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Updated census of Sun-like stars

Artist's impression of a star like our Sun

Artist's impression of a star like our Sun with an orbiting planet in the foreground. The Kepler mission is studying such stars by tracking changes in their brightness.

  • Kepler is a space observatory that measures star brightnesses
  • Brightness oscillations reveal secrets of stars’ sizes, ages and composition

USING NASA’S KEPLER SPACE TELESCOPE, scientists have detected changes in brightness in 500 Sun-like stars, giving a much better idea about the nature and evolution of the stars.

Prior to Kepler’s launch in March 2009, astronomers had identified changes in brightness, or oscillations, of only about 25 stars similar to our Sun in size, age, composition and location within the Milky Way galaxy.

Although Kepler’s primary job is to find Earth-like planets that might be able to support life, it also provides a big boost to ‘asteroseismology’…the study of stars by measuring their natural oscillations.

Those oscillations provide clues about star basics such as mass, radius and age, as well as clues about their internal structure.

“This helps us understand more about the formation of stars and how they evolve,” said Steve Kawaler, an Iowa State University professor of physics and astronomy, a co-author of the research paper and a leader of the Kepler Asteroseismic Investigation.

“These new observations allow us to measure the detailed properties of stars at an accuracy that wasn’t possible before.”

Kepler is orbiting the Sun carrying a photometer, or light meter, to measure changes in star brightnesses. The photometer includes a telescope 94cm in diameter connected to a 95-megapixel CCD camera.

Artist's impression of the Kepler spacecraft

Artist's impression of the Kepler spacecraft

The instrument is pointed at the Cygnus-Lyra region of the Milky Way. It is expected to continuously observe about 170,000 stars for at least three and a half years.

Golden age for star studies

The Kepler Asteroseismic Investigation is using Kepler data to study different kinds of stars.

Kepler has provided astronomers with so much new information, the scientists say they’re “entering a golden era for stellar physics.”

Data from the 500 Sun-like stars gives astronomers a much better understanding of the stars, their properties and their evolution. It also gives astronomers data to test their theories, models and predictions about the stars and the galaxy. And it gives astronomers enough data to make meaningful statistical studies of the stars.

“But this is just the start of things,” Kawaler said. “This is a first broad-brush analysis of the data we’ve seen. This is a preview of this new tool and the kind of detailed census that we’ll be able to do.”

Among the projects to come are studies to determine the ages of all these Sun-like stars, and studies of the host stars of the Earth-like planets.

The investigation is led by a four-member steering committee: Kawaler, Chair Ron Gilliland of the Space Telescope Science Institute based in Baltimore, Jorgen Christensen-Dalsgaard and Hans Kjeldsen, both of Aarhus University in Denmark.

Adapted from information issued by Iowa State University. Illustration by Gabriel Perez Diaz, Instituto de Aastrofisica de Canarias (MultiMedia Service). Kepler illustration courtesy NASA.

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Starquakes reveal stars’ inner secrets

Oscillations in red giant stars

Oscillations in starlight reveal information about the internal structure of stars, in much the same way that seismologists use earthquakes to probe the Earth's interior.

  • Turbulence in stars’ interiors cause continuous ‘starquakes’
  • Long-term monitoring of starlight picks up the quakes
  • Provides a window into the internal life of stars

AUSTRALIAN ASTROPHYSICISTS from the University of Sydney are behind a major breakthrough in the study of stars known as red giants, finding a way to peer deep into their cores to discover which ones are in early infancy, which are fresh-faced teenagers, and which are facing their dying days.

The discovery, published in the latest edition of the journal Nature and made possible by observations using NASA’s powerful Kepler space telescope, is shedding new light on the evolution of stars, including our own Sun.

“Red giants are evolved stars that have exhausted the supply of hydrogen in their cores that powers nuclear fusion, and instead burn hydrogen in a surrounding shell,” said Professor Tim Bedding, the paper’s lead author. Then, “towards the end of their lives, red giants begin burning the helium in their cores.”

The Kepler space telescope has enabled Professor Bedding and colleagues to continuously study starlight from hundreds of red giants at an unprecedented level of precision for nearly a year, giving a window into the stars’ cores.

“The changes in brightness at a star’s surface is a result of turbulent motions inside that cause continuous star-quakes, creating sound waves that travel down through the interior and back to the surface,” Professor Bedding said.

Size comparison of the Sun and red giant

Red giant stars are the focus of University of Sydney research in 'asteroseismology', which aims to probe the internal life of stars.

“Under the right conditions, these waves interact with other waves trapped inside the star’s helium core,” he adds. “It is these ‘mixed’ oscillation modes that are the key to understanding a star’s particular life stage.

“By carefully measuring very subtle features of the oscillations in a star’s brightness we can see that some stars have run out of hydrogen in the centre and are now burning helium, and therefore at a later stage of life.”

Astronomer Travis Metcalfe of the US National Centre for Atmospheric Research, in a companion piece in the same Nature issue which highlights the discovery’s significance, compares red giants to Hollywood stars, whose age is not always obvious from the surface.

“During certain phases in a star’s life, its size and brightness are remarkably constant, even while profound transformations are taking place deep inside,” said Dr Metcalfe.


Professor Bedding and his colleagues work in an emerging field called asteroseismology. “In the same way that geologists use earthquakes to explore Earth’s interior, we use star quakes to explore the internal structure of stars,” he explained.

Professor Bedding said: “We are very excited about the results. We had some idea from theoretical models that these subtle oscillation patterns would be there, but this confirms our models. It allows us to tell red giants apart, and we will be able to compare the fraction of stars that are at the different stages of evolution in a way that we couldn’t before.”

Daniel Huber, a PhD student working with Professor Bedding, added: “This shows how wonderful the Kepler satellite really is. The main aim of the telescope was to find Earth-sized planets that could be habitable, but it has also provided us with a great opportunity to improve our understanding of stars.”

Adapted from information issued by the University of Sydney. Images courtesy ESO / NASA.

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Family of planets found

Artist's conception of the Kepler-11 system

This artist's conception of the newly discovered planetary system shows six planets orbiting the Sun-like star Kepler-11.

  • The star Kepler-11 has six planets
  • Some of them are not much bigger than Earth
  • Discovery was made using the Kepler space telescope

A REMARKABLE PLANETARY SYSTEM discovered by NASA’s Kepler space telescope has six planets circling a Sun-like star, including five small planets in tightly packed orbits.

Astronomers at the University of California, Santa Cruz, and their co-authors analysed the orbital characteristics of the system to determine the sizes and masses of the planets, and figure out their likely compositions.

All of the information gleaned is based on Kepler’s measurements of the changing brightness of the host star (called Kepler-11) as the planets passed in front of it, producing mini-eclipses called ‘transits’.

The five inner planets in the Kepler-11 system range from 2.3 to 13.5 times the mass of the Earth. Their orbital periods are all less than 50 Earth days, which means they are very close to their host star…so close that they would fit inside the orbit of Mercury in our Solar System.

The sixth planet is larger and farther out, with an orbital period of 118 days and an undetermined mass.

“Not only is this an amazing planetary system, it also validates a powerful new method to measure the masses of planets,” said Daniel Fabrycky, a Hubble postdoctoral fellow at UC Santa Cruz, who led the orbital analysis.

Fabrycky and Jack Lissauer, a scientist at NASA’S Ames Research Centre, are the lead authors of a paper on Kepler-11 published in the February 3 issue of Nature.

“Of the six planets, the most massive are potentially like Neptune and Uranus, but the three lowest mass planets are unlike anything we have in our Solar System,” said Jonathan Fortney, assistant professor of astronomy and astrophysics at UCSC, who led the work on understanding the structure and composition of the planets, along with UCSC graduate students Eric Lopez and Neil Miller.

Comparison of Kepler planet sizes with Earth and Jupiter

Planet comparisons. The six newly discovered Kepler-11 planets are represented by the circles on the bottom row. Earth, Jupiter, and estimated sizes of other Kepler planets are shown above. (RE stands for 'radius of Earth'.)

How the work was done

The Kepler space telescope detects planets that “transit” or pass in front of their host star, causing periodic dips in the brightness of the star as measured by the telescope’s sensitive photometer.

The amount of the brightness reduction tells scientists how big the planet is in terms of its radius. The time between transits tells them its orbital period.

To determine the planets’ masses, Fabrycky analysed slight variations in the orbital periods caused by gravitational interactions among the planets.

The timing of the transits is not perfectly regular, which is an indication that the planets are gravitationally interacting, says Fabrycky. The scientists’ computer models show that the system can remain stable on time scales of millions of years.

Previously, detections of transiting planets have been followed up with observations from powerful ground-based telescopes to confirm the planet and determine its mass using Doppler spectroscopy, which measures the “wobble” in the motion of the star caused by the gravitational tug of the planet.

Artist's drawing of the Kepler space telescope

Artist's drawing of the Kepler space telescope

With Kepler-11, however, the planets are too small and the star (2,000 light-years away) is too faint for this method to work.

And this is likely to be the case with many of the planets detected by the Kepler mission, the main goal of which is to find small, Earth-size planets in the habitable zones of their stars. See our related story.

A remarkable system

More than 100 transiting planets have been observed by Kepler and other telescopes, but the vast majority of them are Jupiter-like gas giants, and almost all of them are in (so far as is known) single-planet systems.

The Kepler-11 system is remarkable in terms of the number of planets, their small sizes, and their closely packed orbits. Before this, astronomers had determined both size and mass for only three exoplanets smaller than Neptune.

Now, this single planetary system has added five more.

The sixth planet in Kepler-11 is separated enough from the others that the orbital perturbation method can’t be used to determine its mass, Fabrycky said.

As is the case in our Solar System, all of the Kepler-11 planets orbit in more or less the same plane. This finding reinforces the idea that planets form in flattened discs of gas and dust (‘stellar discs’) spinning around a star, and the disc pattern is conserved after the planets have formed, Fabrycky said.

“The ‘coplanar’ orbits in our Solar System inspired this theory in the first place, and now we have another good example. But that and the Sun-like star are the only parts of Kepler-11 that are like the Solar System,” he said.

What are they like?

The densities of the planets provide clues to their compositions. All six planets have densities lower than Earth’s.

“It looks like the inner two could be mostly water, with possibly a thin skin of hydrogen-helium gas on top, like mini-Neptunes,” Fortney said. “The ones farther out have densities less than water, which seems to indicate significant hydrogen-helium atmospheres.”

Diagram comparing the Kepler-11 system to our Solar System

Diagram comparing the Kepler-11 system to our Solar System, showing how Kepler-11's five small, inner planets would fit within the orbit of Mercury in our Solar System.

That’s surprising, because a small, hot planet should have a hard time holding onto a lightweight atmosphere.

These planets are pretty hot because of their close orbits, and the hotter it is the more gravity you need to keep the atmosphere,” Fortney said.

“My students and I are still working on this, but our thoughts are that all these planets probably started with more massive hydrogen-helium atmospheres, and we see the remnants of those atmospheres on the ones farther out,” Fortney added. “The ones closer in have probably lost most of it.”

Comparing Neptunes and Jupiters

One reason a six-planet system is so exciting is that it allows scientists to make these kinds of comparisons among planets within the same system.

“That’s really powerful, because we can work out what’s happened to this system as a whole,” Fortney said. “Comparative planetary science is how we’ve come to understand our Solar System, so this is much better than just finding more solitary hot Jupiters around other stars.”

For example, the presence of small planets with hydrogen-helium atmospheres suggests that this system formed relatively quickly, he said. Studies indicate that stellar discs lose their hydrogen and helium gas within about 5 million years.

“So it tells us how quickly planets can form,” Fortney said.

The inner planets are so close together that it seems unlikely they formed where they are now, he added.

“At least some must have formed farther out and migrated inward. If a planet is embedded in a disc of gas, the drag on it leads to the planet spiralling inward over time. So formation and migration had to happen early on.”

Adapted from information issued by UCSC.

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Planets galore!

Artist's impression of an exoplanet system

Artist's impression of an exoplanet system. NASA's Kepler space telescope has now found 1,235 planet candidates within a small region of our Milky Way galaxy.

NASA’S KEPLER SPACE TELESCOPE has discovered its first Earth-size planet candidates and its first candidates in the habitable zone, a region where liquid water could exist on a planet’s surface.

Five of the potential planets are nearly Earth-size and orbit in the habitable zone of smaller, cooler stars than our Sun.

Candidates require follow-up observations to verify they are actual planets.

Kepler also found six confirmed planets orbiting a Sun-like star, Kepler-11. This is the largest group of transiting planets orbiting a single star yet discovered outside our Solar System. See our related story.

“In one generation we have gone from extraterrestrial planets being a mainstay of science fiction, to the present, where Kepler has helped turn science fiction into today’s reality,” said NASA Administrator Charles Bolden.

The discoveries are part of several hundred new planet candidates identified in new Kepler mission science data, released on Tuesday, February 1.

Diagram explaining a Kepler transit observation

Kepler detects the dip in starlight as a planet passes in front of a star, called a 'transit'.

A multitude of planets

Kepler looks for signs of planets by measuring tiny decreases in the brightness of stars caused by planets crossing in front of them. This is known as a transit.

The planets are too far away and too small for images of them to be made, but Kepler’s transit method can detect their presence.

The findings increase the number of planet candidates identified by Kepler to-date to 1,235. Of these, 68 are approximately Earth-size; 288 are super-Earth-size; 662 are Neptune-size; 165 are the size of Jupiter and 19 are larger than Jupiter.

Of the 54 new planet candidates found in the habitable zone, five are nearly Earth-sized. The remaining 49 habitable zone candidates range from super-Earth size—up to twice the size of Earth—to larger than Jupiter.

The findings are based on the results of observations conducted May 12 to September 17, 2009, of more than 156,000 stars in Kepler’s field of view, which covers approximately 1/400 of the sky.

“The fact that we’ve found so many planet candidates in such a tiny fraction of the sky suggests there are countless planets orbiting Sun-like stars in our galaxy,” said William Borucki of NASA’s Ames Research Centre, the mission’s science principal investigator.

“We went from zero to 68 Earth-sized planet candidates and zero to 54 candidates in the habitable zone, some of which could have moons with liquid water.”

Star with a large family

Among the stars with planetary candidates, 170 show evidence of multiple planetary candidates.

Kepler-11, located approximately 2,000 light years from Earth, is the most tightly packed planetary system yet discovered. All six of its confirmed planets have orbits smaller than Venus, and five of the six have orbits smaller than Mercury’s.

The only other star with more than one confirmed transiting planet is Kepler-9, which has three.

Diagram comparing Kepler-11 system with the Solar System

A comparison of the orbits of the six planets in the Kepler-11 system with the orbits of Mercury and Venus in our Solar System.

“Kepler-11 is a remarkable system whose architecture and dynamics provide clues about its formation,” said Jack Lissauer, a planetary scientist and Kepler science team member at Ames.

“These six planets are mixtures of rock and gases, possibly including water. The rocky material accounts for most of the planets’ mass, while the gas takes up most of their volume,” he added.

“By measuring the sizes and masses of the five inner planets, we determined they are among the lowest-mass confirmed planets beyond our Solar System.”

All of the planets orbiting Kepler-11 are larger than Earth, with the largest ones being comparable in size to Uranus and Neptune. The innermost planet, Kepler-11b, is ten times closer to its star than Earth is to the Sun.

Moving outward, the other planets are Kepler-11c, Kepler-11d, Kepler-11e, Kepler-11f, and the outermost planet, Kepler-11g, which is half as far from its star as Earth is from the Sun.

The planets Kepler-11d, Kepler-11e and Kepler-11f have a significant amount of light gas, which indicates that they formed within a few million years of the system’s formation.

Kepler field-of-view star chart

This star chart illustrates the large patch of sky that NASA's Kepler mission is staring at during its three-and-a-half-year mission. The planet hunter's full field of view covers 100 square degrees.

Follow up needed

Since transits of planets in the habitable zone of Sun-like stars occur about once a year and require three transits for verification, it is expected to take three years to locate and verify Earth-size planets orbiting Sun-like stars.

The Kepler science team uses ground-based telescopes and the Spitzer Space Telescope to review observations on planetary candidates and other objects of interest the spacecraft finds.

The star field that Kepler observes in the constellations Cygnus and Lyra can only be seen from ground-based observatories in spring through early fall. The data from these other observations help determine which candidates can be validated as planets.

“The historic milestones Kepler makes with each new discovery will determine the course of every exoplanet mission to follow,” said Douglas Hudgins, Kepler program scientist at NASA Headquarters in Washington.

Adapted from information issued by NASA. Image credits NASA, Tim Pyle, Software Bisque, G. Bacon/STScI.

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Join the hunt for alien planets!

Artist's impression of exoplanets

Artist's impression of exoplanets. Planet Hunters will let members of the public join the hunt for alien worlds.

  • Citizen scientists being asked to help identify alien Earths
  • Planet Hunters uses a simple Web-based tool to analyse data
  • Number of planets expected to quadruple in the next few years

WEB USERS AROUND THE GLOBE will be able to help professional astronomers in their search for Earth-like planets thanks to a new online citizen science project called Planet Hunters.

Planet Hunters, which is the latest in the Zooniverse citizen science project collection, will ask users to help analyse data taken by NASA’s Kepler mission. The space telescope has been searching for planets beyond our own Solar System—called exoplanets—since its launch in March 2009.

“The Kepler mission has given us another mountain of data to sort through,” said Kevin Schawinski, a Yale University astronomer and Planet Hunters co-founder.

Schawinski also helped create the Galaxy Zoo citizen science project several years ago, which enlisted hundreds of thousands of Web users around the world to help sort through and classify a million images of galaxies taken by a robotic telescope.

The Kepler space telescope is continually monitoring nearly 150,000 stars, recording their brightness over time. Astronomers analyse these images, looking for any stars that show a slight dimming of their brightness. This dimming could represent a planet passing in front of its host star, blocking a tiny fraction of its light as seen from Kepler’s vantage point in space.

Those stars that periodically dim are the best candidates for hosting relatively small planets that tightly orbit their stars, similar to Earth.

Screenshot from Planet Hunters

Screenshot from the Planet Hunters web site. Citizen scientists have to spot out-of-the-ordinary data in the plots of light intensity from a star. Such data could indicate the presence of a planet.

“The Kepler mission will likely quadruple the number of planets that have been found in the last 15 years, and it’s terrific that NASA is releasing this amazing data into the public domain,” said Debra Fischer, a Yale astronomer and leading exoplanet hunter.

Human brain better than machines

Although Planet Hunters is not tied directly to the Kepler mission, the website will serve as a complement to the work being done by the Kepler team to analyse the data.

Because of the huge amount of data being made available by Kepler, astronomers rely on computers to help them sort through the data and search for possible planet candidates.

“But computers are only good at finding what they’ve been taught to look for,” said Meg Schwamb, another Yale astronomer and Planet Hunters co-founder, “whereas the human brain has the uncanny ability to recognise patterns and immediately pick out what is strange or unique, far beyond what we can teach machines to do.”

After the success of the Galaxy Zoo project, the Yale team decided to enlist Web users once again to create what they hope will become a global network of human computing power.

When users log on to the Planet Hunters website, they’ll be asked to answer a series of simple questions about one of the stars’ light curves—a graph displaying the amount of light emitted by the star over time—to help the astronomers determine whether it displays a repetitive dimming of light, identifying it as an exoplanet candidate.

“The great thing about this project is that it gives the public a front row seat to participate in frontier scientific research,” Schwamb said.

Artist's impression of exoplanets

"The search for planets is the search for life": Debra Fisher, astronomer.

Hunt for alien life

The possibility of Earth-like planets beyond our own Solar System has captured the collective human imagination for centuries.

Today, astronomers have discovered more than 500 planets orbiting stars other than the Sun—yet almost all of these so-called exoplanets are large gas giants, similar to Jupiter, which bear little resemblance to Earth.

Ever since the first exoplanet was discovered in 1995, astronomers have raced to find ever smaller planets closer to our own world.

The search for planets is the search for life,” Fischer said. “And at least for life as we know it, that means finding a planet similar to Earth.”

Scientists believe Earth-like planets are the best place to look for life because they are the right size and orbit their host stars at the right distance to support liquid water, an essential ingredient for every form of life found on Earth.

Yet Fischer is quick to caution that, even with the exceptional data from the Kepler telescope, it will be extremely difficult to pick out the weak signal created by such a small planet as it dims its host star.

Planet Hunters is an experiment—we’re looking for the needle in the haystack,” she said.

Still, Galaxy Zoo proved that ordinary people can make extraordinary discoveries. Several Galaxy Zoo users were listed as co-authors on more than 20 published scientific papers that resulted from the citizen science project, most of whom had no prior knowledge of astronomy.

“The point of citizen science is to actively involve people in real research,” Schawinski said. “When you join Planet Hunters, you’re contributing to actual science—and you might just make a real discovery.”


Adapted from information issued by Yale University. Image credits: NASA / ESA / G. Bacon (STScI) / Planet Hunters.

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More planets found

Artist's impression of two Saturn-like planets orbiting very close to the star Kepler-9b.

Artist's impression of two Saturn-like planets orbiting very close to the star Kepler-9b.

Two planets, similar in size to Saturn, have been spotted by NASA’s Kepler space telescope moving in front of, or transiting, their star.

The transit technique has been used to detect many so-called exoplanets (ones that orbit stars beyond our Solar System), but this is the first confirmed detection via this method of two planets transiting the same star.

The star is known as Kepler-9, and the planets have been dubbed Kepler-9b and Kepler-9c.

Launched in early 2009, Kepler is focusing on over 150,000 candidate stars in the hunt for small, Earth-sized planets. In particular, the aim is to find Earth-sized planets in stars’ habitable zones…orbital slots that are neither too near nor too far from the stars, so that the planets are neither too hot nor too cold.

Kepler can’t see planets—they’re too small and too far away—but its sensitive instruments can see the small dip in starlight as a planet moves in front of its star. From the amount and duration of the dip, the size and orbit of each planet can be derived.

And if there are small deviations in the regularity of the dips, they could indicate the presence of other planets that aren’t on orbits that produce transits.

In June 2010, Kepler scientists published data on 700 potential planets seen in data collected in the first 43 days of the mission. A handful of the stars seemed to have more than one transiting planet.

“Kepler’s high quality data and round-the-clock coverage of transiting objects enable a whole host of unique measurements to be made of the parent stars and their planetary systems,” said Doug Hudgins, the Kepler program scientist at NASA Headquarters in Washington.

Artist's impression of NASA's Kepler space telescope

NASA's Kepler space telescope is dedicated to hunting for planets orbiting distant stars by spotting the dip in starlight as a planet moves in front, or transits, a star.

Scorching orbits

Follow up observations using the giant telescopes of the W.M. Keck Observatory in Hawaii have shown that both planets are slightly less massive than Saturn, and that Kepler-9b is the larger of the two.

The planets orbit very close to their star, which means their years are very short—just 19 Earth days in the case of Kepler-9b, and 38 days for Kepler-9c. Their close proximity to the star means the planets also would be very, very hot.

Seven months of observations have revealed slight variations in the timing of the two planet’s transits, which is exactly what would be expected as each planet gravitationally tugs on the other.

Their observations of the Kepler-9 system also have given the scientists tantalising hints that there could be another, much smaller planet in orbit around the star. The data suggest it could be about 1.5 times the size of Earth and orbiting scorchingly close to the star, taking just 1.6 Earth days to complete one orbit. This would make it a “hot Earth”.

The scientists still have work to do to confirm the presence of the third planet, though, as what can sometimes seem to be a transiting planet can turn out to be an unrelated phenomenon.

Story by Jonathan Nally, editor Images courtesy NASA.

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