Astronomers using the NASA/ESA Hubble Space Telescope have seen dramatic changes in the upper atmosphere of a faraway planet. Just after a violent flare on its parent star bathed it in intense X-ray radiation, the planet’s atmosphere gave off a powerful burst of evaporation. The observations give a tantalising glimpse of the changing climates and weather on planets outside our Solar System.
Dramatic change spotted on a faraway planet - ESA Hubble science release
Astronomer Alain Lecavelier des Etangs (CNRS-UPMC, France) and his team used Hubble to observe the atmosphere of exoplanet HD 189733b during two periods in early 2010 and late 2011, as it was silhouetted against its parent star. While backlit in this way, the planet’s atmosphere imprints its chemical signature on the starlight, allowing astronomers to decode what is happening on scales that are too tiny to image directly. The observations were carried out in order to confirm what the team had previously seen once before in a different planetary system: the evaporation of an exoplanet’s atmosphere.HD 189733b has a blue sky, but that’s where the similarities with Earth stop. The planet is a huge gas giant similar to Jupiter, but it lies extremely close to its star, just one thirtieth the distance Earth is from the Sun. Even though its star is slightly smaller and cooler than the Sun, this makes the planet’s climate exceptionally hot, at above 1000 degrees Celsius, and the upper atmosphere is battered by energetic extreme-ultraviolet and X-ray radiation. As such, it is an excellent candidate to study the effects of a star on a planetary atmosphere.
“The first set of observations were actually disappointing,” Lecavelier says, “since they showed no trace of the planet’s atmosphere at all. We only realised we had chanced upon something more interesting when the second set of observations came in.”
The team’s follow-up observations, made in 2011, showed a dramatic change, with clear signs of a plume of gas being blown from the planet at a rate of at least 1000 tonnes per second. “We hadn’t just confirmed that some planets’ atmospheres evaporate,” Lecavelier explains, “we had watched the physical conditions in the evaporating atmosphere vary over time. Nobody had done that before.”
Despite the extreme temperature of the planet, the atmosphere is not hot enough to evaporate at the rate seen in 2011. Instead the evaporation is thought to be driven by the intense X-ray and extreme-ultraviolet radiation from the parent star, HD 189733A, which is about 20 times more powerful than that of our own Sun. Taking into account also that HD 189733b is a giant planet very close to its star, then it must suffer an X-ray dose 3 million times higher than the Earth.
Evidence to support X-ray driven evaporation comes from simultaneous observations of HD 189733A with the Swift satellite, which, unlike Hubble, can observe the star’s atmosphere-frying X-rays. A few hours before Hubble observed the planet for the second time, Swift recorded a powerful flash of radiation coming from the surface of the star, in which the star briefly became 4 times brighter in X-rays.
“X-ray emissions are a small part of the star’s total output, but it is the part that it is energetic enough to drive the evaporation of the atmosphere,” explains Peter Wheatley (University of Warwick, UK), one of the co-authors of the study. “This was the brightest X-ray flare from HD 189733A of several observed to date, and it seems very likely that the impact of this flare on the planet drove the evaporation seen a few hours later with Hubble.”
X-rays are energetic enough to heat the gas in the upper atmosphere to tens of thousands of degrees, hot enough to escape the gravitational pull of the giant planet. A similar process occurs, albeit less dramatically, when a space weather event such as a solar flare hits the Earth’s ionosphere, disrupting communications. While the team believes that the flash of X-rays is the most likely cause of the atmospheric changes they saw on HD 189733b, there are other possible explanations. For example, it may be that the baseline level of X-ray emission from the star increased between 2010 and 2011, in a seasonal process similar to the Sun’s 11-year sunspot cycle.
Regardless of the details of exactly what happened to HD 189733b’s atmosphere, which the team hope to clarify using future observations with Hubble and ESA’s XMM-Newton X-ray space telescope, there is no question that the planet was hit by a stellar flare, and no question that the rate of evaporation of the planet’s atmosphere shot up.
This research has relevance not only for the study of Jupiter-like planets. Several recent discoveries of rocky “super Earths” near their parent stars are thought to be the remnants of planets like HD 189733b, after the complete evaporation of their atmospheres.
The study is presented in a paper entitled “Temporal variations in the evaporating atmosphere of the exoplanet HD189733b” which will be published in a forthcoming issue of Astronomy and Astrophysics.
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| A star field image showing the star HD 189733 (centre). The NASA/ESA Hubble Space Telescope has given astronomers a fascinating new insight into the atmosphere of HD 189733b. To the right of the star is the notable planetary nebula Messier 27. The field-of-view is approximately 0.9 x 0.6 degrees. Credit: NASA, ESA, and the Digitized Sky Survey 2. Acknowledgment: Davide De Martin (ESA/Hubble) |
Notes
[1] HD 189733b is a ‘hot Jupiter’ exoplanet orbiting the star HD 189733A,
located around 60 light-years from Earth. Hot Jupiters are gas giant planets
which orbit close to their parent stars. HD 189733b lies very close to its
star, at only one thirtieth the distance between the Sun and the Earth,
meaning it experiences temperatures of above 1000 degrees Celsius and orbits
its parent star every 53 hours. It has around 10% more mass than Jupiter.
Even Mercury, the closest planet to the Sun, is around 10 times further
away. The planet has a hazy atmosphere made up primarily of hydrogen, which
scatters short wavelengths of light, meaning it would appear blue. Its star,
HD 189733A, is around 80% of the mass of the Sun, just over three quarters
its diameter, around 800 degrees Celsius cooler and slightly redder in
colour. It is part of a double star system with the star HD 189733B (not to
be confused with the planet, HD 189733b), however this companion star is
several thousand times further from HD 189733A, and much smaller than HD
189733A, and so has little or no effect on the planet.
[2] Only a
small fraction of exoplanets can be studied using the transit method, as it
relies on the planet’s orbit being seen perfectly side-on from our
perspective. However, for those planets where it is possible, observing
transits is an extremely powerful tool. These observations were carried out
using Hubble’s Space Telescope Imaging Spectrograph, an instrument which,
much like a prism, splits light into its constituent colours. The relative
brightnesses of different wavelengths of light carry a lot of information
including the fingerprint of the types, properties, abundances and even
motion of gases it has passed through. In this case, the team were looking
for hydrogen gas (the predominant component of HD 189733b’s atmosphere)
being blown off the atmosphere.
[3] The Swift satellite is an
international mission bringing together NASA, the UK Science and Technology
Facilities Council and the Italian Space Agency (ASI). Its primary purpose
is detecting and studying gamma-ray bursts, but its X-ray and
ultraviolet/optical telescopes are also used for other astronomical
observations.
[4] Super Earths are a class of rocky exoplanets
that are similar in composition to the Earth, but with a few times the mass.
Super Earths within their stars’ habitable zones (where temperatures allow
liquid water) are considered to be good candidates for life. Exoplanets
Kepler-10b and CoRoT-7b are classed as super Earths, but are far too close
to their stars to maintain liquid water. They are thought to be the rocky
cores of planets similar to HD 189733b which have lost their entire
atmospheres to evaporation.
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| Swift's Ultraviolet/Optical Telescope captured this view of HD 189733b's star on Sept. 14, 2011. The image is 6 arcminutes across. Credit: NASA/Swift/Stefan Immler |
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| Strong X-ray emission makes the host star of HD 189733b stand out in this view from Swift's X-Ray Telescope. The image combines data from Sept. 13 and 14, 2011, and is 6 arcminutes across. Credit: NASA/Swift/Stefan Immler |
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| This artist's rendering illustrates the evaporation of HD 189733b's atmosphere in response to a powerful eruption from its host star. NASA's Hubble Space Telescope detected the escaping gases and NASA's Swift satellite caught the stellar flare. Credit: NASA's Goddard Space Flight Center |
Stellar Flare Blasts Exoplanet – NASA Science News
An international team of astronomers using data from NASA's Hubble Space Telescope has made an unparalleled observation, detecting significant changes in the atmosphere of a planet located beyond our solar system."The multiwavelength coverage by Hubble and Swift has given us an unprecedented view of the interaction between a flare on an active star and the atmosphere of a giant planet," said lead researcher Alain Lecavelier des Etangs at the Paris Institute of Astrophysics (IAP), part of the French National Scientific Research Center located at Pierre and Marie Curie University in Paris.
The exoplanet is HD 189733b, a gas giant similar to Jupiter, but about 14 percent larger and more massive. The planet circles its star at a distance of only 3 million miles, or about 30 times closer than Earth's distance from the sun, and completes an orbit every 2.2 days. Its star, named HD 189733A, is about 80 percent the size and mass of our sun.
Astronomers classify the planet as a "hot Jupiter." Previous Hubble observations show that the planet's deep atmosphere reaches a temperature of about 1,900 degrees Fahrenheit (1,030 C).
The system is just 63 light-years away, so close that its star can be seen with binoculars near the famous Dumbbell Nebula. This makes HD 189733b an ideal target for studying the processes that drive atmospheric escape.
"Astronomers have been debating the details of atmospheric evaporation for years, and studying HD 189733b is our best opportunity for understanding the process," said Vincent Bourrier, a doctoral student at IAP and a team member on the new study.
In April 2010, the researchers observed a single transit using Hubble's Space Telescope Imaging Spectrograph (STIS), but they detected no trace of the planet's atmosphere. Follow-up observations in September 2011 showed a surprising reversal, with striking evidence that a plume of gas was streaming away from the exoplanet at 300,000 mph. At least 1,000 tons of gas were leaving the planet's atmosphere every second.
This turn of events was explained by data from Swift's X-ray Telescope. On Sept. 7, 2011, just eight hours before Hubble was scheduled to observe the transit, Swift was monitoring the star when it unleashed a powerful flare.
"The planet's close proximity to the star means it was struck by a blast of X-rays tens of thousands of times stronger than the Earth suffers even during an X-class solar flare, the strongest category," said co-author Peter Wheatley, a physicist at the University of Warwick in England. After accounting for the planet's enormous size, the team notes that HD 189733b encountered about 3 million times as many X-rays as Earth receives from a solar flare at the threshold of the X class.
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| Artist's interpretation of what the exoplanet, flare, and atmosphere loss might have looked like. Credit: NASA/Goddard Space Flight Center/Conceptual Image Lab |
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| Credit: NASA/Goddard Space Flight Center/Conceptual Image Lab |
Sources:
- Dramatic change spotted on a faraway planet, ESA Hubble Science Release, June 28, 2012
- Hubble, Swift Detect First-Ever Changes in an Exoplanet Atmosphere, NASA, June 28, 2012
- Stellar Flare Blasts Exoplanet, NASA Science News, NASA, June 28, 2012
- HD 189733b Exoplanet Animation, Conceptual Image Lab, NASA Goddard F.S.C., June 28, 2012
- Hubble, Swift Detect First-ever Changes in an Exoplanet Atmosphere, Goddard Multimedia, June 28, 2012
