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)

“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 Alain Lecavelier des Etangs c/0 Paris Institute of Astrophysics.

They’re 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).  HD 189733b periodically passes across, or transits, its parent star, and these events give astronomers an opportunity to probe its atmosphere and environment. In a previous study, a group led by Lecavelier des Etangs used Hubble to show that hydrogen gas was escaping from the planet’s upper atmosphere.

The finding made HD 189733b only the second-known “evaporating” exoplanet at the time. 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. Source: NASA


New Way of Probing Exoplanet Atmospheres

The new techniques will eventually let astronomers measure the atmospheres of smaller planets around dimmer stars

For the first time a clever new technique has allowed astronomers to study the atmosphere of an exoplanet in detail – even though it does not pass in front of its parent star. An international team has used ESO’s Very Large Telescope to directly catch the faint glow from the planet Tau Bootis b. They have studied the planet’s atmosphere and measured its orbit and mass precisely for the first time – in the process solving a 15-year old problem.

Surprisingly, the team also finds that the planet’s atmosphere seems to be cooler higher up, the opposite of what was expected. The results will be published in the 28 June 2012 issue of the journal Nature.

A Groundbreaking ‘First’

The planet Tau Bootis b [1] was one of the first exoplanets to be discovered back in 1996, and it is still one of the closest exoplanets known. Although its parent star is easily visible with the naked eye, the planet itself certainly is not, and up to now it could only be detected by its gravitational effects on the star. Tau Bootis b is a large “hot Jupiter” planet orbiting very close to its parent star.

Like most exoplanets, this planet does not transit the disc of its star (like the recent transit of Venus). Up to now such transits were essential to allow the study of hot Jupiter atmospheres: when a planet passes in front of its star it imprints the properties of the atmosphere onto the starlight. As no starlight shines through Tau Bootis b’s atmosphere towards us, this means the planet’s atmosphere could not be studied before.

But now, after 15 years of attempting to study the faint glow that is emitted from hot Jupiter exoplanets, astronomers have finally succeeded in reliably probing the structure of the atmosphere of Tau Bootis b and deducing its mass accurately for the first time.

VLT To The Rescue

The team used the CRIRES [2] instrument on the Very Large Telescope (VLT) at ESO’s Paranal Observatory in Chile. They combined high quality infrared observations (at wavelengths around 2.3 microns) [3] with a clever new trick to tease out the weak signal of the planet from the much stronger one from the parent star [4].

Lead author of the study Matteo Brogi (Leiden Observatory, the Netherlands) explains: “Thanks to the high quality observations provided by the VLT and CRIRES we were able to study the spectrum of the system in much more detail than has been possible before. Only about 0.01% of the light we see comes from the planet, and the rest from the star, so this was not easy”.

The majority of planets around other stars were discovered by their gravitational effects on their parent stars, which limits the information that can be gleaned about their mass: they only allow a lower limit to be calculated for a planet’s mass [5].

The new technique pioneered here is much more powerful. Seeing the planet’s light directly has allowed the astronomers to measure the angle of the planet’s orbit and hence work out its mass precisely.

Currently, more than 400 exoplanets are known. Most are gaseous like Jupiter, but some “super-Earths” are thought to be large terrestrial, or rocky, worlds.

By tracing the changes in the planet’s motion as it orbits its star, the team has determined reliably for the first time that Tau Bootis b orbits its host star at an angle of 44 degrees and has a mass six times that of the planet Jupiter in our own solar system.

“The new VLT observations solve the 15-year old problem of the mass of Tau Bootis b. And the new technique also means that we can now study the atmospheres of exoplanets that don’t transit their stars, as well as measuring their masses accurately, which was impossible before”, says Ignas Snellen (Leiden Observatory, the Netherlands), co-author of the paper. “This is a big step forward.”

As well as detecting the glow of the atmosphere and measuring Tau Bootis b’s mass, the team has probed its atmosphere and measured the amount of carbon monoxide present, as well as the temperature at different altitudes by means of a comparison between the observations and theoretical models.

Ground Based technology

A surprising result from this work was that the new observations indicated an atmosphere with a temperature that falls higher up. This result is the exact opposite of the temperature inversion – an increase in temperature with height – found for other hot Jupiter exoplanets [6] [7].

The VLT observations show that high resolution spectroscopy from ground-based telescopes is a valuable tool for a detailed analysis of non-transiting exoplanets’ atmospheres.

The detection of different molecules in future will allow astronomers to learn more about the planet’s atmospheric conditions. By making measurements along the planet’s orbit, astronomers may even be able to track atmospheric changes between the planet’s morning and evening.

“This study shows the enormous potential of current and future ground-based telescopes, such as the E-ELT. Maybe one day we may even find evidence for biological activity on Earth-like planets in this way”, concludes Ignas Snellen.

[1] The name of the planet, Tau Bootis b, combines the name of the star (Tau Bootis) with the letter “b” indicating that this is the first planet found around this star. The designation Tau Bootis a is used for the star itself.

[2] CRyogenic InfraRed Echelle Spectrometer

[3] At infrared wavelengths, the parent star emits less light than in the optical regime, so this is a wavelength regime favorable for separating out the dim planet’s signal.

[4] This method uses the velocity of the planet in orbit around its parent star to distinguish its radiation from that of the star and also from features coming from the Earth’s atmosphere. The same team of astronomers tested this technique before on a transiting planet, measuring its orbital velocity during its crossing of the stellar disc.

[5] This is because the tilt of the orbit is normally unknown. If the planet’s orbit is tilted relative to the line of sight between Earth and the star then a more massive planet causes the same observed back and forth motion of the star as a lighter planet in a less tilted orbit and it is not possible to separate the two effects.

[6] Thermal inversions are thought to be characterized by molecular features in emission in the spectrum, rather than in absorption, as interpreted from photometric observations of hot Jupiters with the Spitzer Space Telescope. The exoplanet HD 209458b is the best-studied example of thermal inversions in the exoplanet atmospheres.

[7] This observation supports models in which strong ultraviolet emission associated to chromospheric activity – similar to the one exhibited by the host star of Tau Bootis b – is responsible for the inhibition of the thermal inversion.  Source: SpaceDaily


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