exoplanets Archives

April 5, 2012April 26, 2016 by Nancy Atkinson

How Would Humans Respond to First Contact from an Alien World?

Artist concept of an exoplanet. Credit: David A. Hardy.

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According to Star Trek lore, it is only 51 years until humans encounter their first contact with an alien species. In the movie “Star Trek: First Contact,” on April 5, 2063, Vulcans pay a visit to an Earth recovering from a war-torn period (see the movie clip below.) But will such a planet-wide, history-changing event ever really take place? If you are logical, like Spock and his Vulcan species, science points towards the inevitability of first contact. This is according to journalist Marc Kaufman, who is a science writer for the Washington Post and author of the book “First Contact: Scientific Breakthroughs in the Hunt for life Beyond Earth.” He writes that from humanity’s point of view, first contact would be a “harbinger of a new frontier in a dramatically changed cosmos.”

What are some of the arguments for and against the likelihood of first contact ever taking place and what would the implications be?

“One argument against first contact is from those who say there is no other life in the Universe,” said Kaufman, speaking to Universe Today via phone, “and with that is the Fermi paradox, which says that if there is so much life out there, why hasn’t it visited us yet? That was first posited back in the 1950’s and with everything we’ve learned since then, it seems rather presumptuous and Earth-centric to say that because no one has come to Earth, there is no life out there.”

Kaufman argues the Universe is so vast, the number of exoplanets is so huge – with the number of exoplanets in habitable zones now gaining in numbers almost daily – and we now understand that all the makings for the building blocks of life are out in space, so it defies logic to argue there is no other life out there.

Another argument against first contact states there might be microbial life elsewhere in the Universe, but it is not intelligent. “This is where the Fermi paradox comes in even more,” Kaufman said. “It certainly is true — as far as we know — that no intelligent life has made contact with Earth. But when you look at the amount of time we’ve been a technologically advanced society, it has only been a few hundred years. In the vastness of time, that is a pitifully small amount of time – truly nothing.”

In the immensity of cosmological time, Kaufman said, it is quite possible that microbial life emerged and evolved a billion years ago on another world and we missed coinciding with it, as civilizations could have come and gone.

“But all the makings are there and unless we want to say that Earth was made through divine creation or only through an unbelievable set of circumstances this is the only place in the Universe where life began, it just seems hugely, hugely implausible,” Kaufman said.

So, Kaufman says, the best, most logical argument is that life exists beyond Earth and in some instances includes what we would consider intelligence.

“If you have microbial life and billions of planets in habitable zones, the logic says that some of them will advance like we did,” Kaufman said. “There’s no reason to say that evolution is exclusive to Earth. It feels very 14th or 15th century-Earth-centric to say that we are the only place where there is intelligent life.”

Our continued scientific understanding, and in particular, the recent ongoing finding of so many exoplanets, has been a real revolution in our understanding of the cosmos, Kaufman said, and it is a huge boost to the logic of finding life elsewhere.

“It was hypothesized for decades, if not centuries that other planets were out there,” he said. “Now that we are finding planets almost every day, from a scientific perspective, it shows us that if the science is pointing in a certain direction, you just need to have the technology and the knowledge catch up to that hypothesis.”

Kaufman says that like the surge in finding exoplanets, astrobiology is likely the next area of science where breakthroughs will happen.

“Scientists almost unanimously believe there is other life out there, but we just don’t have the technology to find it yet,” he said. “Even with the recent potential cuts in NASA’s budget for planetary missions, and even if NASA is not able to send up as many missions, there is a broad movement going on in college campuses and institutes – from working on synthetic life, to studies in cosmology, and astrochemistry — all of those things are moving forward because there is a real sense that something is within reach. This area of science is just going to blossom.”

So if tomorrow (or on April 5, 2063) a spaceship shows up, how would we respond?

“On one level, I’d hope there would be a huge amount of wonder and awe and a recognition of the vastness of the Universe. But I also imagine there would be a lot of defensiveness, as well,” said Kaufman, referring to some, like Stephen Hawking, who say we shouldn’t send messages out into space — because if a more technically advanced civilization comes to Earth, the outcome for the less advanced (us) would likely be bad.

But Kaufman has hope that Earthlings would welcome a visit.

“Look at the continuing fascination of Roswell or UFOs,” he said. “Throughout history, humans have looked to the skies and thought that we’ve experienced something ‘out there’ – be it angels or gods or spaceships. There is, I believe, a deep human craving that we aren’t alone, and that would be a significant part of our response.”

For more information see Kaufman’s book, and website,”Habitable Zones”

March 28, 2012December 23, 2015 by Jason Major

“Tidal Venuses” May Have Been Wrung Out To Dry

Extreme heating from tidal stresses may render a "Tidal Venus" planet inhabitable

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Earth-sized exoplanets within a distant star’s habitable zone could still be very much uninhabitable, depending on potential tidal stresses — either past or present — that could have “squeezed out” all the water, leaving behind a bone-dry ball of rock.

New research by an international team of scientists suggests that even a moderately eccentric orbit within a star’s habitable zone could exert tidal stress on an Earth-sized planet, enough that the increased surface heating due to friction would boil off any liquid water via extreme greenhouse effect.

Such planets are dubbed “Tidal Venuses”, due to their resemblance to our own super-heated planetary neighbor. This evolutionary possibility could be a factor in determining the actual habitability of an exoplanet, regardless of how much solar heating (insolation) it receives from its star.

The research, led by Dr. Rory Barnes of the University of Washington in Seattle, states that even an exoplanet currently in a circular, stable orbit could have formed with a much more eccentric orbit, thus subjecting it to tidal forces. Any liquid water present after formation would then have been slowly but steadily evaporated and the necessary hydrogen atoms lost to space.

The risk of such a “desiccating greenhouse” effect would be much greater on exoplanets orbiting lower-luminosity stars, since any potential habitable zone would be closer in to the star and thus prone to stronger tidal forces.

And as far as such an effect working to create habitable zones further out in orbit than otherwise permissible by stellar radiation alone… well, that wouldn’t necessarily be the case.

Even if an exoplanetary version of, say, Europa, could be heated through tidal forces to maintain liquid water on or below its surface, a rocky world the size of Earth (or larger) would still likely end up being rather inhospitable.

“One couldn’t do it for an Earthlike planet — the tidal heating of the interior would likely make the surface covered by super-volcanoes,” Dr. Barnes told Universe Today.

So even though the right-sized exoplanets may be found in the so-called “Goldilocks zone” of their star, they may still not be “just right” for life as we know it.

The team’s full paper can be found here.

March 2, 2012December 23, 2015 by Jason Major

Is There Life on Earth?

An Earthshine-lit moon sets over ESO's Paranal Observatory in Chile.

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It may seem like a silly question — of course there’s life on Earth — but what if we didn’t know that? What if we were looking at Earth from another vantage point, from another planet in another star system, perhaps? Would we be able to discern then if Earth were in fact teeming with life? All we’d have to go on would be the tiniest bit of light reflected off Earth, nearly lost in the intense glare of the Sun.

Researchers have found that the secret is knowing what kind of light to look for. And they discovered this with a little help from the Moon.

eso1210c — How Earthshine works. (ESO/L. Calçada)

By using Earthshine — sunlight light reflected off Earth onto the Moon — astronomers with the European Southern Observatory have been able to discern variations that correlate with identifying factors of our planet as being a happy home for life.

In observations made with ESO’s Very Large Telescope (VLT), the presence of oceans, clouds, atmospheric gases and even plants could be detected in the reflected Earthshine.

The breakthrough method was the use of spectropolarimetry, which measures polarized light reflected from Earth. Like polarized sunglasses are able to filter out reflected glare to allow you to see clearer, spectropolarimetry can focus on light reflected off a planet, allowing scientists to more clearly identify important biological signatures.

“The light from a distant exoplanet is overwhelmed by the glare of the host star, so it’s very difficult to analyze — a bit like trying to study a grain of dust beside a powerful light bulb,” said Stefano Bagnulo of the Armagh Observatory, Northern Ireland, and co-author of the study. “But the light reflected by a planet is polarized, while the light from the host star is not. So polarimetric techniques help us to pick out the faint reflected light of an exoplanet from the dazzling starlight.”

Since we have fairly reliable proof that life does in fact exist on Earth, this provides astronomers with a process and a benchmark for locating evidence of life on other distant worlds — life as we know it, anyway.

Life in the Universe, Reflected by the Moon

This view shows the thin crescent Moon setting over ESO’s Paranal Observatory in Chile. Credit: ESO/B. Tafreshi/TWAN

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Earthshine – a poetic, fanciful word for the soft, faint glow on the Moon when the light from the Sun is reflected from the Earth’s surface, onto the dark part of the Moon. And as unlikely as it might seem, astronomers have used Earthshine to verify there’s life in the Universe: Us. While we already know about life on our own world, this technique validates that faint light from distant worlds could also be used to find potential alien life.

“We used a trick called earthshine observation to look at the Earth as if it were an exoplanet,” said Michael Sterzik from the European Southern Observatory. “The Sun shines on the Earth and this light is reflected back to the surface of the Moon. The lunar surface acts as a giant mirror and reflects the Earth’s light back to us — and this is what we have observed with the VLT (Very Large Telescope).”

Sterzik and his team said the fingerprints of life, or biosignatures, are hard to find with conventional methods, but they have now pioneered a new approach that is more sensitive. The astronomers used Earth as a benchmark for the future search for life on planets beyond our Solar System. They can analyze the faint planetshine light to look for indicators, such as certain combinations of gases in the atmosphere – as they found looking at earthshine – to find telltale signs of organic life.

Looking at earthshine, they found strong bio-signatures such as molecular oxygen and methane, as well as the presence of a ‘red edge’ caused by surface vegetation.

Instead of just looking at the planet’s reflected light, astronomers can also use spectropolarimetry, which looks at the polarization of the light. Using this approach, the biosignatures in the reflected light from Earth show up very strongly.

“The light from a distant exoplanet is overwhelmed by the glare of the host star, so it’s very difficult to analyze — a bit like trying to study a grain of dust beside a powerful light bulb,” said co-author Stefano Bagnulo from Armagh Observatory in Northern Ireland. “But the light reflected by a planet is polarised, while the light from the host star is not. So polarimetric techniques help us to pick out the faint reflected light of an exoplanet from the dazzling starlight.”

By looking at earthshine, the team was able to deduce that the Earth’s atmosphere is partly cloudy, that part of its surface is covered by oceans and — crucially — that there is vegetation present. They could even detect changes in the cloud cover and amount of vegetation at different times as different parts of the Earth reflected light towards the Moon.

“These observations allow us to determine the fractional contribution of clouds and ocean surface, and are sensitive to Spectropolarimetry unveils strong biosignatures, visible areas of vegetation as small as 10%,” the team wrote in their paper.

“Finding life outside the Solar System depends on two things: whether this life exists in the first place, and having the technical capability to detect it,” said co-author Enric Palle from Instituto de Astrofisica de Canarias, Tenerife, Spain. “This work is an important step towards reaching that capability.”

“Spectropolarimetry may ultimately tell us if simple plant life — based on photosynthetic processes — has emerged elsewhere in the Universe,” said Sterzik. “But we are certainly not looking for little green men or evidence of intelligent life.”

The astronomers said that future telescopes such as the E-ELT (the European Extremely Large Telescope), could provide more detail about the type of life beyond planets that may exists on another world.

Read the team’s paper, (pdf) which was published in Nature.

Source: ESO

February 23, 2012December 24, 2015 by Nancy Atkinson

‘Nomad’ Planets Could Outnumber Stars 100,000 to 1

An artistic rendition of a nomad object wandering the interstellar medium. Credit: Greg Stewart / SLAC National Accelerator Laboratory

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Could the number of wandering planets in our galaxy – planets not orbiting a sun — be more than the amount of stars in the Milky Way? Free-floating planets have been predicted to exist for quite some time and just last year, in May 2011, several orphan worlds were finally detected. But now, the latest research concludes there could be 100,000 times more free-floating planets in the Milky Way than stars. Even though the author of the study, Louis Strigari from the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), called the amount “an astronomical number,” he said the math is sound.

“Even though this is a large number, it is actually consistent with the amount of mass and heavy elements in our galaxy,” Strigari told Universe Today. “So even though it sounds like a big number, it puts into perspective that there could be a lot more planets and other ‘junk’ out in our galaxy than we know of at this stage.”

And by the way, these latest findings certainly do not lend any credence to the theory of a wandering planet named Nibiru.

Several studies have suggested that our galaxy could perhaps be swarming with billions of these wandering “nomad” planets, and the research that actually found a dozen or so of these objects in 2011 used microlensing to identify Jupiter-sized orphan worlds between 10,000 and 20,000 light-years away. That research concluded that based on the number of planets identified and the area studied, they estimated that there could literally be hundreds of billions of these lone planets roaming our galaxy….literally twice as many planets as there are stars.

But the new study from Kavli estimates that lost, homeless worlds may be up to 50,000 times more common than that.

Using mathematical extrapolations and relying on theoretical variables, Strigari and his 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 be distributed into objects ranging from the size of Pluto to larger than Jupiter.

“What we did was we put together the observations of what the galaxy is made of, what kind of elements it has, as well as how much mass there could possibly be that has been deduced from the gravitational pull from the stars we observed,” Stigari said via phone. “There are a couple of general bounds we used: you can’t have more nomads in the galaxy than the matter we observe, as well as you probably can’t have more than the amount of so called heavy elements than we observe in the galaxy (anything greater that helium on the periodic table).”

But any study of this type is limited by the lack of understanding of planetary formation.

“We don’t at this stage have a good theory that tells us how planets form,” Strigari said, “so it is difficult to predict from a straight theoretical model how many of these objects might be wandering around the galaxy.”

Strigari said their approach was largely empirical. “We asked how many could there possibly be, consistent with the broad constraints, that gives us a limit to how many these objects could possibly exist.”

So, in absence of any theory that really predicts how many of these things should exist, the estimate of 100,000 times the amount of stars in the Milky Way is an upper limit.

“A lot of times in science and astronomy, in order to learn what the galaxy and universe is made of, we first have to ask questions, what is it not made of, and so you start from an upper bound of how many of these planets there could be,”Strigari said. “Maybe when our data gets better we will start reducing this limit and then we can start learning from empirical observations and start having more constrained observations that go into your theoretical models.”

In other words, Strigari said, it doesn’t mean this is the final answer, but this is the state of our knowledge right now. “It kind of quantifies our ignorance, you could say,” he said.

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.

So, where did all these potential free range planets come from? One option is that they formed like stars, directly from the collapse of interstellar gas clouds. According to Strigari some were probably ejected from solar systems. Some research has indicated that ejected planets could be rather common, as planets tend to migrate over time towards the star, and as they plow through the material left over from the solar system’s formation, any other planet between them and their star will be affected. Phil Plait explained it as, “some will shift orbit, dropping toward the star themselves, others will get flung into wide orbits, and others still will be tossed out of the system entirely.”

Don’t worry – our own solar system is stable now, but it could have happened in the past, and some research has suggested we originally started out with more planets in our solar system, but some may have been ejected.

Of course, when discussing planets, the first thing to pop into many people’s minds is if a wandering planet could be habitable.

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

As far as a Nibiru-type wandering world in our solar system right now the answer is no. There is no evidence or scientific basis whatsoever for such a planet. If it was out there and heading towards Earth for a December 21, 2012 meetup, we would have seen it or its effects by now.

Sources: Stanford University, conversation with Louis Strigari

February 13, 2012December 24, 2015 by Paul Scott Anderson

Scientists Find New Clues About the Interiors of ‘Super-Earth’ Exoplanets

Artist's conception of "Super-Earth" exoplanet Kepler-22b, which is about 2.4 times larger than Earth. Credit: NASA.

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As we learned in science class in school, the Earth has a molten interior (the outer core) deep beneath its mantle and crust. The temperatures and pressures are increasingly extreme, the farther down you go. The liquid magmas can “melt” into different types, a process referred to as pressure-induced liquid-liquid phase separation. Graphite can turn into diamond under similar extreme pressures. Now, new research is showing that a similar process could take place inside “Super-Earth” exoplanets, rocky worlds larger than Earth, where a molten magnesium silicate interior would likely be transformed into a denser state as well.

Simply put, the magnesium silicate undergoes what’s called a phase change while in the liquid state. The scientists were able to replicate the extreme temperatures and pressures that would be found inside those exoplanets by using the Janus laser at the Lawrence Livermore National Laboratory and OMEGA at the University of Rochester. A powerful laser pulse generated a shock wave as it passed through the samples. Changes in the velocity of the shock and the temperature of the sample indicated when a phase change was detected.

Interestingly, the different liquid states of the silicate magma in the experiments showed different physical properties under high pressures and temperatures, even though they were still of the same composition. Due to varying densities, the different liquid states tended to want to separate, much like oil and water.

The findings should help to better understand the interiors of terrestrial-type exoplanets, whether they are “Super-Earths” or smaller, like Earth or Mars.

Lead scientist Dylan Spaulding, at the University of California, Berkeley, states: “Phase changes between different types of melts have not been taken into account in planetary evolution models. But they could have played an important role during Earth’s formation and may indicate that extra-solar ‘Super-Earth’ planets are structured differently from Earth.”

The paper was published in the February 10, 2012 edition of the journal Physical Review Letters.

February 10, 2012December 24, 2015 by Paul Scott Anderson

Tidal Heating on Some Exoplanets May Leave Them Waterless

Venus as photographed by the Pioneer spacecraft in 1978. Some exoplanets may suffer the same fate as this scorched world. Credit: NASA/JPL/Caltech

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As the number of exoplanets being discovered continues to increase dramatically, a growing number are now being found which orbit within their stars’ habitable zones. For smaller, rocky worlds, this makes it more likely that some of them could harbour life of some kind, as this is the region where temperatures (albeit depending on other factors as well) can allow liquid water to exist on their surfaces. But there is another factor which may prevent some of them from being habitable after all – tidal heating, caused by the gravitational pull of one star, planet or moon on another; this effect which creates tides on Earth’s oceans can also create heat inside a planet or moon.

The findings were presented at the January 11 annual meeting of the American Astronomical Society in Austin, Texas.

The habitability factor is determined primarily by the amount of heat coming from the planet’s star. The closer a planet is to its star, the hotter it will be, and the farther out it is, the cooler it will be. Simple enough, but tidal heating adds a new wrinkle to the equation. According to Rory Barnes, a planetary scientist and astrobiologist at the University of Washington, “This has fundamentally changed the concept of a habitable zone. We figured out you can actually limit a planet’s habitability with an energy source other than starlight.”

This effect could cause planets to become “tidal Venuses.” In these cases, the planets orbit smaller, dimmer stars, where in order to be in that star’s habitable zone, they would need to orbit much closer in to the star than Earth does with the Sun. The planets would then be subjected to greater tidal heating from the star, enough perhaps to cause them to lose all of their water, similar to what is thought to have happened with Venus in our own solar system (ie. a runaway greenhouse effect). So even though they are within the habitable zone, they would lack oceans or lakes.

What’s problematic is that these planets could subsequently actually have their orbits altered by the tidal heating so that they are no longer affected by it. They would then be more difficult to distinguish from other planets in those solar systems which may still be habitable. While technically still within the habitable zone, they would have effectively been sterilized by the tidal heating process.

Planetary scientist Norman Sleep at Stanford University adds: “We’ll have to be careful when assessing objects that are very near dim stars, where the tides are much stronger than we feel on present-day Earth. Even Venus now is not substantially heated by tides, and neither is Mercury.”

In some cases, tidal heating can be a good thing though. The tidal forces exerted by Jupiter on its moon Europa, for example, are thought to create enough heat to allow a liquid water ocean to exist beneath its outer ice crust. The same may be true for Saturn’s moon Enceladus. This makes these moons still potentially habitable even though they are far outside of the habitable zone around the Sun.

By design, the first exoplanets being found by Kepler are those that orbit closer in to their stars as they are easier to detect. This includes smaller, dimmer stars as well as ones more like our own Sun. The new findings, however, mean that more work will need to be done to determine which ones really are life-friendly and which ones are not, at least for “life-as-we-know-it” anyway.

February 7, 2012December 24, 2015 by Paul Scott Anderson

When Stars Play Planetary Pinball

Artist's conception of a binary star sunset seen from the planet Kepler-16b. Credit: NASA/Ames Research Center/Kepler Mission

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Many of us remember playing pinball at the local arcade while growing up; it turns out that some stars like it as well. Binary stars can play tug-of-war with an unfortunate planet, flinging it into a wide orbit that allows it to be captured by first one star and then the other, in effect “bouncing” it between them before it is eventually flung out into deep space.

The new paper, by Nick Moeckel and Dimitri Veras of the University of Cambridge, will be published in a future issue of Monthly Notices of the Royal Astronomical Society.

The gravitational pull of large gas giant planets can affect the orbits of smaller planets; that scenario is thought to have occurred in our own solar system. In some cases, the smaller planet may be flung into a much wider orbit, perhaps even 100 times wider than Pluto’s. In the case of single stars, that’s normally how it ends. In a binary star system, however, the two stars may play a game of “cosmic pinball” with the poor planet first.

Moeckel and Dimitri conducted simulations of binary star systems, with two sun-like stars orbiting each other at distances between 250 and 1,000 times the distance of the Earth from the Sun. Each star had its own set of planets. The planetary systems would often become unstable, resulting in one of the planets being flung out, where it could be subsequently captured by the other star’s gravity. Since the new orbit around the second star would also tend to be quite wide, the planet would be vulnerable to recapture again by the first star. This could continue for a long time, and the simulations indicated that more than half of all planets initially ejected would get caught in this game of “cosmic pinball.”

In the end, some planets would settle back into an orbit around one of the stars, but the majority would escape both stars altogether, finally being flung out into deep space forever.

According to Moeckel, “Once a planet starts transitioning back and forth, it’s almost certainly at the beginning of a trip that will end in deep space.”

We are fortunate to live in a solar system where our planet is in a nice, stable orbit. For others out there who may not be so lucky, it would be like living through a disaster movie played out over eons.

The paper is available here.

February 1, 2012December 24, 2015 by Tammy Plotner

Russia Will Begin Hunt For Extrasolar Planets

Russia to Start Own Search for Extrasolar Planets - Photo: Paul A. Kempton

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Located just south of Saint Petersburg on Pulkovo Heights, one of the greatest Russian Observatories of all times – the Pulkovo Observatory – is about to embark on a very noble study. According to the head of the Institute for Space Research, Lev Zelyony, the Soviet telescopes are about to turn their eyes towards deep skies in search of extrasolar planets. “Scientists from the Pulkovo Observatory are planning to use ground-based instruments to study the transit of planets around their parent stars,” Zelyony said at a roundtable meeting at RIA Novosti headquarters in Moscow.

The observatory was absolutely state-of-the-art when it opened for business in 1839 and employed Wilhelm von Struve as its director. It houses some of the largest refractor telescopes in the world, including a 38-cm (15 in.) aperture refractor and a 30-inch (76 cm) refractor – both built by Alvan Clarke and Sons. Fifty years later, they added an astrophysical laboratory, a mechanical workshop and installed one of Europe’s largest lensed telescope, a 76-cm refractor (30 inch). Later additions to the observatory included a Littrow spectrograph and horizontal solar telescope and the facility blossomed into a world leader in stellar spectroscopy, cataloging and more. Modern improvements include astrograph equipment, an interferometer, radio telescope and even an additional 65-cm (26-inch) refractor. The Pulkovo Observatory is up to the task.

Pulkovo_observatory_2004 — Pulkovo Observatory 2004 - Credit: Vladimir Ivanov

The hunt for exoplanets is one of the most popular aspects of modern astronomy and one of the fastest growing fields. In less than 25 years, 755 and an ever-increasing number of planets have been cataloged… and the research just doesn’t end. The United States Kepler Mission and French CoRoT space telescope have had their share of fun, but using a ground-based telescope could also be a viable source of planet detection, Zelyony said. He also cited the example of the Hungarian Automated Telescope Network (HATNet) which so far has discovered 29 exoplanets. By using the transit detection method, the Russian astronomers are eager to begin observations where a small change in magnitude could mean a big change in the way their telescopes perceive the stars.

“It is an interesting research, which should be pursued,” Zelyony said. “It will also help us look at our Solar System from a different perspective.”

Original Story Source: Rionovosti News Release.

January 11, 2012December 24, 2015 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.

microlens1 — 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.”

eso1204b — 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