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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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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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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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Over 50 new planets discovered

ASTRONOMERS HAVE ANNOUNCED a rich haul of more than 50 new exoplanets, including 16 super-Earths, one of which orbits at the edge of the habitable zone of its star. By studying the properties of all the planets found so far by this project, the team has found that about 40% of stars similar to the Sun have at least one planet lighter than Saturn.

The discoveries were made using the HARPS spectrograph on the 3.6-metre telescope at the European Southern Observatory’s (ESO) La Silla Observatory in Chile.

The new exoplanets orbit nearby stars, and include sixteen super-Earths. This is the largest number of such planets ever announced at one time.

Planets with a mass between one and ten times that of the Earth are called super-Earths. There are no such planets in our Solar System, but they appear to be very common around other stars. Discoveries of such planets in the habitable zones around their stars are very exciting because—if the planet were rocky and had water, like Earth—they could potentially be an abode of life.

One of the newly discovered planets, HD 85512 b, is estimated to be only 3.6 times the mass of the Earth and is located at the edge of its star’s habitable zone — a narrow zone around a star in which water may be present in liquid form if conditions are right.

Adapted from information issued by ESO / M. Kornmesser.

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Fertile ground for other Earths

Artist's impression of a white dwarf

Astronomers have suggested that white dwarfs—old, cool, burned out stars—could be good places to look for orbiting habitable planets. (Artist's impression)

  • White dwarfs are Sun-like stars reaching the end of their lives
  • Small and cool, they could be ideal places for habitable planets
  • Sky surveys should show if white dwarfs have such planets

PLANET HUNTERS HAVE FOUND hundreds of planets outside the Solar System in the last decade, though it is unclear whether even one might be habitable.

But it could be that the best place to look for planets that can support life is around dim, dying stars called white dwarfs.

In a new paper published online in The Astrophysical Journal Letters, Eric Agol, a University of Washington associate professor of astronomy, suggests that potentially habitable planets orbiting white dwarfs could be much easier to find—if they exist—than other exoplanets located so far.

White dwarfs, cooling stars believed to be in the final stage of life, typically have about 60 percent of the mass of the Sun, but by volume they are only about the size of Earth.

Though born hot, they eventually become cooler than the Sun and emit just a fraction of its energy, so the habitable zones for their planets are significantly closer than Earth is to the Sun.

“If a planet is close enough to the star, it could have a stable temperature long enough to have liquid water at the surface—if it has water at all—and that’s a big factor for habitability,” Agol said.

A planet so close to its star could be seen using an Earth-based telescope as small as one metre wide (the largest telescope are now 8-10 metres), as the planet passes in front of, and dims the light from, the white dwarf, he said.

Red giant to white dwarf

White dwarfs evolve from stars like the Sun. When such a star’s core can no longer produce nuclear reactions that convert hydrogen to helium, it starts burning hydrogen outside the core.

That begins the transformation to a red giant, with a greatly expanded outer atmosphere that typically envelops—and destroys—any planets as close as Earth.

Finally the star sheds its outer atmosphere, leaving the glowing, gradually cooling, core as a white dwarf, with a surface temperature around 5,000 degrees Celsius.

Life cycle of a sun-like star

A star like our Sun goes through many different stages during its life, ending up as a white dwarf surrounding by a cloud of gas.

At that point, the star produces heat and light in the same way as a dying fireplace ember, though the star’s ember could last for three billion years.

Once the red giant sheds its outer atmosphere, more distant planets that were beyond the reach of that atmosphere could begin to migrate closer to the white dwarf, Agol said.

New planets also possibly could form from a ring of debris left behind by the star’s transformation.

In either case, a planet would have to move very close to the white dwarf to be habitable, perhaps 800,000 to 3.2 million kilometres from the star. That’s less than one percent of the distance from Earth to the Sun (150 million kilometres) and substantially closer than Mercury is to the Sun.

“From the planet, the star would appear slightly larger than our Sun, because it is so close, and slightly more orange, but it would look very, very similar to our Sun,” Agol said.

The planet also would be ‘tidally locked’, so the same side would always face the star and the opposite side would always be in darkness. The likely areas for habitation, he said, might be toward the edges of the light zone, nearer the dark side of the planet.

How to find other Earths

Candidate white dwarf in NGC 6397.

A candidate white dwarf star (marked in red) within the star cluster NGC 6397.

The nearest white dwarf to Earth is Sirius B at a distance of about 8.5 light years (a light year is about 9.5 trillion kilometres). It is believed to once have been five times more massive than the Sun, but now it has about the same mass as the Sun packed into the same volume as Earth.

Agol is proposing a survey of the 20,000 white dwarfs closest to Earth. Using a 1-metre telescope, he said, one star could be surveyed in 32 hours of observation.

If there is no telltale dimming of light from the star in that time, it means no planet orbiting closely enough to be habitable is passing in front of the star so that it is easily observable from Earth.

Ideally, the work could be carried out by a network of telescopes that would make successive observations of a white dwarf as it progresses through the sky.

“This could take a huge amount of time, even with such a network,” he said.

The same work could be accomplished by larger specialty telescopes, such as the Large Synoptic Survey Telescope that is planned for operations later this decade in Chile, of which the UW is a founding partner.

If it turns out that the number of white dwarfs with potential Earth-like planets is very small—say one in 1,000—that telescope still would be able to track them down efficiently.

Finding an Earth-like planet around a white dwarf could provide a meaningful place to look for life, Agol said. But it also would be a potential lifeboat for humanity if Earth, for some reason, becomes uninhabitable.

“Those are the reasons I find this project interesting,” he said. “And there’s also the question of, ‘Just how special is Earth?'”

Adapted from information issued by the University of Washington. Images courtesy ESA / Hubble Information Centre / ESO / NASA / G. Bacon (STScI) / S. Steinhöfel.

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Earth-sized Planets could be everywhere

Artist's impression of Earth-like planets

  • Galaxy could have more than 46 billion Earth-size planets
  • Small planets outnumber larger ones
  • Findings challenge theories of planet formation

Nearly one in four stars similar to the Sun may host planets as small as Earth, according to a new study funded by NASA and the University of California.

The study is the most extensive and sensitive planetary census of its kind. Astronomers used the W.M. Keck Observatory in Hawaii for five years to search 166 Sun-like stars near our Solar System for planets of various sizes, ranging from three to 1,000 times the mass of Earth.

All of the planets in the study orbit close to their stars. The results show more small planets than large ones, indicating small planets are more prevalent in our Milky Way galaxy.

“We studied planets of many masses—like counting boulders, rocks and pebbles in a canyon—and found more rocks than boulders, and more pebbles than rocks. Our ground-based technology can’t see the grains of sand, the Earth-size planets, but we can estimate their numbers,” said Andrew Howard of the University of California, Berkeley, lead author of the new study.

W.M. Keck Observatory

The W.M. Keck Observatory, atop Mauna Kea in Hawaii, was used to survey 166 Sun-like stars for planets of different sizes.

“Earth-size planets in our galaxy are like grains of sand sprinkled on a beach—they are everywhere.”

The study appears in the October 29 issue of the journal Science.

The research provides a tantalising clue that potentially habitable planets could also be common. These hypothesised Earth-size worlds would orbit farther away from their stars, where conditions could be favourable for life.

NASA’s Kepler spacecraft is also surveying Sun-like stars for planets and is expected to find the first true Earth-like planets in the next few years.

Small planets outnumber large ones

Howard and his planet-hunting team, which includes principal investigator Geoff Marcy, also of the University of California, Berkeley, looked for planets within 80-light-years of Earth, using the radial velocity, or “wobble,” technique.

They measured the numbers of planets falling into five groups, ranging from 1,000 times the mass of Earth, or about three times the mass of Jupiter, down to three times the mass of Earth.

The search was confined to planets orbiting close to their stars—within 0.25 astronomical units, or a quarter of the distance between our Sun and Earth.

A distinct trend jumped out of the data—smaller planets outnumber larger ones. Only 1.6 percent of stars were found to host giant planets orbiting close in. That includes the three highest-mass planet groups in the study, or planets comparable to Saturn and Jupiter.

About 6.5 percent of stars were found to have intermediate-mass planets, with 10 to 30 times the mass of Earth—planets the size of Neptune and Uranus. And 11.8 percent had the so-called “super-Earths,” weighing in at only three to 10 times the mass of Earth.

“During planet formation, small bodies similar to asteroids and comets stick together, eventually growing to Earth-size and beyond. Not all of the planets grow large enough to become giant planets like Saturn and Jupiter,” Howard said. “It’s natural for lots of these building blocks, the small planets, to be left over in this process.”

Diagram indicating numbers of different sized planets in the Galaxy

A new survey, funded by NASA and the University of California, reveals that small planets are more common than large ones.

Life in the hot zone

The astronomers extrapolated from these survey data to estimate that 23 percent of Sun-like stars in our galaxy host even smaller planets, the Earth-sized ones, orbiting in the hot zone close to a star.

“This is the statistical fruit of years of planet-hunting work,” said Marcy. “The data tell us that our galaxy, with its roughly 200 billion stars, has at least 46 billion Earth-size planets, and that’s not counting Earth-size planets that orbit farther away from their stars in the habitable zone.”

The findings challenge a key prediction of some theories of planet formation.

Models predict a planet “desert” in the hot-zone region close to stars, or a drop in the numbers of planets with masses less than 30 times that of Earth. This desert was thought to arise because most planets form in the cool, outer region of solar systems, and only the giant planets were thought to migrate in significant numbers into the hot inner region.

The new study finds a surplus of close-in, small planets where theories had predicted a scarcity.

“We are at the cusp of understanding the frequency of Earth-sized planets among planetary systems in the solar neighbourhood,” said Mario R. Perez, Keck program scientist at NASA Headquarters in Washington.

“This work is part of a key NASA science program and will stimulate new theories to explain the significance and impact of these findings.”

Adapted from information issued by NASA / JPL-Caltech / UC Berkeley / WMKO.

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“Goldilocks” planet discovered

Artist's impression of a planet orbiting Gliese 581

An artist's impression of a planet orbiting the red dwarf star Gliese 581. Astronomers have just discovered one in the star's "habitable zone", where temperatures could be right for liquid water to exist.

  • Gliese 581g orbits in its star’s “Goldilocks” zone
  • Temperature okay for liquid water – not too hot, not too cold
  • If confirmed, will be first potentially habitable planet yet found

A team of planet-hunting astronomers, utilising the HIRES spectrometer on the W.M. Keck Observatory’s Keck I Telescope, has announced the discovery of an Earth-sized planet orbiting a nearby red dwarf star.

The new planet, known as Gliese 581g, is at a distance that places it squarely in the middle of the star’s “habitable zone” where liquid water could exist on the planet’s surface.

If confirmed, this would be the most Earth-like exoplanet and the first bona fide potentially habitable one yet discovered.

To astronomers, a “potentially habitable” planet is one that could sustain life—even the simplest of life—and not necessarily one that humans would consider a nice place to live. Habitability depends on many factors, but liquid water and an atmosphere are among the most important.

The discovery by the team, led by astronomers at the University of California, Santa Cruz, and the Carnegie Institution of Washington DC, is based on 11 years of observations made at the Keck Observatory atop Mauna Kea mountain on the Big Island of Hawaii.

“Our findings offer a very compelling case for a potentially habitable planet,” said Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz. “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.”

Diagram showing a star's habitable zone

Earth is in our Solar System's habitable or "Goldilocks" zone (blue band) where the temperature is neither too hot nor too cold for liquid water to exist on a rocky planet's surface. For hotter (whiter) or cooler (red) stars (shown at left), the zone is at a different distance.

“Advanced techniques combined with old-fashioned ground-based telescopes continue to lead the exoplanet revolution,” added Paul Butler of the Carnegie Institution.

“Our ability to find potentially habitable worlds is now limited only by our telescope time.”

Vogt and Butler lead the Lick-Carnegie Exoplanet Survey. The team’s new findings are reported in a paper to be published in the Astrophysical Journal.

Planet of perpetual night and day

The astronomers have deduced that the planet is tidally locked to the star, meaning that one side is always facing the star and basking in perpetual daylight, while the side facing away from the star is in perpetual darkness.

One effect of this is to stabilise the planet’s surface climates, according to Vogt. The most habitable zone on the planet’s surface would be the line between shadow and light (known as the “terminator”), with surface temperatures decreasing toward the dark side and increasing toward the light side.

“Any emerging life forms would have a wide range of stable climates to choose from and to evolve around, depending on their longitude,” Vogt said.

The researchers estimate that the average surface temperature of the planet is between -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.

Artist's impression of planets orbiting Gliese 581

It is now thought there are six planets circling the star Gliese 581, making it the most Solar System-like place discovered so far in the cosmos.

If Gliese 581g has a rocky composition similar to the Earth’s, its diameter would be about 1.2 to 1.4 times that of the Earth. 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.

In fact, the scientists have reported the discovery of not one but two new planets circling Gliese 581. This brings to six the number of known planets around this star, the most yet discovered in a planetary system other than our own solar system.

Like our Solar System, the planets of Gliese 581 have nearly circular orbits. Gliese 581g has a mass 3 to 4 times that of the Earth and an orbital period of just under 37 days. Its mass indicates that it is probably a rocky planet with a definite surface and that it has enough gravity to hold on to an atmosphere, according to Vogt.

A difficult discovery

Although the planets themselves can’t be seen, the effect of their gravitational pull on their parent star can be measured. It shows up as a slight movement, or radial velocity change, in the star.

Multiple planets induce complex wobbles in the star’s motion, and astronomers use sophisticated analyses to distinguish the effects of the planets and determine their orbits and masses.

“It’s really hard to detect a planet like this,” Vogt said. “Every time we measure the radial velocity, that’s an evening on the telescope, and it took more than 200 observations with a precision of about 1.6 meters per second to detect this planet.”

W.M. Keck Observatory

Domes of the twin giant telescopes of the W.M. Keck Observatory on Mauna Kea mountain on the Big Island of Hawaii.

To get that many radial velocity measurements (238 in total), Vogt’s team combined their HIRES observations with published data from another group led by the Geneva Observatory (HARPS, the High Accuracy Radial velocity Planetary Search project).

In addition to the radial velocity observations, co-authors Gregory Henry and Michael Williamson of Tennessee State University made precise night-to-night brightness measurements of the star with one of Tennessee State University’s robotic telescopes.

“Our brightness measurements verify that the radial velocity variations are caused by the new orbiting planet and not by any process within the star itself,” Henry said.

How many habitable planets are out there?

Given the relatively small number of stars that have been carefully monitored by planet hunters, this discovery has come surprisingly soon.

“If these are rare, we shouldn’t have found one so quickly and so nearby,” Vogt said.

“The number of systems with potentially habitable planets is probably on the order of ten or 20 percent, and when you multiply that by the hundreds of billions of stars in the Milky Way, that’s a large number. There could be tens of billions of these systems in our galaxy.”

Gliese 581, located 20 light years away from Earth, has a somewhat chequered history of habitable-planet claims. Two previously detected planets in the system 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 sceptical. The newly discovered planet g, however, lies right in the middle of the habitable zone.

“It’s the Goldilocks planet,” Vogt said. “That’s a well-worn analogy, but in this case it fits. We had planets on both sides of the habitable zone—one too hot and one too cold—and now we have one in the middle that’s just right.”

Adapted from information issued by W.M. Keck Observatory / ESO / L. Calçada / NASA / ESA / G. Bacon (STScI).

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