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Cloudy weather on an alien world

WEATHER FORECASTERS on exoplanet GJ 1214b would have an easy job. Today’s forecast: cloudy. Tomorrow: overcast. Extended outlook: more clouds.

The planet, which is known as GJ 1214b, is classified as a super-Earth because its mass is intermediate between that of Earth and Neptune. Recent searches for planets orbiting other stars (‘exoplanets‘) have shown that super-Earths like GJ 1214b are among the most common type of planets in the Milky Way galaxy. Because no such planets exist in our Solar System, the physical nature of super-Earths is largely unknown.

Previous studies of GJ 1214b yielded two possible interpretations of the planet’s atmosphere: it could consist entirely of water vapour or some other type of heavy molecule, or it could contain high-altitude clouds that prevent the observation of what lies underneath.

Artist's view of exoplanet GJ 1214b

An artist’s view of exoplanet GJ 1214b. The weather forecast is for cloudy skies.

But now a team of astronomers led by Laura Kreidberg and Jacob Bean of the Department of Astronomy and Astrophysics at the University of Chicago, have detected clear evidence of clouds in the atmosphere of GJ 1214b from data collected with the Hubble Space Telescope. The Hubble observations used 96 hours of telescope time spread over 11 months. This was the largest Hubble programme ever devoted to studying a single exoplanet.

The researchers describe their work as an important milestone on the road to identifying potentially habitable, Earth-like planets beyond our Solar System. The results appear in the January 2 issue of the journal Nature.

Pushing the limits

“We really pushed the limits of what is possible with Hubble to make this measurement,” said Kreidberg, a third-year graduate student and first author of the new paper. “This advance lays the foundation for characterising other Earths with similar techniques.”

“I think it’s very exciting that we can use a telescope like Hubble that was never designed with this in mind, do these kinds of observations with such exquisite precision, and really nail down some property of a small planet orbiting a distant star,” explained Bean, an assistant professor and the project’s principal investigator.

GJ 1214b is located just 40 light-years from Earth, in the direction of the constellation Ophiuchus. Because of its proximity to our Solar System and the small size of its host star, GJ 1214b is the most easily observed of the known super-Earths. It transits, or passes in front of its parent star, every 38 hours, giving scientists an opportunity to study its atmosphere as starlight filters through it.

Kreidberg, Bean and their colleagues used Hubble to precisely measure the spectrum of GJ 1214b in near-infrared light, finding what they consider definitive evidence of high clouds blanketing the planet. These clouds hide any information about the composition and behaviour of the lower atmosphere and surface.

Four planets in a row

An artist’s rendering comparing the size of GJ 1214b, another, larger exoplanet, and Earth and Neptune.

Unearthly weather

The planet was discovered in 2009 by the MEarth Project, which monitors 2,000 red dwarf stars for transiting planets. The planet was next targeted for follow-up observations to characterise its atmosphere. The first spectra, which were obtained by Bean in 2010 using a ground-based telescope, suggested that the planet’s atmosphere either was predominantly water vapour or hydrogen-dominated with high-altitude clouds.

More precise Hubble observations made in 2012 and 2013 enabled the team to distinguish between these two scenarios. The news is about what they didn’t find. The Hubble spectra revealed no chemical fingerprints whatsoever in the planet’s atmosphere. This allowed the astronomers to rule out cloud-free atmospheres made of water vapour, methane, nitrogen, carbon monoxide, or carbon dioxide.

The best explanation for the new data is that there are high-altitude clouds in the atmosphere of the planet, though their composition is unknown. Models of super-Earth atmospheres predict clouds could be made out of potassium chloride or zinc sulphide at the scorching temperatures of 230 degrees Celsius found on GJ 1214b. “You would expect very different kinds of clouds to form than you would expect, say, on Earth,” Kreidberg said.

The launch of NASA’s next major space telescope, the James Webb Space Telescope (JWST), later this decade should reveal more about such worlds, Kreidberg said. “Looking forward, JWST will be transformative,” she said. “The new capabilities of this telescope will allow us to peer through the clouds on planets like GJ 1214b. But more than that, it may open the door to studies of Earth-like planets around nearby stars.”

Adapted from information issued by the University of Chicago. Images courtesy NASA, ESA and G. Bacon.

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Pluto has a CO glow

Artist's impression of Pluto and Charon

An artist's impression of Pluto and its largest moon, Charon. Astronomers have detected thin traces of carbon monoxide gas in the dwarf planet's atmosphere.

ASTRONOMERS HAVE DISCOVERED carbon monoxide in the atmosphere of Pluto, capping off nearly two decades of work to detect the gas in the ‘air’ of the distant, icy world.

Pluto, discovered in 1930, was long considered the Sun’s smallest and most distant planet. Since 2006, though, it has been regarded by many astronomers as a ‘dwarf planet’…one of a handful of such bodies with sizes of hundreds of kilometres that orbit in the distant reaches of the Solar System, out beyond Neptune.

Pluto is the only dwarf planet known to have an atmosphere. The thin layer of gases was detected in 1988 when it dimmed the light of a distant star as Pluto passed in front of it.

The new results, obtained using the 15-metre James Clerk Maxwell Telescope in Hawaii, show a strong signal of carbon monoxide gas.

Team leader Dr Jane Greaves of the University of St Andrews will present the new discovery today at the UK National Astronomy Meeting in Wales.

Fragile atmosphere

Previously, Pluto’s atmosphere was known to be over a hundred kilometres thick, but the new data raise this height to more than 3,000 kilometres—a quarter of the way out to Pluto’s largest moon, Charon.

In 1989 Pluto made its closest approach to the Sun, a comparatively recent event given that it takes 248 years to complete each orbit. The gases probably result from solar heating of surface ice, which sublimates (goes directly from ice to gas) as a consequence of the slightly higher temperatures during this period.

The resulting atmosphere is probably the most fragile in the Solar System, with the top layers blowing away into space.

“The height to which we see the carbon monoxide agrees well with models of how the solar wind strips Pluto’s atmosphere,” commented team member Dr Christiane Helling, also of the University of St Andrews.

Artist's impression of the view from the surface of Pluto

Artist's impression of the view from the surface of Pluto, showing a thin, hazy atmosphere.

Deep space cold snap

The gas is extremely cold, about -220 degrees Celsius. A big surprise for the team was that the CO measurement was more than twice as strong as an upper limit obtained by another group, who used the IRAM 30-metre telescope in Spain in 2000.

“It was thrilling to see the signal gradually emerge as we added in many nights of data”, said Dr Jane Greaves, the team leader from the University of St Andrews.

“The change in brightness over the last decade is startling,” she added. “We think the atmosphere may have grown in size, or the carbon monoxide abundance may have been boosted.”

Such changes have been seen with Pluto before, but only in the lower atmosphere, where methane—the only other gas ever positively identified—has also been seen to vary.

Critical balance

Unlike the greenhouse gas carbon dioxide, carbon monoxide acts as a coolant, while methane absorbs sunlight and so produces heating. The balance between the two gases—which are just trace elements in what is thought to be a nitrogen-dominated atmosphere—is critical for its fate during the many-decades long seasons.

The newly discovered carbon monoxide may hold the key to slowing the loss of Pluto’s atmosphere. But if the chilling effect is too great, it could result in nitrogen snowfalls and all the gases freezing back onto the ground.

“Seeing such an example of extra-terrestrial climate-change is fascinating”, says Dr Greaves. “This cold, simple atmosphere that is strongly driven by the heat from the Sun could give us important clues to how some of the basic physics works, and act as a contrasting test-bed to help us better understand the Earth’s atmosphere.”

The JCMT is operated jointly by the UK, Canada and the Netherlands and is approaching its twenty-fifth anniversary.

The team has another Pluto observing run scheduled at the JCMT for the end of April, and in the long-term, they hope to continue tracking the changes in the atmosphere at least up to the fly-by of NASA’s New Horizons space probe in 2015.

Adapted from information issued by RAS. Images courtesy ESO / L. Calcada.

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“Gateway to space” mission lives on

Artist's concept of Earth's atmosphere

Artist's concept of Earth's atmosphere from 0 to 700km, showing the mesosphere, thermosphere and ionosphere (MTI) regions. It also shows various MTI atmospheric phenomenon and the atmospheric range in which they occur. The TIMED spacecraft orbits at an approximate altitude of 680 km.

  • TIMED satellite explores little-known region high above Earth
  • Important for studying effect on the Sun on our planet
  • Spacecraft still healthy and mission extended for a fourth time

Nine years after beginning its unprecedented look at the gateway between Earth’s environment and space—not to mention collecting more data on the upper atmosphere than any other satellite—NASA’s Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission has been extended again.

Before the launch of TIMED, the mesosphere and lower thermosphere/ionosphere—which help protect us from harmful solar radiation—had been one of the least explored and understood regions of our environment.

“The middle part of the atmosphere was the part we kind of ignored,” says John Sigwarth, the deputy project scientist for TIMED at NASA’s Goddard Space Flight Centre in Greenbelt, MD.

“It’s too high for balloons and too low for spacecraft. So the understanding of this middle atmosphere and its impact on the upper atmosphere has been tremendously increased due to TIMED.”

The primary science objective of the TIMED mission is to understand the energy transfer into and out of the Mesosphere and Lower Thermosphere/Ionosphere (MLTI) region of the Earth’s atmosphere (energetics), as well as the basic structure (i.e., pressure, temperature, and winds) that results from the energy transfer into the region.

Artist's concept of TIMED spacecraft

Artist's concept of the TIMED spacecraft, which orbits Earth at 680km altitude.

The mission will now continue to study the influences of the Sun and humans on our upper atmosphere. TIMED began its extended mission on October 1, 2010, and will collect data through 2014. This is its fourth extension since the original 2-year mission began in January 2002.

A new focus

TIMED will focus this time on a problem that has long puzzled scientists: differentiating between human-induced and naturally occurring changes in this atmospheric region. This extension also allows TIMED to continue collecting data for longer than a full 11-year solar cycle.

“The Sun is a variable star with an 11 year cycle,” says Sigwarth. “So, if things change in the mesosphere, you don’t know if it’s because the Sun changed or because human activity has caused the change.”

“By getting back to the same point in the cycle, we can compare what it was like then, and what it’s like now, and see if there’s a long term trend of changes that’s not solar related.”

The key instrument performing this work is known as SABER (or Sounding of the Atmosphere using Broadband Emission Radiometry), built by Hampton University in Hampton, Virginia. SABER can remotely sense composition and temperature in the mesosphere.

In addition to checking for effects from humans, TIMED scientists would like to understand how cooling temperatures in the middle atmosphere are causing the thermosphere to become less dense and its composition to change.

With fewer particles in the thermosphere, there’s less drag on satellites in space, which affects how long spacecraft and space debris stay in orbit—information that must be integrated into calculations for orbit models.

Composition changes in the thermosphere can also alter ionospheric structures that affect radio wave propagation and communications. To help with this is an instrument called SEE (or the Solar EUV Experiment) built at the University of Colorado, which looks at the Sun’s x-rays and extreme ultraviolet rays to see how they impact our atmosphere.

TIMED will also collaborate with NASA’s newest eye on the Sun, the Solar Dynamics Observatory, which provides continuing solar radiation measurements and new views of how solar activity is created.

Adapted from information issued by NASA GSFC / JPL/APL.

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Sunset in space

An edge-on image of Earth's atmosphere

An edge-on image of Earth's atmosphere, taken by an astronaut aboard the International Space Station, reveals different layers and clouds.

  • Edge-on view of Earth’s atmosphere
  • Atmospheric layers, clouds visible
  • Astronauts see 16 sunsets and sunrises per day!

This spectacular image of sunset over the Indian Ocean was taken by astronauts aboard the International Space Station (ISS).

The image presents an edge-on, or limb view, of the Earth’s atmosphere as seen from orbit.

The Earth’s curvature is visible along the horizon line, or limb, that extends across the image from centre left to lower right. Above the darkened surface of the Earth, a brilliant sequence of colours roughly denotes several layers of the atmosphere.

Deep oranges and yellows appear in the troposphere, which extends from the Earth’s surface to 6–20 km high. This layer contains over 80 percent of the mass of the atmosphere and almost all of the water vapour, clouds, and precipitation. Several dark cloud layers are visible within this layer.

Variations in the colours are due mainly to varying concentrations of either clouds or aerosols (airborne particles or droplets).

See the full-size image here (new window).

The pink to white region above the clouds appears to be the stratosphere; this atmospheric layer generally has few or no clouds, and it extends up to approximately 50 km above the Earth’s surface.

Above the stratosphere, blue layers mark the upper atmosphere (including the mesosphere, thermosphere, ionosphere, and exosphere), as it gradually fades into the blackness of outer space.

The ISS was located over the southern Indian Ocean when this picture was taken, with the astronaut looking towards the west. Astronauts aboard the ISS see 16 sunrises and sunsets per day due to their high orbital velocity (greater than 28,000 km per hour).

The multiple chances for photography are fortunate because at that speed, each sunrise or sunset only lasts a few seconds!

Astronaut photograph provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Centre. Caption adapted from information issued by William L. Stefanov, NASA-JSC.