Posted on by Paul Scott Anderson

New Study Shows How Trace Elements Affect Stars’ Habitable Zones

Comparison of the habitable zone around the Sun in our solar system and around the star Gliese 581. Credit: ESO

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Habitable zones are the regions around stars, including our own Sun, where conditions are the most favourable for the development of life on any rocky planets that happen to orbit within them. Generally, they are regions where temperatures allow for liquid water to exist on the surface of these planets and are ideal for “life as we know it.” Specific conditions, due to the kind of atmosphere, geological conditions, etc. must also be taken into consideration, on a case-by-case basis.

Now, by examining trace elements in the host stars, researchers have found clues as to how the habitable zones evolve, and how those elements also influence them. To determine what elements are in a star, scientists study the wavelengths of its light. These trace elements are heavier than the hydrogen and helium gases which the star is primarily composed of. Variations in the composition of these stars are now thought to affect the habitable zones around them.

The study was led by Patrick Young, a theoretical astrophysicist and astrobiologist at Arizona State University. Young and his team presented their findings on January 11, 2012 at the annual meeting of the American Astronomical Society in Austin, Texas. He and his colleagues have examined more than a hundred dwarf stars so far.

An abundance of these elements can affect how opaque a star’s plasma is. Calcium, sodium, magnesium, aluminum and silicon have been found to also have small but significant effects on a star’s evolution – higher levels tended to result in cooler, redder stars. As Young explains, “The persistence of stars as stable objects relies on the heating of plasma in the star by nuclear fusion to produce pressure that counteracts the inward force of gravity. A higher opacity traps the energy of fusion more efficiently and results in a larger radius, cooler star. More efficient use of energy also means that nuclear burning can proceed more slowly, resulting in a longer lifetime for the star.”

The lifetime of a star’s habitable zone can also be influenced by another element – oxygen. Young continues: “The habitable lifetime of an orbit the size of Earth’s around a one-solar-mass star is only 3.5 billion years for oxygen-depleted compositions but 8.5 billion years for oxygen-rich stars. For comparison, we expect the Earth to remain habitable for another billion years or so, for about 5.5 billion years total, before the Sun becomes too luminous. Complex life on Earth arose some 3.9 billion years after its formation, so if Earth is at all representative, low-oxygen stars are perhaps less than ideal targets.”

As well as the habitable zone, the composition of a star can determine the eventual composition of any planets that form. The carbon-oxygen and magnesium-silicon ratios of stars can affect whether a planet will have magnesium or silicon-loaded clay minerals such as magnesium silicate (MgSiO3), silicon dioxide (SiO2), magnesium orthosilicate (Mg2SiO4), and magnesium oxide (MgO). A star’s composition can also play a role in whether a rocky planet might have carbon-based rock instead of silicon-based rock like our planet. Even the interior of planets could be affected, as radiocative elements would determine whether a planet has a molten core or a solid one. Plate tectonics, thought to be important for the evolution of life on Earth, depend on a molten interior.

Young and his team are now looking at 600 stars, ones that are already being targeted in exoplanet searches. They plan to produce a list of the 100 best stars which could have potentially habitable planets.

Posted on by Nancy Atkinson

Potential ‘Goldilocks’ Planet Found

A planet 6 times the mass of Earth orbits around the star Gliese 667 C, which belongs to a triple system. Credit: ESO

A new-found planet is in a ‘just-right’ location around its star where liquid water could possibly exist on the planet’s surface. A team of international astronomers have discovered a potentially habitable super-Earth orbiting a nearby star in a habitable zone, where it isn’t too hot or too cold for liquid water to exist. The planet, GJ 667Cc, has an orbital period of about 28 days and with a mass about 4.5 times that of the Earth. The star that it orbits is quite interesting. It is an M-class dwarf star and is a member of a triple star system and appears to be quite different from our Sun, relatively lacking in metallic elements.

The team said this discovery demonstrates that habitable planets could form in a greater variety of environments than previously believed.

“This was expected to be a rather unlikely star to host planets,” said Steven Vogt from UC Santa Cruz, one of the scientists involved in the discovery. “Yet there they are, around a very nearby, metal-poor example of the most common type of star in our galaxy. The detection of this planet, this nearby and this soon, implies that our galaxy must be teeming with billions of potentially habitable rocky planets.”

“This planet is the new best candidate to support liquid water and, perhaps, life as we know it,” said Guillem Anglada-Escudé, from the University of Gottingen in Germany. He was with the Carnegie Institute for Science when the planet was first discovered.

The planet orbits quite close to its parent star at 0.12 astronomical units, which is much closer than Mercury to the Sun. However, the Planetary Habitability Laboratory says the star is much dimmer and provides enough energy for the planet to possibly maintain similar terrestrial temperatures. There’s a caveat, though, that astronomers aren’t sure what the planet’s composition is, because they have not been able to measure its size; therefore, it could be a either a rocky or a gas planet. I would need to have a radius between about 1.7 and 2.2 Earth radii to be a rocky world.

The team used public data from the European Southern Observatory combined with observations from the Keck Observatory in Hawaii and the new Carnegie Planet Finder Spectrograph at the Magellan II Telescope in Chile. To follow up and verify the findings, the team used the radial velocity method to measures the small wobbles in the star’s motion caused by the gravitational tug of a planet.

“With the advent of a new generation of instruments, researchers will be able to survey many M dwarf stars for similar planets and eventually look for spectroscopic signatures of life in one of these worlds,” said Anglada-Escudé.

The star, GJ 667C is 22 light years away. It has much lower abundance of elements heavier than helium, such as iron, carbon, and silicon, as does our Sun. The other two stars (GJ 667A and B) are a pair of orange K dwarfs, with a concentration of heavy elements only 25% that of our Sun’s. Such elements are the building blocks of terrestrial planets so it was thought to be unusual for metal-depleted star systems to have an abundance of low mass planets.

Diagram of the planets orbiting the star GJ 667C. Credit: UC Santa Cruz

GJ 667C had previously been observed to have another super-Earth (GJ 667Cb) with a period of 7.2 days, although this finding was never published. This orbit is too tight, and thus hot, to support life. The new study started with the aim of obtaining the orbital parameters of this super-Earth, and came to find an additional planet.

The new planet receives 90% of the light that Earth receives. However, because most of its incoming light is in the infrared, a higher percentage of this incoming energy should be absorbed by the planet. When both these effects are taken into account, the planet is expected to absorb about the same amount of energy from its star that the Earth absorbs from the Sun. This would allow surface temperatures similar to Earth and perhaps liquid water, but this extreme cannot be confirmed without further information on the planet’s atmosphere.

The team said there is a possibility of other planets in the system, potentially a gas-giant planet and an additional super-Earth with an orbital period of 75 days. However, further observations are needed to confirm these two possibilities.

This is the fourth potentially habitiable extrasolar planet. Three were found in 2011: Gliese 581d, which scientists say is likely a rocky world about 20 ight-years away; HD 85512 b, another planet orbiting in a habitable zone is about 36 light-years away from Earth; and Kepler 22b, about 600 light-years away. Vogt was involved in the discovery of another planet in 2010 (Gliese 581g) in which he said the “chances of life on this planet are 100%,” but other astronomers have cast doubt on whether that planet even exists.

Papers:
The HARPS search for southern extra-solar planets XXXI. The M-dwarf sample, and A planetary system around the nearby M dwarf GJ 667C with at least one super-Earth in its habitable zone (will add link when it becomes available)

Sources: UC Santa Cruz, Carnegie Institute for Science, Planetary Habitability Laboratory

Posted on by Bruce Dorminey

Casting Swords into Space Observatories

Earth as seen from lunar orbit. Credit: NASA

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Editor’s note – Bruce Dorminey is a science journalist and author of Distant Wanderers: The Search for Planets beyond the Solar System.

Planet hunter extraordinaire Geoff Marcy recently let his frustration surface about the current state of the search for other habitable solar systems. Despite the phenomenal planet-finding success of NASA’s Kepler mission, Marcy, an astronomer at the University of California at Berkeley, correctly pointed out that NASA budget cuts have severely hampered the hunt for extrasolar life.

A decade ago, only a few dozen extrasolar planets had been detected. Today, by some recent gravitational microlensing estimates, there are more planets than stars in the Milky Way. But without the ability to characterize these extrasolar planetary atmospheres from space, we are astrobiologically hamstrung.

NASA’s goal had been that by 2020, we would have a pretty good idea about how frequently terrestrial Earth-mass planets orbit other stars — whether those planets have atmospheres that resemble our own; and, more crucially, whether those atmospheres exhibit the telltale signs of planets harboring life.

But consider how the federal government spends our tax dollars on a daily basis. Each and every day for more than a decade, the U.S. military spent roughly $1 billion a day funding congressionally-undeclared wars in Iraq and Afghanistan.

In contrast, NASA’s cancelled SIM and TPF missions were both originally estimated to have cost less than $1.5 billion dollars each.

Artist concept of the now-cancelled Terrestrial Planet Finder mission. Credit: NASA

SIM, the Space Interferometry Mission, was to have focused on finding extrasolar earths in a targeted search; its follow-on mission, NASA’s TPF, the Terrestrial Planet Finder mission, was to have characterized the atmospheres of these earth twins in an attempt to remotely detect the signatures of life.

The astronomical community continues to be resourceful as it can in working around these problems. But if NASA had followed through with the SIM and TPF missions in the timeframe that it first announced, we would have a very good idea of our own earth’s galactic pecking order by now.

Instead, war-funding has taken priority. On the home front, we’ve let the attacks of 9/11 take us down a road that has resulted in our airports resembling Orwellian netherworlds. Most of us now accept that we must basically disrobe and be physically prodded before boarding an aircraft.

Kids born at the beginning of what was supposed to be a great new millennium — remember 2001: A Space Odyssey, anyone? — have instead grown up accustomed to running the gauntlet just to take their teddy bears onto the plane with them.

Contrast the country’s current poisoned national mood with the heady days of euphoria surrounding this country’s Moon shots.

Dare we attempt to again turn at least a portion of our swords back into ploughshares?

If the U.S. is going to continue to lead the world in science and technology, the country will have to quit living in a state of perpetual geopolitical paranoia and take space seriously again.

No one wants to turn a blind eye to our national defense and NASA may never return to its glory days. But something is amiss when within a generation, we’ve gone from John F. Kennedy pointedly challenging the nation to test its mettle by safely sending a man to the moon and back before the end of the decade to this current era of national teeth gnashing.

Newt Gingrich was openly ridiculed on the morning TV news shows for advocating that the U.S. use private enterprise to help us put a manned lunar colony on the moon. Mitt Romney responded that he’d fire any employee that walked into his office and suggested such a plan.

Perhaps Gingrich is not the ideal messenger for jumpstarting a long dormant manned lunar program. But our country has reached a sad nadir when a presidential candidate is publicly mocked for advocating the hard work of boldly revamping our national space policy.

Posted on by Paul Scott Anderson

New Research Suggests Fomalhaut b May Not Be a Planet After All

The Fomalhaut b photograph. Credit: NASA, ESA, and P. Kalas (University of California, Berkeley, USA)

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When the Hubble Space Telescope photographed the apparent exoplanet Fomalhaut b in 2008, it was regarded as the first visible light image obtained of a planet orbiting another star. The breakthrough was announced by a research team led by Paul Kalas of the University of California, Berkeley. The planet was estimated to be approximately the size of Saturn, but no more than three times Jupiter’s mass, or perhaps smaller than Saturn according to some other studies, and might even have rings. It resides within a debris ring which encircles the star Fomalhaut, about 25 light-years away.

Another team at Princeton, however, has just announced that they believe the original findings are in error, and that the planet is actually a dust cloud, based on new observations by the Spitzer Space Telescope. Their paper has just been accepted by the Astrophysical Journal.

According to the abstract:

The nearby A4-type star Fomalhaut hosts a debris belt in the form of an eccentric ring, which is thought to be caused by dynamical influence from a giant planet companion. In 2008, a detection of a point-source inside the inner edge of the ring was reported and was interpreted as a direct image of the planet, named Fomalhaut b. The detection was made at ~600–800 nm, but no corresponding signatures were found in the near-infrared range, where the bulk emission of such a planet should be expected. Here we present deep observations of Fomalhaut with Spitzer/IRAC at 4.5 µm, using a novel PSF subtraction technique based on ADI and LOCI, in order to substantially improve the Spitzer contrast at small separations. The results provide more than an order of magnitude improvement in the upper flux limit of Fomalhaut b and exclude the possibility that any flux from a giant planet surface contributes to the observed flux at visible wavelengths. This renders any direct connection between the observed light source and the dynamically inferred giant planet highly unlikely. We discuss several possible interpretations of the total body of observations of the Fomalhaut system, and find that the interpretation that best matches the available data for the observed source is scattered light from transient or semi-transient dust cloud.

Kalas has responded to the new study, saying that they considered the dust cloud possibility but ruled it out for various reasons. For one thing, Spitzer lacks the light sensitivity to detect a Saturn-sized planet, and bright rings could also explain the optical characteristics observed. He says, “We welcome the new Spitzer data, but we don’t really agree with this interpretation.”

The Princeton team, interestingly, thinks that there may be a real planet orbiting Fomalhaut, but still hiding from detection. From the paper:

In particular, we find that there is almost certainly no direct flux from a planet contributing to the visible-light signature. This, in combination with the existing body of data for the Fomalhaut system, strongly implies that the dynamically inferred giant planet companion and the visible-light point source are physically unrelated. This in turn implies that the ‘real’ Fomalhaut b still hides in the system. Although we do find a tentative point source in our images that could in principle correspond to this object, its significance is too low to distinguish whether it is real or not at this point.

A resolution to the debate may come from the James Webb Space Telescope, scheduled to launch in 2018.

Of course it will be disappointing if Fomalhaut b does turn out to not be a planet after all, but let’s not forget that thousands of other ones are being discovered and confirmed. There may occasionally be hits-and-misses, but so far the planetary hunt overall has been nothing short of a home run…

The paper is available here.

Posted on by Mike Simonsen

Goldilocks Moons

The Goldilocks Zones around various type stars. Credit: NASA/JPL-Caltech

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The search for extraterrestrial life outside our Solar System is currently focused on extrasolar planets within the ‘habitable zones’ of exoplanetary systems around stars similar to the Sun. Finding Earth-like planets around other stars is the primary goal of NASA’s Kepler Mission.

The habitable zone (HZ) around a star is defined as the range of distances over which liquid water could exist on the surface of a terrestrial planet, given a dense enough atmosphere. Terrestrial planets are generally defined as rocky and similar to Earth in size and mass. A visualization of the habitable zones around stars of different diameters and brightness and temperature is shown here. The red region is too hot, the blue region is too cold, but the green region is just right for liquid water. Because it can be described this way, the HZ is also referred to as the “Goldilocks Zone”.

Normally, we think of planets around other stars as being similar to our solar system, where a retinue of planets orbits a single star. Although theoretically possible, scientists debated whether or not planets would ever be found around pairs of stars or multiple star systems. Then, in September, 2011, researchers at NASA’s Kepler mission announced the discovery of Kepler-16b, a cold, gaseous, Saturn-sized planet that orbits a pair of stars, like Star Wars’ fictional Tatooine.

This week I had the chance to interview one of the young guns studying exoplanets, Billy Quarles. Monday, Billy and his co-authors, professor Zdzislaw Musielak and associate professor Manfred Cuntz, presented their findings on the possibility of Earth-like planets inside the habitable zones of Kepler 16 and other circumbinary star systems, at the AAS meeting in Austin, Texas.

The Goldilocks Zones around various type stars. Credit: NASA/JPL-Caltech

“To define the habitable zone we calculate the amount of flux that is incident on an object at a given distance,” Billy explained. “We also took into account that different planets with different atmospheres will retain heat differently. A planet with a really weak greenhouse effect can be closer in to the stars. For a planet with a much stronger greenhouse effect, the habitable zone will be further out.”

“In our particular study, we have a planet orbiting two stars. One of the stars is much brighter than the other. So much brighter, that we ignored the flux coming from the smaller fainter companion star altogether. So our definition of the habitable zone in this case is a conservative estimate.”

Quarles and his colleagues performed extensive numerical studies on the long-term stability of planetary orbits within the Kepler 16 HZ. “The stability of the planetary orbit depends on the distance from the binary stars,” said Quarles. “The further out the more stable they tend to be, because there is less perturbation from the secondary star.”

For the Kepler 16 system, planetary orbits around the primary star are only stable out to 0.0675 AU (astronomical units). “That is well inside the inner limit of habitability, where the runaway greenhouse effect takes over,” Billy explained. This all but rules out the possibility of habitable planets in close orbit around the primary star of the pair. What they found was that orbits in the Goldilocks Zone farther out, around the pair of Kepler 16’s low-mass stars, are stable on time scales of a million years or more, providing the possibility that life could evolve on a planet within that HZ.

Kepler 16's orbit from Quarles et al

Kepler 16b’s roughly circular orbit, about 65 million miles from the stars, is on the outer edge of this habitable zone. Being a gas giant, 16b is not a habitable terrestrial planet. However, an Earth-like moon, a Goldilocks Moon, in orbit around this planet could sustain life if it were massive enough to retain an Earth-like atmosphere. “We determined that a habitable exomoon is possible in orbit around Kepler-16b,” Quarles said.

I asked Quarles how stellar evolution impacts these Goldilocks Zones. He told me, “There are a number of things to consider over the lifetime of a system. One of them is how the star evolves over time. In most cases the habitable zone starts out close and then slowly drifts out.”

During a star’s main sequence lifetime, nuclear burning of hydrogen builds up helium in its core, causing an increase in pressure and temperature. This occurs more rapidly in stars that are more massive and lower in metallicity. These changes affect the outer regions of the star, which results in a steady increase in luminosity and effective temperature. The star becomes more luminous, causing the HZ to move outwards. This movement could result in a planet within the HZ at the beginning of a star’s main sequence lifetime, to become too hot, and eventually, uninhabitable. Similarly, an inhospitable planet originally outside the HZ, may thaw out and enable life to commence.

“For our study, we ignored the stellar evolution part,” said lead author, Quarles. “We ran our models for a million years to see where the habitable zone was for that part of the star’s life cycle.”

Being at the right distance from its star is only one of the necessary conditions required for a planet to be habitable. Habitable conditions on a planet require various geophysical and geochemical conditions. Many factors can prevent, or impede, habitability. For example, the planet may lack water, gravity may be too weak to retain a dense atmosphere, the rate of large impacts may be too high, or the minimum ingredients necessary for life (still up for debate) may not be there.

One thing is clear. Even with all the requirements for life as we know it, there appear to be plenty of planets around other stars, and very likely, Goldilocks Moons around planets, orbiting within the habitable zones of stars in our galaxy, that detecting the signature of life in the atmosphere of a planet or moon around another Sun seems like only a matter of time now.

Posted on by Amy Shira Teitel

Astronomers Find Saturn’s Possible Cosmic Doppelgänger

Credit: Michael Osadciw/University of Rochester

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By analyzing the silhouette of an exoplanet passing in front of its parent star some 420 light years from Earth, a team of astrophysicists has discovered an exoplanet that just might turn out to be Saturn’s cosmic doppelgänger. 

Assistant Professor of Physics and Astronomy at Rochester University Eric Mamajek and graduate student Mark Pecaut studied data from the international SuperWASP (Wide Angle Search for Planets) and All Sky Automated Survey (ASAS) project.

An artist's impression of a brown dwarf surrounded by a cloud of proto-planet dust. Image credit: JPL

They were looking at the star’s light pattern; periodic dimming is a telltale sign that a planet is passing in front of it. A spherical planet will dim a star’s light regularly. As seen from Earth, the star’s light will dim as the planet starts to cross it, getting darker until it reaches a point of maximum dimness – the point when the planet is directly between the Earth and the star. Then, the light will get brighter at the same pace as it previously dimmed.

But in December 2010, they noticed something odd. As they analyzed data gathered over a 54 day period in early 2007, the star 1SWASP J140747.93-394542.6 dimmed irregularly. The object passing in front of it couldn’t be a spherical planet, so what was it?

The object had an elliptical silhouette, it was blocking the star’s light in an intermittent and irregular pattern, and was obscuring a significant portion of the star’s light. At one point in the pass, 95 percent of the star’s light was obscured, most likely by dust.

“When I first saw the light curve, I knew we had found a very weird and unique object,” said Mamajek. “After we ruled out the eclipse being due to a spherical star or a circumstellar disk passing in front of the star, I realized that the only plausible explanation was some sort of dust ring system orbiting a smaller companion—basically a ‘Saturn on steroids.'” Rings were the likeliest culprit of the oscillating dimness in the star’s light.

“This marks the first time astronomers have detected an extrasolar ring system transiting a Sun-like star, and the first system of discrete, thin, dust rings detected around a very low-mass object outside of our solar system,” said Mamajek. But there are still some major questions about what exactly has been discovered.

A size comparison between the Sun, a low mass star, a brown dwarf, Jupiter, and the Earth. Image credit: NASA

It could be a very low-mass star, brown dwarf, or gas giant planet. But it’s still too early to know either way. To arrive at some answer, they will need to determine the object’s mass.

A planet that size will exert a gravitational pull on its star in the same way that Jupiter tugs on the Sun. The amount of wobbling this gravitational interaction creates can reveal the mass of the object and give astronomers a clue about what it might be. If it has a mass between 13 and 75 times that of Jupiter, it will likely be a brown dwarf. If it’s any smaller, astronomers will know that it’s likely a planet more similar to Saturn.

Two of Saturn's shepherd moons keep the planet's F ring in check. Image Credit: NASA/JPL

The object itself isn’t the only interesting part of the find; Mamajek is particularly interested in the gaps between the apparent rings that are optically similar to those around Saturn. Gaps are usually indicative of objects within the rings with enough gravitational influence to shape them, like Saturn’s shepherd moons.

But even if the rings turn out to be a cloud of dust, the discovery will be no less exciting. If the object turns out to be a brown dwarf with a cloud of dust, Mamajek thinks it’s likely his team has observed the late stages of planet formation. Or, if the object is a large planet, they may be observing the formation of moons around the giant planet.

Either way it’s an awesome discovery. As cool as finding Saturn’s twin would be, watching moons form around another planet would be equally fascinating. The team’s findings will be published in an upcoming issue of the Astronomical Journal.

Source: University of Rochester.

Posted on by Paul Scott Anderson

Tatooine the Sequel: Kepler Finds Two More Exoplanets Orbiting Binary Stars

Artist's conception of the Kepler-35 system. Lynette Cook / extrasolar.spaceart.org

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For exoplanet fans, this week has been an exciting one, with some amazing new discoveries being announced at the American Astronomical Society meeting in Austin, Texas – our galaxy is brimming with planets, probably billions, and the smallest known planets have been found (again), with one about the size of Mars. But that’s not all; it was also announced that Kepler has found not one but two more planets orbiting binary stars!

The two star systems are Kepler-34 and Kepler-35; they consist of double stars which orbit each other and are about 4,900 and 5,400 light-years from Earth. The two new planets, Kepler-34b and Kepler-35b, each orbit one of these pairs of stars and are both about the size of Saturn. Since they orbit fairly close to their stars, they are not in the habitable zones; Kepler 34-b completes an orbit in 289 days and Kepler-35b in 131 days. It’s more the fact that they orbit double stars that makes them so interesting.

This is now the third planet found in a binary star system. The first, Kepler-16b, was nicknamed Tatooine as it was reminiscent of the world orbiting two suns in the Star Wars films. Until recently, it was unknown if any such star systems had planetary companions. It was considered possible, although unlikely, and remained only a theory. But now, the view is that there may indeed be a lot of them out there, just as planets are now apparently common around single stars. That’s good news for planet-hunters, as most stars in our galaxy are binaries.

According to William Welsh of San Diego State University who participated in the study, “This work further establishes that such ‘two sun’ planets are not rare exceptions, but may in fact be common, with many millions existing in our galaxy. This discovery broadens the hunting ground for systems that could support life.”

Eric B. Ford, associate professor of astronomy at the University of Florida, stated: “We have long believed these kinds of planets to be possible, but they have been very difficult to detect for various technical reasons. With the discoveries of Kepler-16b, 34b and 35b, the Kepler mission has shown that the galaxy abounds with millions of planets orbiting two stars.”

The hope now is that Kepler will continue until 2016 to be able to further refine its findings so far. That will require a mission extension, but scientists involved are optimistic they will get it.

According to Ford, “Astronomers are practically begging NASA to extend the Kepler mission until 2016, so it can characterize the masses and orbits of Earth-size planets in the habitable zone. Kepler is revolutionizing so many fields, not just planetary science. It would be a shame not to maximize the scientific return of this great observatory. Hopefully common sense will prevail and the mission will continue.”

Yes, indeed.

The study was published January 11, 2012 in the journal Nature (payment or subscription required for access to full article).

See also PhysOrg.com for a good overview of the new findings.

Posted on by Nancy Atkinson

Microlensing Study Says Every Star in the Milky Way has Planets

This artists’s cartoon view gives an impression of how common planets are around the stars in the Milky Way. The planets, their orbits and their host stars are all vastly magnified compared to their real separations. A six-year search that surveyed millions of stars using the microlensing technique concluded that planets around stars are the rule rather than the exception. The average number of planets per star is greater than one. Credit: ESO/M. Kornmesser

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How common are planets in the Milky Way? A new study using gravitational microlensing suggests that every star in our night sky has at least one planet circling it. “We used to think that the Earth might be unique in our galaxy,” said Daniel Kubas, a co-lead author of a paper that appears this week in the journal Nature. “But now it seems that there are literally billions of planets with masses similar to Earth orbiting stars in the Milky Way.”

Over the past 16 years, astronomers have detected more than 3,035 exoplanets – 2,326 candidates and 709 confirmed planets orbiting other stars. Most of these extrasolar planets have been discovered using the radial velocity method (detecting the effect of the gravitational pull of the planet on its host star) or the transit method (catching the planet as it passes in front of its star, slightly dimming it.) Those two methods usually tend to find large planets that are relatively close to their parent star.

But another method, gravitational microlensing — where the light from the background star is amplified by the gravity of the foreground star, which then acts as a magnifying glass — is able to find planets over a wide range of mass that are further away from their stars.

Gravitational microlensing method requires that you have two stars that lie on a straight line in relation to us here on Earth. Then the light from the background star is amplified by the gravity of the foreground star, which thus acts as a magnifying glass.

An international team of astronomers used the technique of gravitational microlensing in six-year search that surveyed millions of stars. “We conclude that stars are orbited by planets as a rule, rather than the exception,” the team wrote in their paper.

“We have searched for evidence for exoplanets in six years of microlensing observations,” said lead author Arnaud Cassan from the Institut de Astrophysique in Paris. “Remarkably, these data show that planets are more common than stars in our galaxy. We also found that lighter planets, such as super-Earths or cool Neptunes, must be more common than heavier ones.”

The Milky Way above the dome of the Danish 1.54-metre telescope at ESO's La Silla Observatory in Chile. The central part of the Milky Way is visible behind the dome of the ESO 3.6-metre telescope in the distance. On the right the Magellanic Clouds can be seen. This telescope was a major contributor to the PLANET project to search for exoplanets using microlensing. The picture was taken using a normal digital camera with a total exposure time of 15 minutes. Credit: ESO/Z. Bardon

The astronomers surveyed millions of stars looking for microlensing events, and 3,247 such events in 2002-2007 were spotted in data from the European Southern Observatory’s PLANET and OGLE searches. The precise alignment needed for microlensing is very unlikely, and statistical results were inferred from detections and non-detections on a representative subset of 440 light curves.

Three exoplanets were actually detected: a super-Earth and planets with masses comparable to Neptune and Jupiter. The team said that by microlensing standards, this is an impressive haul, and that in detecting three planets, they were either incredibly lucky despite huge odds against them, or planets are so abundant in the Milky Way that it was almost inevitable.

The astronomers then combined information about the three positive exoplanet detections with seven additional detections from earlier work, as well as the huge numbers of non-detections in the six years’ worth of data (non-detections are just as important for the statistical analysis and are much more numerous, the team said.) The conclusion was that one in six of the stars studied hosts a planet of similar mass to Jupiter, half have Neptune-mass planets and two thirds have super-Earths.

This works out to about 100 billion exoplanets in our galaxy.

The survey was sensitive to planets between 75 million kilometers and 1.5 billion kilometers from their stars (in the Solar System this range would include all the planets from Venus to Saturn) and with masses ranging from five times the Earth up to ten times Jupiter.

This also shows that microlensing is a viable way to find exoplanets. Astronomers hope to use other methods in the future to find even more planets.

“I have a list of 17 different ways to find exoplanets and only five have been used so far,” said Virginia Trimble from the University of California, Irvine and the Las Cumbres Observatory, providing commentary at the American Astronomical Scoeity meeting this week, “I expect we’ll be finding many more planets in the future.”

Sources: Nature, ESO, AAS briefing

Posted on by Paul Scott Anderson

Analysis of the First Kepler SETI Observations

Example of signals KOI 817 and KOI 812. Credit: The Search for Extra Terrestrial Intelligence at UC Berkeley

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As the Kepler space telescope begins finding its first Earth-sized exoplanets, with the ultimate goal of finding ones that are actually Earth-like, it would seem natural that the SETI (Search for Extraterrestrial Intelligence) program would take a look at them as well, in the continuing search for alien radio signals. That is exactly what SETI scientists are doing, and they’ve started releasing some of their preliminary results.

They are processing the data taken by Kepler since early 2011; some interesting signals have been found (a candidate signal is referred to as a Kepler Object of Interest or KOI), but as they are quick to point out, these signals so far can all be explained by terrestrial interference. If a single signal comes from multiple positions in the sky, as these ones do, it is most likely to be interference.

They do, however, also share characteristics which would be expected of alien artificial signals.

A couple of examples are from KOI 817 and KOI 812. They are of a very narrow frequency, as would be expected from a signal of artificial origin. They also change in frequency over time, due to the doppler effect – the motion of the alien signal source relative to the radio telescope on Earth. If a signal is found with these characteristics but also does not appear to be just interference, that would be a good candidate for an actual artificial signal of extraterrestrial origin.

These are only the results of the first observations and many more will come during the next weeks and months.

Looking for signals has always been like looking for a needle in the cosmic haystack; until now we were searching pretty much blind, starting even before we knew if there were any other planets out there or not. What if our solar system was the only one? Now we know that it is only one of many, with new estimates of billions of planets in our galaxy alone, based on early Kepler data. Plus the fact that the majority of those are thought to be smaller, rocky worlds like Earth, Mars, etc. How many of them are actually habitable is still an open question, but finding them narrows down the search, providing more probable actual targets to turn the radio telescopes toward instead of just trying to search billions of stars overall.

All twelve signal examples so far can be downloaded here (PDF).

Posted on by Paul Scott Anderson

Four New Exoplanets to Start Off the New Year!

Artist's conception of a gas giant orbiting close to its star. Credit: NASA/JPL-Caltech/T. Pyle (SSC)

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It’s only a few days into 2012 and already some new exoplanet discoveries have been announced. As 2011 ended, there were a total of 716 confirmed exoplanets and 2,326 planetary candidates, found by both orbiting space telescopes like Kepler and ground-based observatories. The pace of new discoveries has accelerated enormously in the past few years. Now there are four more confirmed exoplanets to add to the list.

The four planets, HAT-P-34b, HAT-P-35b, HAT-P-36b, HAT-P-37b all have very tight orbits around their (four different) stars, taking only 5.5, 3.6, 1.3 and 2.8 days to complete an orbit. Compare that to Mercury, which takes 87.969 days and 365 days of course for Earth.

They were found by astronomers with the Harvard-Smithsonian Center for Astrophysics which operates a network of ground-based telescopes known as the HATNet project. The first exoplanet discovery by HATNet, the planet HAT-P-1b, was in 2006.

They are all “hot jupiter” type planets, gas giants which orbit very close to their stars and so are much hotter than Earth, like Mercury in our own solar system. Mercury though, of course, is a small rocky world, but in some alien solar systems, gas giants have been found orbiting just as close to their stars, or even closer, than Mercury does here. HAT-P-34b however, may have an “outer component” and is in a very elongated orbit. The other three are more typical hot Jupiters. They were discovered using the transit method, when a planet is aligned in its orbit so that it passes in front of its star, from our viewpoint.

So what does this mean? If exoplanet discoveries continue to grow exponentially as expected, then 2012 should be a good year, not only for yet more new planets being found, but also for our understanding of these alien worlds and how such a wide variety of solar systems came to be. We’ve come a long way from 1992 and the first exoplanet discoveries and things promise to only get more exciting in the future.

If you want to get your exoplanet news quickly this year, I recommend the Exoplanet App for iPhone, iPad and iPod Touch. You can also follow @ExoplanetApp on Twitter.

The abstract and paper are here.