Super-Earth's atmosphere comes into view

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Dec 1, 2010

A team of astronomers has made the first direct measurement of the atmosphere of an exoplanetary "super-Earth". The findings suggest that the exoplanet named GJ 1214b has either an atmosphere swarming with clouds or one enveloped in water vapour.

Since the discovery of the first extra-solar planet – or exoplanet – in 1995, over 500 more have subsequently been unveiled. While most of these are gas giants like Jupiter, astronomers are getting better at finding smaller exoplanets that could be more similar to Earth.

GJ 1214b weighs in at 6.5 Earth masses and is a so called super-Earth because it tips the scales at between twice and ten times the mass of our own planet. Discovered in 2009, and circling a star approximately 40 light-years from Earth, the exoplanet's low density implied that it is blanketed by an atmosphere. However, until this latest research, led by Jacob Bean at the Harvard-Smithsonian Center for Astrophysics, US, direct measurements of this atmosphere had remained elusive.

Bean and colleagues used a spectrograph, attached to the Very Large Telescope (VLT), to analyse light from the parent star as the planet passed in front of it. During such a transit some starlight passes through the planet's atmosphere and can be soaked up by its constituent chemicals. This produces a spectrum containing tell-tale fingerprints – gaps at wavelengths where light is absorbed by the atmosphere. Crucially, Bean's spectrum for GJ 1214b was featureless: there were no gaps in the data.

Such a result rules out models suggesting the possibility of a cloud-free, hydrogen-rich atmosphere similar in composition to Neptune. Hydrogen, the lightest element, doesn't cling very tightly to a planet, giving it a better chance of absorbing incoming sunlight. "A hydrogen-dominated atmosphere would be very 'puffy'," Bean told physicsworld.com. "It is this puffiness that would have given a very strong signature in the spectrum that we measured," he added.

The lack of such a signature leaves two rival explanations fighting to explain Bean's finding. "The featureless spectrum tells us that it is probably a very dense atmosphere. However, the alternative is that it does have a puffy atmosphere but with thick, high clouds that we can't see through, similar to Venus, or [Saturn's largest Moon] Titan," explained Bean.

I think we'll get the answer within a year, maybe even sooner Jacob Bean, Harvard-Smithsonian Center for Astrophysics

Should it turn out to be the former, the most likely chemical candidate is water vapour; GJ 1214b orbits so close to its host star that it could well be shrouded in steam. Bean is confident of nailing the answer soon: "I think we'll get the answer within a year, maybe even sooner, we just need longer wavelength observations. Whilst clouds and hazes give a uniform absorption over the wavelength range we used, over very large wavelengths you would expect a difference," he said.

However, some researchers are cautious. "They've done this looking through the Earth's atmosphere, which is never a friend to astronomy," Carole Haswell, an exoplanet researcher at the Open University, told physicsworld.com. "What they've done is very difficult; any slight systematic effects are going to have a huge effect on the conclusions that you draw. It's good, solid and exciting stuff but I'd like to see it checked from space, e.g. with Hubble," she added.

Should Bean's findings be confirmed, Haswell sees this area of research as a crucial part of finding a "second Earth". "If you can measure the composition of the atmospheres of planets like GJ 1214b then you are getting quite close to saying how similar they are to Earth. This is a big step in addressing the question of whether Earth is unique," she explained.

This is a pretty major stepping stone in getting to the end goal of finding an Earth-like planet with signatures of life David Sing, University of Exeter

David Sing, who researches exoplanet atmospheres at the University of Exeter, agrees. "There have been a number of spectral studies of so-called 'hot-Jupiters' but this is the first time it's been done for a terrestrial-type planet," he said. "This is a pretty major stepping stone in getting to the end goal of finding an Earth-like planet with signatures of life," he added.

And Haswell believes we've come along way in a short period of time, telling physicsworld.com: "The fact that in 1995 we didn't know of any planets around other stars and now we're measuring the atmospheres of planets in the same ball park as the Earth is amazing."

The findings are described in a paper published in Nature 468 669.

Colin Stuart is a science communicator, writer and broadcaster based in London

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'Best evidence yet' for dark matter comes from Milky Way centre

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Oct 29, 2010

Energetic radiation pulsing from the belly of the Milky Way is the clearest signal yet of dark matter. That is according to a pair of astrophysicists in the US who reach this conclusion after scrutinising the public data collected by NASA's orbiting Fermi Observatory. "I certainly think it's the best evidence we've seen so far," says Dan Hooper, one half of the team, based at the University of Chicago.

It is a huge claim because for over 70 years astrophysicists have debated the existence of dark matter, which is thought to make up 80% of the universe's mass, yet they have failed to gather any definitive evidence, either direct or indirect, for its existence. But with several hints for dark matter published in recent years – all received with scrutiny by the wider astrophysics community – the US pair will have a hard time convincing others that their signal is what they think it is.

Hooper and his colleague Lisa Goodenough of New York University have analysed the spectra of gamma rays coming from the centre of our galaxy, as collected by the Large Area Telescope onboard the Fermi observatory. Although dark matter does not couple to light, it should annihilate with itself to produce gamma rays, and the amount of annihilation should increase rapidly towards the galactic centre as dark-matter density increases.

Last year Hooper and Goodenough compared the Fermi spectra of gamma rays with a simple computer model of dark matter, and suggested that an excess of gamma rays coming from the galactic centre might be evidence of dark-matter annihilation. At that time other researchers weren't convinced because there were other possible origins for the signal, such as high-energy photons striking interstellar gas. In their latest analysis, however, Hooper and Goodenough have tried to allay these concerns using a far more complex methodology that looks at specific components making up the background of gamma rays.

The US pair break down the gamma-ray background into three parts: a narrow emission from the galaxy's disc; an emission from known point sources; and a spherical or "bulge" emission around the galactic centre. According to their model, no matter what parameters one chooses for dark matter, there should always be a threshold within the bulge emission where dark-matter annihilation begins to outshine other gamma-ray sources. This is because – unlike other sources – emission from dark-matter annihilation follows a square law, so that doubling the density increases the annihilation four-fold.

Hooper and Goodenough examined the Fermi spectra at many regions inside the gamma-ray bulge, and found the data always matched the model's prediction of normal emission – except right at the galactic centre. Here, in a narrow region spanning less than one-quarter of a degree, the emission was far stronger than the model predicted, and had a more lopsided spectrum. Those characteristics, the US pair claims, point to a dark-matter particle – a weakly interacting massive particle, or WIMP – in a mass range of 7.3–9.2 GeV.

This light mass is partly what lends the analysis credence. For years physicists working on the DAMA experiment in Italy claim to have found WIMPs colliding with sodium-iodide nuclei, while those working on the CoGeNT collaboration in the US have tentatively revealed similar WIMP signals coming from germanium detectors – and many believe the only way to reconcile these signals is to assume a WIMP with a mass around 8 GeV.

"Until I had seen this latest paper from Hooper and Goodenough, I was kind of thinking with the light WIMP scenario – nah," says Alex Murphy, a particle astrophysicist who works on the ZEPLIN-III dark-matter experiment in the UK. "But now I've seen it, I'm starting to think – hmm, maybe. Perhaps now we should be looking at other ways to confirm or disprove this proposal."

Murphy voices scepticism about the strength of the claim, however, because he is not convinced Hooper and Goodenough understand the idiosyncrasies of the Fermi instrumentation sufficiently well. Although the Fermi team has published its own preprint revealing an excess of gamma rays near the galactic centre, it has so far stopped short of interpreting this as dark matter.

Ronaldo Bellazzini, the principal investigator on Fermi's Italian team, warns that Hooper and Goodenough's analysis of the galactic centre could still be prone to misinterpretation. "Unfortunately, this region, and whatever [Fermi] observes along the line of sight to it, is rich with astrophysical sources that can mimic signals similar to dark-matter annihilation, like pulsars and supernovae remnants" he says.

Meanwhile, Michael Kuhlen, a dark-matter theorist at the University of California at Berkeley, believes there is "probably a good reason" why the Fermi collaboration has held back from making conclusions on the gamma-ray excess. "They're certainly aware of it, but probably just haven't been able to convince themselves that they fully understand the instrument's behaviour, or the backgrounds, or the kinds of possible astrophysical sources that could produce the signal," he says.

But Kulen adds: "Really they're just trying to stir the pot, and get people to seriously consider the possibility that Fermi may have already detected a dark-matter annihilation signal. This is a good thing."

A preprint of the paper is available at arXiv: 1010.2752.

Jon Cartwright is a freelance journalist based in Bristol, UK

View the original article here

'Best evidence yet' for dark matter comes from Milky Way centre

To enjoy free access to all high-quality "In depth" content, including topical features, reviews and opinion sign up

Oct 29, 2010

Energetic radiation pulsing from the belly of the Milky Way is the clearest signal yet of dark matter. That is according to a pair of astrophysicists in the US who reach this conclusion after scrutinising the public data collected by NASA's orbiting Fermi Observatory. "I certainly think it's the best evidence we've seen so far," says Dan Hooper, one half of the team, based at the University of Chicago.

It is a huge claim because for over 70 years astrophysicists have debated the existence of dark matter, which is thought to make up 80% of the universe's mass, yet they have failed to gather any definitive evidence, either direct or indirect, for its existence. But with several hints for dark matter published in recent years – all received with scrutiny by the wider astrophysics community – the US pair will have a hard time convincing others that their signal is what they think it is.

Hooper and his colleague Lisa Goodenough of New York University have analysed the spectra of gamma rays coming from the centre of our galaxy, as collected by the Large Area Telescope onboard the Fermi observatory. Although dark matter does not couple to light, it should annihilate with itself to produce gamma rays, and the amount of annihilation should increase rapidly towards the galactic centre as dark-matter density increases.

Last year Hooper and Goodenough compared the Fermi spectra of gamma rays with a simple computer model of dark matter, and suggested that an excess of gamma rays coming from the galactic centre might be evidence of dark-matter annihilation. At that time other researchers weren't convinced because there were other possible origins for the signal, such as high-energy photons striking interstellar gas. In their latest analysis, however, Hooper and Goodenough have tried to allay these concerns using a far more complex methodology that looks at specific components making up the background of gamma rays.

The US pair break down the gamma-ray background into three parts: a narrow emission from the galaxy's disc; an emission from known point sources; and a spherical or "bulge" emission around the galactic centre. According to their model, no matter what parameters one chooses for dark matter, there should always be a threshold within the bulge emission where dark-matter annihilation begins to outshine other gamma-ray sources. This is because – unlike other sources – emission from dark-matter annihilation follows a square law, so that doubling the density increases the annihilation four-fold.

Hooper and Goodenough examined the Fermi spectra at many regions inside the gamma-ray bulge, and found the data always matched the model's prediction of normal emission – except right at the galactic centre. Here, in a narrow region spanning less than one-quarter of a degree, the emission was far stronger than the model predicted, and had a more lopsided spectrum. Those characteristics, the US pair claims, point to a dark-matter particle – a weakly interacting massive particle, or WIMP – in a mass range of 7.3–9.2 GeV.

This light mass is partly what lends the analysis credence. For years physicists working on the DAMA experiment in Italy claim to have found WIMPs colliding with sodium-iodide nuclei, while those working on the CoGeNT collaboration in the US have tentatively revealed similar WIMP signals coming from germanium detectors – and many believe the only way to reconcile these signals is to assume a WIMP with a mass around 8 GeV.

"Until I had seen this latest paper from Hooper and Goodenough, I was kind of thinking with the light WIMP scenario – nah," says Alex Murphy, a particle astrophysicist who works on the ZEPLIN-III dark-matter experiment in the UK. "But now I've seen it, I'm starting to think – hmm, maybe. Perhaps now we should be looking at other ways to confirm or disprove this proposal."

Murphy voices scepticism about the strength of the claim, however, because he is not convinced Hooper and Goodenough understand the idiosyncrasies of the Fermi instrumentation sufficiently well. Although the Fermi team has published its own preprint revealing an excess of gamma rays near the galactic centre, it has so far stopped short of interpreting this as dark matter.

Ronaldo Bellazzini, the principal investigator on Fermi's Italian team, warns that Hooper and Goodenough's analysis of the galactic centre could still be prone to misinterpretation. "Unfortunately, this region, and whatever [Fermi] observes along the line of sight to it, is rich with astrophysical sources that can mimic signals similar to dark-matter annihilation, like pulsars and supernovae remnants" he says.

Meanwhile, Michael Kuhlen, a dark-matter theorist at the University of California at Berkeley, believes there is "probably a good reason" why the Fermi collaboration has held back from making conclusions on the gamma-ray excess. "They're certainly aware of it, but probably just haven't been able to convince themselves that they fully understand the instrument's behaviour, or the backgrounds, or the kinds of possible astrophysical sources that could produce the signal," he says.

But Kulen adds: "Really they're just trying to stir the pot, and get people to seriously consider the possibility that Fermi may have already detected a dark-matter annihilation signal. This is a good thing."

A preprint of the paper is available at arXiv: 1010.2752.

Jon Cartwright is a freelance journalist based in Bristol, UK

View the original article here