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Spotting the stars that eat Earths

SOME SUN-LIKE STARS are ‘earth-eaters.’ During their development they ingest large amounts of the kind of rocky material from which ‘terrestrial’ planets like Earth and Mars and are made.

Trey Mack, a graduate student in astronomy at Vanderbilt University, has developed a model that estimates the effect that such a diet has on a star’s chemical composition and has used it to analyse a pair of twin stars that both have their own planets.

“Trey has shown that we can actually model the chemical signature of a star in detail, element by element, and determine how that signature is changed by the ingestion of Earth-like planets,” said Vanderbilt Professor of Astronomy Keivan Stassun, who supervised the study.

A star’s chemical make-up is determined by analysing is light after it has been split into a high-resolution spectrum.

This ability will add substantially to astronomers’ understanding of the process of planet formation as well as assisting in the ongoing search for Earth-like exoplanets, according to the astronomers.

Heavy metal stars

First, some background: Stars consist of more than 98 percent hydrogen and helium. All the other elements make up less than 2 percent of their mass.

Astronomers have arbitrarily defined all the elements heavier than hydrogen and helium as ‘metals’ and have coined the term ‘metallicity’ to refer to the ratio of the relative abundance of iron to hydrogen in a star’s chemical makeup.

Since the mid-1990s, when astronomers developed the capability to detect extrasolar planets in large numbers, there have been several studies that attempt to link stars’ metallicity factor with the likelihood of planets forming in orbit around them.

In one such study, researchers at Los Alamos National Laboratory argued that stars with high metallicity are more likely to develop planetary systems than those with low metallicity.

Another study concluded that hot, Jupiter-sized planets are found predominantly circling stars with high metallicity while smaller planets are found circling stars with a wide range of metal content.

Twin star study

Building on the work of colleague Simon Schuler of the University of Tampa, , who expanded the examination of stars’ chemical composition beyond their iron content, Mack looked at the abundance of 15 specific elements relative to that of the Sun

He was particularly interested in elements like aluminium, silicon, calcium and iron that have melting points higher than 600 degrees Celsius because these are the materials that serve as building blocks for Earth-like planets.

Mack, Schuler and Stassun decided to apply this technique to the planet-hosting binary pair of stars designated HD 20781 and HD 20782. Both stars should have condensed out of the same cloud of dust and gas and so both should have started with the same chemical compositions.

This particular binary pair is the first one discovered where both stars have planets of their own.

Both of the stars in the binary pair are G-class dwarf stars similar to the Sun. One star is orbited closely by two Neptune-size planets. The other possesses a single Jupiter-size planet that follows a highly eccentric orbit.

The difference in their planetary systems make the two stars ideal for studying the connection between exoplanets and the chemical composition of their stellar hosts.

Artist’s impression of a Jupiter-like planet orbiting a star. Courtesy ESO/L. Calçada.

Artist’s impression of a Jupiter-like planet orbiting a star. Courtesy ESO/L. Calçada.

Voracious planet eaters

When they analysed the spectrum of the two stars the astronomers found that the relative abundance of the refractory elements was significantly higher than that of the Sun.

They also found that the higher the melting temperature of a particular element, the higher was its abundance, a trend that serves as a compelling signature of the ingestion of Earth-like rocky material.

They calculated that each of the twins would have had to consume an additional 10-20 Earth-masses of rocky material to produce the chemical signatures.

Specifically, the star with the Jupiter-sized planet appears to have swallowed an extra ten Earth masses while the star with the two Neptune-sized planets wolfed down an additional 20.

The results support the proposition that a star’s chemical composition and the nature of its planetary system are linked.

“Imagine that the star originally formed rocky planets like Earth. Furthermore, imagine that it also formed gas giant planets like Jupiter,” said Mack.

“The rocky planets form in the region close to the star where it is hot and the gas giants form in the outer part of the planetary system where it is cold. However, once the gas giants are fully formed, they begin to migrate inward and, as they do, their gravity begins to pull and tug on the inner rocky planets.”

“With the right amount of pulling and tugging, a gas giant can easily force a rocky planet to plunge into the star. If enough rocky planets fall into the star, they will stamp it with a particular chemical signature that we can detect.”

Systems like our own?

Following this logic, it is unlikely that either of the binary twins possesses terrestrial planets.

For one star, the two Neptune-sized planets are orbiting the star quite closely, at one-third the distance between the Earth and the Sun. For the other star the trajectory of the Jupiter-sized planet grazes the star, taking it closer than orbit of Mercury at the point of closest approach.

The astronomers speculate that the reason the star with the two Neptune-size planets ingested more terrestrial material than its twin was because the two planets were more efficient at pushing material into their star than the single Jupiter-sized planet was at pushing material into its star.

If the chemical signature of G-class stars that swallow rocky planets proves to be universal, “when we find stars with similar chemical signatures, we will be able to conclude that their planetary systems must be very different from our own and that they most likely lack inner rocky planets,” said Mack.

“And when we find stars that lack these signatures, then they are good candidates for hosting planetary systems similar to our own.”

Adapted from information issued by Vanderbilt University.

Lone star has two planets

A NEW GIANT PLANET has been located in a star system about 250 light-years from Earth. The planet, perhaps twice the mass of Jupiter, could help researchers learn more about how extrasolar planets are formed.

An extrasolar (‘beyond the Solar System’) planet is one that belongs to a star system other than our own.

The star system harbouring the new planet contains only one star, as do the other three systems with extrasolar planets analysed by San Francisco State University astronomer Stephen Kane, an assistant professor of physics and astronomy, and his colleagues. It is a surprising finding, given the high rate of multiple-star systems in our solar neighbourhood.

“There is a great interest in these stars that are known to host planets,” Kane explained, since astronomers suspect that planet formation in a multi-star system would be very different from planet formation in a single-star system like our own.

False-colour image of star HD 4230

This false-colour image shows the star HD 4230, located about 250 light-years from Earth. Measurements indicate the presence of two, unseen, planets orbiting it.

A multiple-star system “might have not one but two” flattened clouds, called discs, where planets form, he said. “Or it could be that having an extra star would be disruptive, and its gravity could cause any protoplanets to pull apart.”

Relatively few extrasolar planets have been found in multiple-star systems, “but we know that they are there,” Kane said.

A wobbling star

In the four systems studied by the researchers, using optical imaging data collected at the Gemini North observatory in Hawaii, there were some intriguing signs that perhaps a second star – or something else – was present.

In each system, the extrasolar planets were discovered by the radial velocity technique, which measures variations in the speed at which a star moves away and toward Earth, “wobbled” by the gravitational pull of a nearby cosmic body. Depending on the radial velocity signature, astronomers can calculate whether the wobble is coming from a planet or star.

In the star systems studied by Kane and his colleagues, there was a part of the radial velocity data that couldn’t be explained entirely by the pull of an orbiting planet. And at the same time, the planets that had already been discovered in these systems followed eccentric orbits, swinging away from their stars in a less circular and more elliptical fashion, “more like that of a comet,” Kane said.

An unexplained velocity

With these two clues, the researchers wondered if the radial velocity and eccentric orbits might be explained by the presence of another star in the system. But when they took a closer look at the systems, they were able to rule out the possibility that another star was perturbing the system.

Depiction of planetary orbits around HD 168443

The two solid circles marked b and c, depict the orbits of planets circling HD 168443, one of the stars studied by Stephen Kane and his colleagues. The dashed circles represent the size of the orbits of Mercury, Venus Earth and Mars in our Solar System.

“I thought we were likely to find stellar companions, and when all four didn’t have a binary star, that did surprise me,” Kane said.

But in the case of one star, HD 4230, the unexplained radial velocity appears to be coming from the pull of a previously undiscovered giant planet, the researchers report. They confirmed the planet’s presence with additional radial velocity data collected at Hawaii’s Keck observatory.

Given that the researchers did not find any stellar companions, Kane says it is very likely that the leftover radial velocity is instead a signal that there are additional planets to be found in all four systems. The researchers feel this is especially true for the system called HD 168443, where their ability to detect a companion star was very strong.

Adapted from information issued by San Francisco State University.

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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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Cloudy weather on an alien world

WEATHER FORECASTERS on exoplanet GJ 1214b would have an easy job. Today’s forecast: cloudy. Tomorrow: overcast. Extended outlook: more clouds.

The planet, which is known as GJ 1214b, is classified as a super-Earth because its mass is intermediate between that of Earth and Neptune. Recent searches for planets orbiting other stars (‘exoplanets‘) have shown that super-Earths like GJ 1214b are among the most common type of planets in the Milky Way galaxy. Because no such planets exist in our Solar System, the physical nature of super-Earths is largely unknown.

Previous studies of GJ 1214b yielded two possible interpretations of the planet’s atmosphere: it could consist entirely of water vapour or some other type of heavy molecule, or it could contain high-altitude clouds that prevent the observation of what lies underneath.

Artist's view of exoplanet GJ 1214b

An artist’s view of exoplanet GJ 1214b. The weather forecast is for cloudy skies.

But now a team of astronomers led by Laura Kreidberg and Jacob Bean of the Department of Astronomy and Astrophysics at the University of Chicago, have detected clear evidence of clouds in the atmosphere of GJ 1214b from data collected with the Hubble Space Telescope. The Hubble observations used 96 hours of telescope time spread over 11 months. This was the largest Hubble programme ever devoted to studying a single exoplanet.

The researchers describe their work as an important milestone on the road to identifying potentially habitable, Earth-like planets beyond our Solar System. The results appear in the January 2 issue of the journal Nature.

Pushing the limits

“We really pushed the limits of what is possible with Hubble to make this measurement,” said Kreidberg, a third-year graduate student and first author of the new paper. “This advance lays the foundation for characterising other Earths with similar techniques.”

“I think it’s very exciting that we can use a telescope like Hubble that was never designed with this in mind, do these kinds of observations with such exquisite precision, and really nail down some property of a small planet orbiting a distant star,” explained Bean, an assistant professor and the project’s principal investigator.

GJ 1214b is located just 40 light-years from Earth, in the direction of the constellation Ophiuchus. Because of its proximity to our Solar System and the small size of its host star, GJ 1214b is the most easily observed of the known super-Earths. It transits, or passes in front of its parent star, every 38 hours, giving scientists an opportunity to study its atmosphere as starlight filters through it.

Kreidberg, Bean and their colleagues used Hubble to precisely measure the spectrum of GJ 1214b in near-infrared light, finding what they consider definitive evidence of high clouds blanketing the planet. These clouds hide any information about the composition and behaviour of the lower atmosphere and surface.

Four planets in a row

An artist’s rendering comparing the size of GJ 1214b, another, larger exoplanet, and Earth and Neptune.

Unearthly weather

The planet was discovered in 2009 by the MEarth Project, which monitors 2,000 red dwarf stars for transiting planets. The planet was next targeted for follow-up observations to characterise its atmosphere. The first spectra, which were obtained by Bean in 2010 using a ground-based telescope, suggested that the planet’s atmosphere either was predominantly water vapour or hydrogen-dominated with high-altitude clouds.

More precise Hubble observations made in 2012 and 2013 enabled the team to distinguish between these two scenarios. The news is about what they didn’t find. The Hubble spectra revealed no chemical fingerprints whatsoever in the planet’s atmosphere. This allowed the astronomers to rule out cloud-free atmospheres made of water vapour, methane, nitrogen, carbon monoxide, or carbon dioxide.

The best explanation for the new data is that there are high-altitude clouds in the atmosphere of the planet, though their composition is unknown. Models of super-Earth atmospheres predict clouds could be made out of potassium chloride or zinc sulphide at the scorching temperatures of 230 degrees Celsius found on GJ 1214b. “You would expect very different kinds of clouds to form than you would expect, say, on Earth,” Kreidberg said.

The launch of NASA’s next major space telescope, the James Webb Space Telescope (JWST), later this decade should reveal more about such worlds, Kreidberg said. “Looking forward, JWST will be transformative,” she said. “The new capabilities of this telescope will allow us to peer through the clouds on planets like GJ 1214b. But more than that, it may open the door to studies of Earth-like planets around nearby stars.”

Adapted from information issued by the University of Chicago. Images courtesy NASA, ESA and G. Bacon.

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Close encounter could reveal planets

NASA’s Hubble Space Telescope will have two opportunities in the next few years to hunt for Earth-sized planets around the red dwarf star Proxima Centauri. The opportunities will occur in October 2014 and February 2016 when Proxima Centauri, the star nearest to our Solar System, passes in front of two other stars. Astronomers plotted Proxima Centauri’s precise path and predicted the two close encounters using data from Hubble.

Red dwarfs are the most common class of stars in our Milky Way galaxy; there are about 10 for every star like our Sun. Red dwarfs are less massive than other stars, and because lower-mass stars tend to have smaller planets, they are ideal places to go hunting for Earth-sized planets.

Previous attempts to detect planets circling Proxima Centauri have not been successful. But astronomers believe they may be able to detect smaller Earth-sized planets, if they exist, by looking for ‘microlensing’ effects during the two rare stellar alignments.

The projected motion of the red dwarf star Proxima Centauri

The projected motion of the red dwarf star Proxima Centauri (green line) over the next decade, as plotted from Hubble Space Telescope observations (the path appears looped due to Earth’s motion around the Sun. In 2014 and 2016 Proxima Centauri will pass almost in front of two background stars, affording astronomers a rare opportunity to study the warping of space by Proxima’s gravity. The amount of warping will be used to calculate a precise mass for Proxima Centauri and look for the gravitational footprint and any planets orbiting the star. Credit: NASA, ESA, K. Sahu and J. Anderson (STScI), H. Bond (STScI and Pennsylvania State University), M. Dominik (University of St. Andrews), and Digitized Sky Survey (STScI/AURA/UKSTU/AAO)

Microlensing occurs when a foreground star (the ‘lens’) passes close to our line of sight to a more distant background star (the ‘source’). The appearance of the background star may be distorted, brightened and multiplied depending on the alignment between the foreground lens and the background source.

These microlensing events, which range in duration from a few hours to a few days, will enable astronomers to precisely measure the mass of Proxima Centauri. Getting a precise determination of mass is critical to understanding a star’s temperature, diameter, intrinsic brightness and longevity.

Astronomers will measure the mass by examining images of each of the background stars to see how far the stars appear to be shifted from their real positions in the sky. The shifts will be the result of Proxima Centauri’s gravitational field warping space. The degree of shift can be used to measure Proxima Centauri’s mass; the greater the shift, the greater the mass. If the red dwarf has any planets, their gravitational fields will produce a second small position shift.

Diagram explaining microlensing as Proxima Centauri appears to pass close to a background star

The upcoming conjunction between the nearest star to our Sun, Proxima Centauri, and a distant background star. Proxima’s gravitational field distorts space like a funhouse mirror and bends the path of light from the background star. The result is that the apparent position of the star will shift slightly during Proxima Centauri’s passage, as seen in the upper right diagram. If an unseen planet is orbiting Proxima Centauri, the star’s apparent position will be further offset, as seen at lower right. Credit: A. Feild (STScI)

At a distance of 4.2 light-years from Earth, Proxima Centauri is just 0.2 light-year from the more distant binary star Alpha and Beta Centauri. These three stars are considered part of the triple-star system, though Proxima Centauri evolved in isolation from the two Sun-like companion stars.

Because Proxima Centauri is so close to Earth, the area of sky warped by its gravitation field is larger than for more distant stars. This makes it easier to look for shifts in apparent stellar position caused by this effect. However, the position shifts will be too small to be perceived by any but the most sensitive telescopes in space and on the ground. The European Space Agency’s Gaia space telescope (due for launch later this year) and the European Southern Observatory’s Very Large Telescope in Chile might be able to make measurements comparable to Hubble’s.

Adapted from information issued by NASA and the Space Telescope Science Institute.

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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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Light of an alien “super-Earth”

Artist's concept of the planet 55 Cancri e

Artist's concept of the planet 55 Cancri e, a world that orbits its star so closely—about 25 times closer than Mercury is to our Sun—that it is tidally locked with one face forever blistering under the heat of its star.

NASA’S SPITZER SPACE TELESCOPE has detected light emanating from a “super-Earth” planet beyond our Solar System for the first time. While the planet is not habitable, the detection is a historic step toward the eventual search for signs of life on other planets.

While no current telescope is able to show us an image of the planet, Spitzer can detect the spectrum of light coming from it.

The planet, called 55 Cancri e, falls into a class of planets termed super Earths, which are more massive than our home world but lighter than giant planets like Neptune. Fifty-five Cancri e is about twice as big and eight times as massive as Earth. The planet orbits a bright star, called 55 Cancri, in a mere 18 hours.

Previously, Spitzer and other telescopes were able to study the planet by analysing how the light from 55 Cancri changed as the planet passed in front of the star. In the new study, Spitzer measured how much infrared light comes from the planet itself.

The results reveal the planet is likely dark and its sun-facing side is more than 1,700 degrees Celsius, hot enough to melt metal.

Distant water world

The new information is consistent with a prior theory that 55 Cancri e is a water world: a rocky core surrounded by a layer of water in a “supercritical” state where it is both liquid and gas, and topped by a blanket of steam.

“It could be very similar to Neptune, if you pulled Neptune in toward our Sun and watched its atmosphere boil away,” said Michael Gillon of Universite de Liege in Belgium, principal investigator of the research, which appears in the Astrophysical Journal. The lead author is Brice-Olivier Demory of the Massachusetts Institute of Technology in Cambridge.

Artist's impression of the Spitzer Space Telescope

Artist's impression of the Spitzer Space Telescope

The 55 Cancri system is relatively close to Earth at 41 light-years away. It has five known planets, with 55 Cancri e being the closest to the star and tidally locked, so one side always faces the star. Spitzer discovered the star-facing side is extremely hot, indicating the planet probably does not have a substantial atmosphere to carry the sun’s heat to the unlit side.

Direct measurements

In 2005, Spitzer became the first telescope to detect light from a planet beyond our Solar System. To the surprise of many, the observatory saw the infrared light of a “hot Jupiter,” a gaseous planet much larger than the solid 55 Cancri e.

Since then, other telescopes, including NASA’s Hubble and Kepler space telescopes, have performed similar feats with gas giants using the same method.

In this method, a telescope gazes at a star as a planet circles behind it. When the planet disappears from view, the light from the star system dips ever so slightly, but enough that astronomers can determine how much light came from the planet itself. This information reveals the temperature of a planet, and, in some cases, its atmospheric components. Most other current planet-hunting methods obtain indirect measurements of a planet by observing its effects on the star.

During Spitzer’s ongoing extended mission, steps were taken to enhance its unique ability to see exoplanets, including 55 Cancri e. Those steps, which included changing the cycling of a heater and using an instrument in a new way, led to improvements in how precisely the telescope points at targets.

NASA’s James Webb Space Telescope, scheduled to launch in 2018, likely will be able to learn even more about the planet’s composition. The telescope might be able to use a similar infrared method as Spitzer to search other potentially habitable planets for signs of molecules possibly related to life.

Adapted from information issued by NASA / JPL-Caltech.

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Billions of super-Earths “out there”

Artist's impression of a planet circling a red dwarf star

Artist's impression of a planet circling the star Gliese 581. Astronomers estimate there could be tens of billions of "super-Earth" planets in our galaxy.

ROCKY PLANETS NOT MUCH BIGGER THAN EARTH are very common in the habitable zones around faint red stars, say astronomers.

The habitable zone is the distance from a star where it is neither too hot nor too cold for liquid water to exist on the surface of a rocky planet.

The international team used a “planet finder” instrument to estimate that there are tens of billions of such planets in the Milky Way galaxy alone, and probably about 100 in the Sun’s immediate neighbourhood.

This is the first direct measurement of the frequency of super-Earths around red dwarfs, which account for 80% of the stars in the Milky Way.

This first direct estimate of the number of light planets circling red dwarf stars used observations made with the HARPS spectrograph on the 3.6-metre telescope at the European Southern Observatory’s La Silla Observatory in Chile.

Super-Earths abound

The HARPS team has been searching for exoplanets orbiting the most common kind of star in the Milky Way—red dwarf stars (also known as M dwarfs. These stars are faint and cool compared to the Sun, but very common and long-lived, and therefore account for 80% of all the stars in the Milky Way.

“Our new observations with HARPS mean that about 40% of all red dwarf stars have a super-Earth orbiting in the habitable zone…,” says Xavier Bonfils (IPAG, Observatoire des Sciences de l’Univers de Grenoble, France), the leader of the team.

Diagram showing the habitable zone for small, medium and large stars.

Diagram showing the habitable zone (green area) varies depending on the size and temperature of the star. Too close in (red area) and it's too hot; too far out (blue area) and it's too cold.

“Because red dwarfs are so common—there are about 160 billion of them in the Milky Way—this leads us to the astonishing result that there are tens of billions of these planets in our galaxy alone.”

The HARPS team surveyed a carefully chosen sample of 102 red dwarf stars in the southern skies over a six-year period. A total of nine super-Earths (planets with masses between one and ten times that of Earth) were found, including two inside the habitable zones of stars Gliese 581 and Gliese 667 C respectively.

The astronomers could estimate how heavy the planets were and how far from their stars they orbited.

By combining all the data, including observations of stars that did not have planets, and looking at the fraction of existing planets that could be discovered, the team has been able to work out how common different sorts of planets are in red dwarf systems.

They find that the frequency of occurrence of super-Earths in the habitable zone is 41% with a range from 28% to 95%.

On the other hand, more massive planets, similar to Jupiter and Saturn in our Solar System, are found to be rare in red dwarf systems. Less than 12% of red dwarfs are expected to have giant planets (with masses between 100 and 1,000 times that of the Earth).

In the zone

As there are many red dwarf stars close to the Sun the new estimate means that there are probably about 100 super-Earth planets in the habitable zones around stars in the neighbourhood of the Sun at distances less than about 30 light-years.

“The habitable zone around a red dwarf, where the temperature is suitable for liquid water to exist on the surface, is much closer to [a red dwarf] star than the Earth is to the Sun,” says Stéphane Udry (Geneva Observatory and member of the team).

“But red dwarfs are known to be subject to stellar eruptions or flares, which may bathe the planet in X-rays or ultraviolet radiation, and which may make life there less likely.”

One of the planets discovered in the HARPS survey of red dwarfs is Gliese 667 Cc. This is the second planet in this triple-star system and it seems to be situated close to the centre of the habitable zone.

Artist’s impression of a sunset seen from the super-Earth Gliese 667 Cc

This artist’s impression shows a sunset seen from the super-Earth Gliese 667 Cc. The brightest star in the sky is the red dwarf Gliese 667 C, which is part of a triple star system. The other two more distant stars, Gliese 667 A and B appear in the sky also to the right. Astronomers have estimated that there are tens of billions of such rocky worlds orbiting faint red dwarf stars in the Milky Way alone.

Although this planet is more than four times heavier than the Earth it is the closest twin to Earth found so far, and almost certainly has the right conditions for the existence of liquid water on its surface.

Gliese 667 Cc is the second super-Earth planet inside the habitable zone of a red dwarf discovered during this HARPS survey, after Gliese 581d was announced in 2007 and confirmed in 2009.

“Now that we know that there are many super-Earths around nearby red dwarfs we need to identify more of them using both HARPS and future instruments,” concludes Xavier Delfosse, another member of the team.

Some of these planets are expected to pass in front of, or transit, their parent star as they orbit, and astronomers can use these transits to learn more about the planets’ atmospheres and look for signs of life.

Adapted from information issued by ESO / L. Calçada.

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Galaxy may swarm with ‘nomad’ planets

Artistic rendition of a nomad planet

A new study suggest there could be 100,000 times more free-floating planets in deep space than there are stars. (Artist's impression)

OUR GALAXY may be awash in homeless planets, wandering through space instead of orbiting a star.

In fact, there may be 100,000 times more “nomad planets” in the Milky Way than stars, according to a new study by researchers at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), a joint institute of Stanford University and the SLAC National Accelerator Laboratory.

If observations confirm the estimate, this new class of celestial objects will affect current theories of planet formation and could change our understanding of the origin and abundance of life.

“If any of these nomad planets are big enough to have a thick atmosphere, they could have trapped enough heat for bacterial life to exist,” said Louis Strigari, leader of the team that reported the result in a paper submitted to the Monthly Notices of the Royal Astronomical Society.

Although nomad planets don’t bask in the warmth of a star, they may generate heat through internal radioactive decay and tectonic activity.

Universe is riddle with planets

Searches over the past two decades have identified more than 500 planets outside our Solar System, almost all of which orbit stars.

Last year, researchers detected about a dozen nomad planets, using a technique called gravitational microlensing, which looks for stars whose light is momentarily refocused by the gravity of passing planets.

Artistic rendition of a nomad planet

This image is an artistic rendition of a nomad object wandering in interstellar space. The object is intentionally blurry to represent uncertainty about whether it has an atmosphere.

The research produced evidence that roughly two nomads exist for every typical, so-called main-sequence star in our galaxy. The new study estimates that nomads may be up to 50,000 times more common than that.

To arrive at what Strigari himself called “an astronomical number,” the KIPAC team took into account the known gravitational pull of the Milky Way galaxy, the amount of matter available to make such objects, and how that matter might divvy itself up into objects ranging from the size of Pluto to larger than Jupiter.

Not an easy task, considering no one is quite sure how these bodies form. According to Strigari, some were probably ejected from planetary systems, but research indicates that not all of them could have formed in that fashion.

“The universe is riddled with unseen planetary-mass objects that we are just now able to detect,” said Alan Boss of the Carnegie Institution for Science, who was not involved in the research.

Target for new telescopes

A good count, especially of the smaller objects, will have to wait for the next generation of big survey telescopes, especially the space-based Wide-Field Infrared Survey Telescope and the ground-based Large Synoptic Survey Telescope, both set to begin operation in the early 2020s.

Artistic rendition of the Large Synoptic Survey Telescope

The Large Synoptic Survey Telescope, set to begin operation in the early 2020s, will be able to discover numerous nomad planets.

A confirmation of the estimate could lend credence to another possibility mentioned in the paper—that as nomad planets roam their starry pastures, collisions could scatter their microbial flocks to seed life elsewhere.

“Few areas of science have excited as much popular and professional interest in recent times as the prevalence of life in the universe,” said co-author and KIPAC Director Roger Blandford.

“What is wonderful is that we can now start to address this question quantitatively by seeking more of these erstwhile planets and asteroids wandering through interstellar space, and then speculate about hitchhiking bugs.”

Adapted from information issued by Stanford University / LSST Corporation / Greg Stewart / SLAC National Accelerator Laboratory / ESO.

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