California.

Not necessarily habitable

Planets one to two times the size of Earth are not necessarily habitable. Painstaking observations by Petigura’s team show that planets two or three times the diameter of Earth are typically like Uranus and Neptune, which have a rocky core surrounded by helium and hydrogen gases and perhaps water. Planets close to a star may even be water worlds – planets with oceans hundreds of kilometres deep above a rocky core.

Nevertheless, planets between one and two times the diameter of Earth may well be rocky and, if located within the Goldilocks orbital zone – not too hot, not too cold, just right for liquid water – could support life.

“Kepler’s one goal is to answer a question that people have been asking since the days of Aristotle: What fraction of stars like the Sun have an Earth-like planet?” said Howard. “We’re not there yet, but Kepler has found enough planets that we can make statistical estimates.”

Plot of Kepler and TERRA exoplanets

Using a computer program called TERRA, scientists have sifted extra exoplanets (red dots) out of the existing Kepler data (grey dots). Image by Erik Petigura and Geoff Marcy, UC Berkeley, and Andrew Howard, Institute for Astronomy, University of Hawaii.

Finding planets in the ‘noise’

The estimates are based on a better understanding of the percentage of big Earth-size planets that Kepler misses because of uncertainties in detection, which the team estimates to be about one in four, or 25 percent.

To find planets, the Kepler telescope captures repeated images of 150,000 stars in a region of the sky in the constellation Cygnus. The data are analysed by computer software – the “pipeline” – in search of stars that dim briefly as a result of a planet passing in front, called a transit.

For planets as large as Jupiter, the star may dim by 1 percent, or one part in 100, which is easily detectable. A planet as small as Earth, however, dims the star by one part in 10,000, which is likely to be lost in the data ‘noise’, Petigura said.

The missing worlds

So Petigura spent the past two years writing a software program called TERRA, which is very similar to Kepler’s pipeline. The team then fed TERRA simulated planets to test how efficiently the software detects Earth-size planets.

After carefully measuring the fraction of planets missed by TERRA, the team corrected for this and then plugged in real Kepler observations freely available on the Internet. They identified 119 Earth-like planets ranging in size from nearly six times the diameter of Earth to the diameter of Mars. Thirty-seven of these planets were not identified in previous Kepler reports.

The analysis confirmed that the number of planets increases as the size decreases, which Howard and the Kepler team reported last year. Perhaps 1 percent of stars have planets the size of Jupiter, while 10 percent have planets the size of Neptune.

Adapted from information issued by the University of California, Berkeley.

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461 new planet candidates

Artist's impression of a planetary system

The number of exoplanet candidates found by the Kepler space observatory, has jumped up by 461. Image: ESO/M. Kornmesser

  • NASA’s Kepler space mission aims to detect Earth-like planets
  • 2,740 planet candidates detected orbiting 2,036 stars
  • Kepler now has 105 confirmed planets

SCIENTISTS WITH NASA’S KEPLER MISSION have announced the discovery of 461 new planet candidates. Four of the potential new planets are less than twice the size of Earth and orbit in their star’s ‘habitable zone, the region in the planetary system where liquid water might exist on the surface of a planet.

Based on observations conducted from May 2009 to March 2011, the findings show a steady increase in the number of smaller-size planet candidates and the number of stars with more than one candidate.

“There is no better way to kickoff the start of the Kepler extended mission than to discover more possible outposts on the frontier of potentially life bearing worlds,” said Christopher Burke, Kepler scientist at the SETI Institute in Mountain View, California, who is leading the analysis.

Flat-pack planetary systems

Since the last Kepler catalogue was released in February 2012, the number of candidates discovered in the Kepler data has increased by 20 percent and now totals 2,740 potential planets orbiting 2,036 stars.

The most dramatic increases are seen in the number of Earth-size and super Earth-size candidates discovered, which grew by 43 and 21 percent respectively.

Plot of exoplanets discovered in Kepler data

Since the last Kepler catalogue was released in February 2012, the number of candidates discovered in the Kepler data has increased by 20 percent and now totals 2,740 potential planets orbiting 2,036 stars. NASA

The new data increases the number of stars discovered to have more than one planet candidate from 365 to 467. Today, 43 percent of Kepler’s planet candidates are observed to have neighbour planets.

“The large number of multi-candidate systems being found by Kepler implies that a substantial fraction of exoplanets reside in flat multi-planet systems,” said Jack Lissauer, planetary scientist at NASA’s Ames Research Center in Moffett Field, California. “This is consistent with what we know about our own planetary neighborhood.”

New Earths – just a question of when

The Kepler space telescope identifies planet candidates by repeatedly measuring the change in brightness of more than 150,000 stars in search of planets that pass in front, or ‘transit,’ their host star. At least three transits are required to verify a signal as a potential planet.

Scientists analysed more than 13,000 transit-like signals to eliminate known spacecraft instrumentation and astrophysical false positives – phenomena that masquerade as planetary candidates – to identify the potential new planets.

Candidates require additional follow-up observations and analyses to be confirmed as planets. At the beginning of 2012, 33 candidates in the Kepler data had been confirmed as planets. Today, there are 105.

“The analysis of increasingly longer time periods of Kepler data uncovers smaller planets in longer period orbits – orbital periods similar to Earth’s,” said Steve Howell, Kepler mission project scientist at Ames. “It is no longer a question of will we find a true Earth analogue, but a question of when.”

The complete list of Kepler planet candidates is available in an interactive table at the NASA Exoplanet Archive. The archive is funded by NASA’s Exoplanet Exploration Program to collect and make public data to support the search for and characterisation of exoplanets and their host stars.

More information:

NASA Exoplanet Archive

Kepler Mission

Adapted from information issued by NASA.

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Three space missions extended

Artist's concepts of Spitzer, Planck and Kepler

Artist's concepts of Spitzer, Planck and Kepler. NASA extended Spitzer and Kepler for two additional years; and the US portion of Planck, a European Space Agency mission, for one year. (Relative sizes not to scale.)

NASA HAS EXTENDED three missions—Kepler, the Spitzer Space Telescope and the US portion of the European Space Agency’s Planck mission—as a result of the 2012 Senior Review of Astrophysics Missions.

“This means scientists can continue using the three spacecraft to study everything from the birth of the universe with Planck, and galaxies, stars, planets, comets and asteroids with Spitzer, while Kepler is determining what percentage of Sun-like stars host potentially habitable Earth-like planets,” said Michael Werner, the chief scientist for astronomy and physics at JPL.

Kepler has been approved for extension through fiscal year 2016, providing four additional years to find Earth-size planets in the habitable zone—the region in a planetary system where liquid water could exist on the surface of the orbiting planet—around Sun-like stars in our galaxy.

Spitzer, launched in 2003, will continue to provide the astronomical community with its unique infrared images for another two years. It has continued to explore the cosmos since running out of coolant, as expected, in 2009.

Among its many duties during its “warm mission”, the observatory is probing the atmospheres of planets beyond our Sun and investigating the glow of some of the most distant galaxies known. As requested by the project, Spitzer received two additional years of operations.

NASA will fund an additional year of US participation in the European Space Agency’s Planck mission. Planck, launched in 2009, is gathering data from the very early universe, shortly after its explosive birth in a big bang. Planck’s observations are yielding insight into the origin, evolution and fate of our universe.

More information:

Kepler mission

Spitzer Space Telescope

Planck mission

Adapted from information issued by JPL. Images courtesy NASA / JPL-Caltech.

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Mini-Solar System has smallest planets found so far

Artist's concept of the KOI-961 system

This artist's concept depicts an itsy bitsy planetary system -- so compact, in fact, that it's more like Jupiter and its moons than a star and its planets. Astronomers using data from NASA's Kepler mission and ground-based telescopes recently confirmed that the system, called KOI-961, hosts the three smallest exoplanets known so far to orbit a star other than our Sun.

ASTRONOMERS USING DATA from NASA’s Kepler mission have discovered the three smallest planets yet detected orbiting a star beyond our Sun. The planets orbit a single star, called KOI-961, and are 0.78, 0.73 and 0.57 times the radius of Earth. The smallest is about the size of Mars.

All three planets are thought to be rocky like Earth but orbit close to their star, making them too hot to be in the habitable zone, which is the region where liquid water could exist.

Of the more than 700 planets confirmed to orbit other stars, called exoplanets, only a handful are known to be rocky.

“Astronomers are just beginning to confirm the thousands of planet candidates uncovered by Kepler so far,” said Doug Hudgins, Kepler programme scientist at NASA Headquarters in Washington. “Finding one as small as Mars is amazing, and hints that there may be a bounty of rocky planets all around us.”

Dramatic revision of planets sizes

Kepler searches for planets by continuously monitoring more than 150,000 stars, looking for telltale dips in their brightness caused by crossing, or transiting, planets. At least three transits are required to verify a dip as a planet.

Follow-up observations from ground-based telescopes also are needed to confirm the discoveries.

The latest discovery comes from a team led by astronomers at the California Institute of Technology in Pasadena. The team used data publicly released by the Kepler mission, along with follow-up observations from the Palomar Observatory, near San Diego, and the W.M. Keck Observatory atop Mauna Kea in Hawaii.

Artist's conception comparing the KOI-961 system to the Jupiter system

In many ways the KOI-961 planetary system is similar to Jupiter and the largest four of its many moons. (Artist's conception)

Their measurements dramatically revised the sizes of the planets from what was originally estimated, revealing their small nature.

The three planets are very close to their star, taking less than two days to orbit around it. The KOI-961 star, which is located about 130 light-years away, is a red dwarf with a diameter one-sixth that of our Sun, making it just 70 percent bigger than Jupiter.

Mini-Solar Systems could be everywhere

“This is the tiniest [planetary] system found so far,” said John Johnson, the principal investigator of the research from NASA’s Exoplanet Science Institute at the California Institute of Technology in Pasadena.

“It’s actually more similar to Jupiter and its moons in scale than any other planetary system. The discovery is further proof of the diversity of planetary systems in our galaxy.”

Red dwarfs are the most common kind of star in our Milky Way galaxy. The discovery of three rocky planets around one red dwarf suggests that the galaxy could be teeming with similar rocky planets.

“These types of systems could be ubiquitous in the universe,” said Phil Muirhead, lead author of the new study from Caltech. “This is a really exciting time for planet hunters.”

First Earth-sized planets

The discovery follows a string of recent milestones for the Kepler mission. In December 2011, scientists announced the mission’s first confirmed planet in the habitable zone of a Sun-like star: a planet 2.4 times the size of Earth called Kepler-22b.

Later in the month, the team announced the discovery of the first Earth-size planets orbiting a Sun-like star outside our Solar System, called Kepler-20e and Kepler-20f.

Chart comparing the smallest known exoplanets with Mars and Earth

The smallest known exoplanets, or planets outside the Solar System, compared with Mars and Earth.

For the latest discovery, the team obtained the sizes of the three planets (called KOI-961.01, KOI-961.02 and KOI-961.03) with the help of a well-studied twin star to KOI-961, Barnard’s Star.

By better understanding the KOI-961 star, they could then determine how big the planets must be to have caused the observed dips in starlight.

In addition to the Kepler observations and ground-based telescope measurements, the team used modelling techniques to confirm the planet discoveries.

Prior to these confirmed planets, only six other planets had been confirmed using the Kepler public data.

Adapted from information issued by NASA / JPL-Caltech.

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First Earth-size planets orbiting a Sun-like star

Comparison of Kepler-20e and Kepler-20f with Venus and Earth

Comparison of newfound planets, Kepler-20e and Kepler-20f, with Venus and Earth from our Solar System. The two Kepler planets are the first Earth-size worlds found circling a Sun-like star elsewhere in our galaxy.

  • First Earth-size planets found orbiting another Sun-like star
  • The system is 1,000 light-years from Earth
  • Three other planets already known in this system

NASA’S KEPLER MISSION has discovered the first Earth-size planets orbiting a Sun-like star outside our Solar System. The planets, called Kepler-20e and Kepler-20f, are too close to their star to be in the so-called habitable zone where liquid water could exist on a planet’s surface, but they are the smallest exoplanets ever confirmed circling a star like our Sun.

The discovery marks the next important milestone in the search for planets like Earth.

The new planets are thought to be rocky. Kepler-20e is slightly smaller than Venus, measuring 0.87 times the radius of Earth. Kepler-20f is a bit larger than Earth, measuring 1.03 times its radius.

Both planets reside in a five-planet system called Kepler-20, approximately 1,000 light-years from Earth.

Kepler-20e orbits its parent star every 6.1 days and Kepler-20f every 19.6 days. These short orbital periods mean the planets circle close to their star, and are therefore very hot, inhospitable worlds.

Kepler-20f, at 800 degrees Fahrenheit, is similar to an average day on the planet Mercury. The surface temperature of Kepler-20e, at more than 760 degrees Celsius, would melt glass.

Earth-size planets now known to exist

“The primary goal of the Kepler mission is to find Earth-sized planets in the habitable zone,” said Francois Fressin of the Harvard-Smithsonian Centre for Astrophysics, and lead author of a new study published in the journal Nature.

“This discovery demonstrates for the first time that Earth-size planets exist around other stars, and that we are able to detect them.”

Artist's impression of Kepler-20e

Artist's impression of Kepler-20e, which is about 0.87 times the radius of Earth.

The Kepler-20 system includes three other planets that are larger than Earth but smaller than Neptune. Kepler-20b, the closest planet, Kepler-20c, the third planet, and Kepler-20d, the fifth planet, orbit their star every 3.7, 10.9 and 77.6 days.

All five planets have orbits lying roughly within Mercury’s orbit in our Solar System. The host star belongs to the same G-type class as our Sun, although it is slightly smaller and cooler.

Odd planetary system

The system has an unexpected arrangement. In our Solar System, small, rocky worlds orbit close to the Sun and large, gaseous worlds orbit farther out. In comparison, the planets of Kepler-20 are organised in alternating size: large, small, large, small and large.

“The Kepler data are showing us some planetary systems have arrangements of planets very different from that seen in our Solar System,” said Jack Lissauer, planetary scientist and Kepler science team member at NASA’s Ames Research Centre.

“The analysis of Kepler data continue to reveal new insights about the diversity of planets and planetary systems within our galaxy.”

Scientists are not certain how the system evolved but they do not think the planets formed in their existing locations.

They theorise the planets formed farther from their star and then migrated inward, likely through interactions with the disc of material from which they originated.

This allowed the worlds to maintain their regular spacing despite alternating sizes.

Artist's impression of Kepler-20f

Artist's impression of Kepler-20f, which is about 1.03 times as wide as Earth.

Cosmic game of hide and seek

The Kepler space telescope detects planets and planet candidates by measuring dips in the brightness of more than 150,000 stars to search for planets crossing in front, or transiting, their stars.

The Kepler science team requires at least three transits to verify a signal as a planet.

On December 5 the team announced the discovery of Kepler-22b in the habitable zone of its parent star. It is likely to be too large to have a rocky surface.

While Kepler-20e and Kepler-20f are Earth-size, they are too close to their parent star to have liquid water on the surface.

“In the cosmic game of hide and seek, finding planets with just the right size and just the right temperature seems only a matter of time,” said Natalie Batalha, Kepler deputy science team lead and professor of astronomy and physics at San Jose State University.

“We are on the edge of our seats knowing that Kepler’s most anticipated discoveries are still to come.”

Adapted from information issued by NASA/Ames/JPL-Caltech.

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Our Sun can expect inner turmoil in old age

Artist's impression of a red giant star

Our Sun will one day become a red giant star (artist's impression), swelling up to be much bigger than it is now. Astronomers have learned that the inside and outsides of such stars are very different.

SCIENTISTS HAVE MADE a new discovery about how old stars called ‘red giants’ rotate, giving an insight into what our Sun will look like in five billion years.

The international team of scientists, including University of Sydney astronomers Professor Tim Bedding and Dr Dennis Stello, has discovered that red giants have slowed down on the outside, while their cores spin at least 10 times faster than their outer layers.

The finding, just published in the prestigious journal Nature, tells us what the Sun will look like in five billion years when it develops into a red giant.

“The heart of a star determines how it evolves, and understanding how a star rotates deep inside helps us to understand how stars like our Sun will grow old,” said Professor Tim Bedding from the University of Sydney’s School of Physics.

Using NASA’s Kepler space telescope, the team “peered” deep inside ageing red giants to make their discovery of the difference in rotation rate between the core and outer layers of the stars.

Stars and the ice skater effect

The team, led by Paul Beck from Leuven University in Belgium, analysed waves inside the stars, which appear as rhythmic variations in the surface brightness of the stars.

The effect of rotation on the frequencies of the waves is so small it took the team nearly two years of almost continuous data gathering from the Kepler satellite to make their discovery.

Cutaway diagram of a red giant star

The cores of red giant stars have been found to spin at least 10 times faster than the outer layers.

“Red giants were once stars like our Sun, but as they age their outer layers expand to more than five times their original size and cool down significantly, so they look red,” explained Dr Dennis Stello, from the University of Sydney’s School of Physics.

“The opposite actually happens to the cores of red giants, as the core contracts and becomes extremely hot and dense,” said Dr Stello.

“We’ve just discovered that the core spins much faster than the outer layers in these old stars, which makes sense when you consider what happens to other spinning things like, say, an ice skater performing pirouettes.”

“A spinning ice skater will slow down if their arms are stretched far out, like the expanded outer layers of the red giants. The ice skater will spin faster if their arms are pulled tightly to the body, like the fast spinning contracted core of red giants.”

Star quakes reveal stellar inner secrets

The Kepler space telescope—one of NASA’s most successful space missions—is searching in the constellation Cygnus for potentially habitable planets by focussing on those similar in size to Earth. It does this by carefully and individually measuring the light coming from over 100,000 stars.

“Kepler is able to detect variations in a star’s brightnessof only a few parts in a million, so its measurements are ideally suited to detect the tiny brightness fluctuations of stars,” explained Dr Stello.

Artist's impression of the Kepler spacecraft

Artist's impression of the Kepler spacecraft

“We study these variations in brightness to work out what’s going on deep inside stars. It’s called asteroseismology—just as geologists use earthquakes to explore Earth’s interior, we use star quakes to explore the interiors of stars,” said Dr Stello.

Different waves reveal information on different parts of the star, and by a detailed comparison of the depth to which these waves travel inside the star the team found the rotation rate dramatically increased towards the stellar core.

In addition to helping us understand how stars age, asteroseismology will help Kepler’s mission of discovering Earth-sized planets outside our Solar System by characterising the host stars around which these planets orbit.

Adapted from information issued by the University of Sydney / ESO / L. Calcada.

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Kepler finds planet in the habitable zone

Artist's conception illustrates Kepler-22b

This artist's conception illustrates Kepler-22b, a planet known to comfortably circle in the habitable zone of a Sun-like star. It is the first planet that NASA's Kepler mission has confirmed to orbit in a star's habitable zone—the region around a star where liquid water, a requirement for life on Earth, could persist. The planet is 2.4 times the size of Earth.

  • “Super Earth” found in its star’s “habitable zone”
  • Located 600 light-years away from our planet
  • Scientists studying 2,326 planet candidates

NASA’S KEPLER MISSION has confirmed its first planet in the “habitable zone,” the region around a star where liquid water could exist on a planet’s surface.

Kepler also has discovered more than 1,000 new planet candidates, nearly doubling its previously known count.

Ten of these candidates are near-Earth-size and orbit in the habitable zone of their host star. Candidates require follow-up observations to verify they are actual planets.

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 is about 2.4 times the radius of Earth.

Scientists don’t yet know if Kepler-22b has a predominantly rocky, gaseous or liquid composition, but its discovery is a step closer to finding Earth-like planets.

Clear confirmation

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 the habitable zone, with orbits more closely resembling those of Venus and Mars.

“This is a major milestone on the road to finding Earth’s twin,” said Douglas Hudgins, Kepler program scientist at NASA Headquarters in Washington.

Artist's impression of the Kepler space telescope

Artist's impression of the Kepler space telescope

“Kepler’s results continue to demonstrate the importance of NASA’s science missions, which aim to answer some of the biggest questions about our place in the universe.”

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.

Follow-up with ground-based telescopes

“Fortune smiled upon us with the detection of this planet,” said William Borucki, Kepler principal investigator at NASA Ames Research Centre, 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.”

The Kepler science team uses ground-based telescopes and NASA’s Spitzer Space Telescope to review observations on planet candidates the spacecraft finds.

The star field that Kepler observes in the constellations Cygnus and Lyra can only be seen from ground-based observatories in the Northern Hemisphere’s spring through early autumn.

The data from these other observations help determine which candidates can be validated as planets.

Over 1,000 new planet candidates

Kepler-22b is located 600 light-years away. 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, although it is slightly smaller and cooler.

Of the 54 habitable zone planet candidates reported in February 2011, Kepler-22b is the first to be confirmed.

The Kepler team is hosting its inaugural science conference at Ames this week, announcing 1,094 new planet candidate discoveries.

Diagram comparing our Solar System to Kepler-22

This diagram compares our own Solar System to Kepler-22, a star system containing the first "habitable zone" planet discovered by NASA's Kepler mission. The habitable zone is the sweet spot around a star where temperatures are right for water to exist in its liquid form. Liquid water is essential for life on Earth.

Since the last catalogue was released in February, the number of planet candidates identified by Kepler has increased by 89 percent and now totals 2,326.

Of these, 207 are approximately Earth-size, 680 are super Earth-size, 1,181 are Neptune-size, 203 are Jupiter-size and 55 are larger than Jupiter.

The findings, based on observations conducted May 2009 to September 2010, show a dramatic increase in the numbers of smaller-size planet candidates.

Abundant Earths out there?

Kepler observed many large planets in small orbits early in its mission, which were reflected in the February data release.

Having had more time to observe three transits of planets with longer orbital periods, the new data suggest that planets one to four times the size of Earth may be abundant in the galaxy.

The number of Earth-size, and super Earth-size candidates, has increased by more than 200 and 140 percent since February, respectively.

There are 48 planet candidates in their star’s habitable zone.

While this is a decrease from the 54 reported in February, the Kepler team has applied a stricter definition of what constitutes a habitable zone in the new catalogue, to account for the warming effect of atmospheres, which would move the zone away from the star, out to longer orbital periods.

“The tremendous growth in the number of Earth-size candidates tells us that we’re honing in on the planets Kepler was designed to detect: those that are not only Earth-size, but also are potentially habitable,” said Natalie Batalha, Kepler deputy science team lead at San Jose State University.

“The more data we collect, the keener our eye for finding the smallest planets out at longer orbital periods.”

Adapted from information issued by NASA/Ames/JPL-Caltech.

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“Star Wars” planet discovered

Artist's concept of Kepler-16b

This artist's concept shows Kepler-16b (black circle), dubbed the most "Tatooine-like" planet yet found in our galaxy because it orbits two stars (a big yellow one, and a small red one). Tatooine is the name of Luke Skywalker's home world in the science fiction movie Star Wars. In this case, the planet it not thought to be habitable. It is a cold world, with a gaseous surface.

THE EXISTENCE OF A WORLD with a double sunset, as portrayed in the film Star Wars more than 30 years ago, is now scientific fact. NASA’s Kepler mission has made the first unambiguous detection of a Saturn-sized ‘circumbinary’ planet—a planet orbiting two stars—200 light-years from Earth.

Unlike Star Wars’ Tatooine, the planet is cold, gaseous and not thought to harbour life, but its discovery demonstrates the diversity of planets in our galaxy.

Previous research has hinted at the existence of circumbinary planets, but clear confirmation proved elusive. Kepler detected such a planet, known as Kepler-16b, by observing transits, where the brightness of a parent star dims from the planet crossing in front of it.

“This discovery confirms a new class of planetary systems that could harbour life,” Kepler principal investigator William Borucki said. “Given that most stars in our galaxy are part of a binary system, this means the opportunities for life are much broader than if planets form only around single stars.”

“This milestone discovery confirms a theory that scientists have had for decades but could not prove until now.”

Hunting in the ‘habitable zone’

A research team led by Laurance Doyle of the SETI Institute in Mountain View, California, used data from the Kepler space telescope, which measures dips in the brightness of more than 150,000 stars, to search for transiting planets.

Kepler is the first NASA mission capable of finding Earth-size planets in or near the “habitable zone,” the region in a planetary system where liquid water can exist on the surface of the orbiting planet.

Artist's concept of Kepler-16b

This artist's concept illustrates Kepler-16b, the first planet known to definitively orbit two stars—what's called a circumbinary planet. The planet, which can be seen in the foreground, was discovered by NASA's Kepler mission.

Scientists detected the new planet in the Kepler-16 system, a pair of orbiting stars that eclipse each other from our vantage point on Earth.

When the smaller star partially blocks the larger star, a primary eclipse occurs, and a secondary eclipse occurs when the smaller star is occulted, or completely blocked, by the larger star.

Astronomers further observed that the brightness of the system dipped even when the stars were not eclipsing one another, hinting at the presence of a third body.

Two stars for the price of one

The additional dimming in brightness events, called the tertiary and quaternary eclipses, reappeared at irregular intervals of time, indicating the stars were in different positions in their orbit each time the third body passed.

This showed the third body was circling, not just one, but both stars, in a wide circumbinary orbit.

The gravitational tug on the stars, measured by changes in their eclipse times, was a good indicator of the mass of the third body. Only a very slight gravitational pull was detected, one that only could be caused by a small mass.

“Most of what we know about the sizes of stars comes from such eclipsing binary systems, and most of what we know about the size of planets comes from transits,” said Doyle, who also is the lead author and a Kepler participating scientist.

“Kepler-16 combines the best of both worlds, with stellar eclipses and planetary transits in one system.”

Here’s a short animation that shows the planet’s strange orbit:

More than we can imagine

This discovery confirms that Kepler-16b is an inhospitable, cold world about the size of Saturn and thought to be made up of about half rock and half gas.

The parent stars are smaller than our Sun. One is 69 percent the mass of the Sun and the other only 20 percent.

Kepler-16b orbits around both stars every 229 days, similar to Venus’ 225-day orbit, but lies outside the system’s habitable zone, where liquid water could exist on the surface, because the stars are cooler than our Sun.

“Working in film, we often are tasked with creating something never before seen,” said visual effects supervisor John Knoll of Industrial Light & Magic, a division of Lucasfilm Ltd., in San Francisco. “However, more often than not, scientific discoveries prove to be more spectacular than anything we dare imagine.”

“There is no doubt these discoveries influence and inspire storytellers,” added Knoll. “Their very existence serves as cause to dream bigger and open our minds to new possibilities beyond what we think we know.’”

Adapted from information issued by NASA. Images courtesy NASA / JPL-Caltech / T. Pyle, and NASA / JPL-Caltech / R. Hurt.

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Alien world is blacker than coal

Artist's conception of exoplanet TrES-2b

Exoplanet TrES-2b (artist's conception) is darker than the blackest coal. This Jupiter-sized world reflects less than 1% of the light that falls on it, making it blacker than any planet or moon in our Solar System.

  • Exoplanet TrES-2b orbits a star 750 light-years from Earth
  • Reflects less than one percent of the starlight falling on it
  • The Jupiter-sized world puts out only a faint red glow

ASTRONOMERS HAVE DISCOVERED the darkest known exoplanet—a distant, Jupiter-sized gas giant known as TrES-2b. Their measurements show that TrES-2b reflects less than one percent of the starlight falling on it, making it blacker than coal or any planet or moon in our Solar System.

“TrES-2b is considerably less reflective than black acrylic paint, so it’s truly an alien world,” said astronomer David Kipping of the Harvard-Smithsonian Centre for Astrophysics (CfA), lead author on the paper reporting the research.

In our Solar System, Jupiter is swathed in bright clouds of ammonia that reflect more than a third of the sunlight reaching it. In contrast, TrES-2b (which was discovered in 2006 by the Trans-Atlantic Exoplanet Survey, or TrES) lacks reflective clouds due to its high temperature.

Pitch black planet … almost

TrES-2b orbits its star at a distance of only 4.8 million kilometres. The star’s intense light heats TrES-2b to a temperature of more than 1,000° Celsius—much too hot for ammonia clouds.

Instead, its exotic atmosphere contains light-absorbing chemicals like vaporised sodium and potassium, or gaseous titanium oxide. Yet none of these chemicals fully explain the extreme blackness of TrES-2b.

“It’s not clear what is responsible for making this planet so extraordinarily dark,” stated co-author David Spiegel of Princeton University. “However, it’s not completely pitch black. It’s so hot that it emits a faint red glow, much like a burning ember or the coils on an electric stove.”

Artist's impression of the Kepler spacecraft

Artist's impression of the Kepler spacecraft

One-sided world

Kipping and Spiegel determined the reflectivity of TrES-2b using data from NASA’s Kepler spacecraft. Kepler is designed to measure the brightnesses of distant stars with extreme precision.

The team monitored the brightness of the TrES-2 system as the planet orbited its star. They detected a subtle dimming and brightening due to the planet’s changing phase.

TrES-2b is believed to be tidally locked like our moon, so one side of the planet always faces the star. And like our moon, the planet shows changing phases as it orbits its star. This causes the total brightness of the star plus planet to vary slightly.

“By combining the impressive precision from Kepler with observations of over 50 orbits, we detected the smallest-ever change in brightness from an exoplanet—just 6 parts per million,” said Kipping. “In other words, Kepler was able to directly detect visible light coming from the planet itself.”

More where this one came from?

The extremely small fluctuations proved that TrES-2b is incredibly dark. A more reflective world would have shown larger brightness variations as its phase changed.

Kepler has located more than 1,200 planetary candidates in its field of view. Additional analysis will reveal whether any other unusually dark planets lurk in that data.

TrES-2b orbits the star GSC 03549-02811, which is located about 750 light-years from Earth. (One light-year is about 10 trillion kilometres.)

Adapted from information issued by CfA. Images courtesy David A. Aguilar (CfA) / NASA.

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