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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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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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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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Discovery triples the number of stars

Side-by-side comparison of stars in the Milky Way and a distant galaxy

Filtering out the light from brighter stars, astronomers detected the faint signature of small, dim red dwarf stars in nearby elliptical galaxies (right), and found these are much more numerous than in our own Milky Way (left). This finding suggests that the total number of stars in the universe could be up to three times higher than previously thought. (Artist's impression)

  • Red dwarf stars seen in distant galaxies for first time
  • There are far more of them than expected
  • Increases the number of potential planets in the cosmos

ASTRONOMERS HAVE DISCOVERED that small, dim stars known as red dwarfs are much more prolific than previously thought—so much so that the total number of stars in the universe is likely three times bigger than realised.

Because red dwarfs are relatively small and dim compared to stars like our Sun, astronomers hadn’t been able to detect them in galaxies other than our own Milky Way and its nearest neighbours before now.

As such, they did not know how much of the total stellar population of the universe is made up of red dwarfs.

Now astronomers have used powerful instruments on the Keck Observatory in Hawaii to detect the faint signature of red dwarfs in eight massive, relatively nearby galaxies called elliptical galaxies, which are located between about 50 million and 300 million light-years away.

They discovered that the red dwarfs, which are only between 10 and 20 percent as massive as the Sun, were much more bountiful than expected.

“No one knew how many of these stars there were,” said Pieter van Dokkum, a Yale University astronomer who led the research, which is described the December 1 advanced online edition of the journal Nature.

“Different theoretical models predicted a wide range of possibilities, so this answers a longstanding question about just how abundant these stars are.”

A galaxy

More red dwarf stars in other galaxies potentially means more habitable planets.

Other galaxies are not like ours

The team discovered that there are about 20 times more red dwarfs in elliptical galaxies than in the Milky Way, said Charlie Conroy of the Harvard-Smithsonian Center for Astrophysics, who was also involved in the research.

“We usually assume other galaxies look like our own. But this suggests other conditions are possible in other galaxies,” Conroy said. “So this discovery could have a major impact on our understanding of galaxy formation and evolution.”

For instance, Conroy said, galaxies might contain less dark matter—a mysterious substance that has mass but cannot be directly observed—than previous measurements of their masses might have indicated. Instead, the abundant red dwarfs could contribute more mass than realised.

In addition to boosting the total number of stars in the universe, the discovery also increases the number of planets orbiting those stars, which in turn elevates the number of planets that might harbour life, van Dokkum said.

In fact, a recently discovered exoplanet that astronomers believe could potentially support life orbits a red dwarf star, called Gliese 581.

“There are possibly trillions of Earths orbiting these stars,” van Dokkum said, adding that the red dwarfs they discovered, which are typically more than 10 billion years old, have been around long enough for complex life to evolve.

“It’s one reason why people are interested in this type of star.”

Adapted from information issued by Yale University / NASA / ESA.

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