While we wait for the last transit of Venus of the century, it’s worth remembering how tricky transit studies can be when we push them out to exoplanetary distances. You would think that catching a transit of a planet like Venus, closer to us than the Sun, would be simplicity itself, but the orbital planes of Venus and the Earth are not precisely enough aligned to allow for more than a pair of transits followed by over a century of waiting for the next. I’ve just received a copy of Mark Anderson’s The Day the World Discovered the Sun (Da Capo Press, 2012) and will be writing about 18th Century transit studies and their impact in coming weeks.
The transits Anderson writes about and the expeditions that ranged the globe to study them played a role in helping astronomers understand the dimensions of the Solar System. And you can see that if Venus is a challenge, tracking planets around other stars will push our technology to its limit. Nonetheless, we’re getting quite good at teasing information out of our data, as the planet known as HD 189733b continues to show us. It was just 16 months ago that carbon dioxide and methane were discovered in the planet’s atmosphere. Moreover, the work was done using a 3-meter instrument located at NASA’s Infrared Telescope Facility at Mauna Kea that ranks no higher than 40th among our ground-based instruments.
Sodium had already been found in the atmosphere of HD 189733b, and now comes word that astronomers working at the Hobby-Eberly Telescope at UT Austin’s McDonald Observatory have identified interactions with the parent star that show the planet is slowly dissolving. 63 light years away in the constellation Vulpecula, the planet is losing hydrogen in great streamers that are much hotter than previously thought, an indication that flare activity on the star is interacting with the atmosphere of the planet. Given the proximity of planet to star, that’s not entirely surprising: HD 189733b, a ‘hot Jupiter,’ orbits 10 times closer to its star than Mercury to our Sun.
Probing an exoplanet’s atmosphere is made possible by its transits as seen from Earth, as Seth Redfield (Wesleyan University) explains:
“Each time the planet passes in front of the star, the planet blocks some of the star’s light. If the planet has no atmosphere, it will block the same amount of light at all wavelengths. However, if the planet has an atmosphere, gases in its atmosphere will absorb some additional light.”
It was Redfield who first identified sodium in HD189733b’s atmosphere using the same instrument, based on observations that compared the spectrum of the light collected during a transit with the light of the star by itself. As huge as this planet is — 20 percent more massive than Jupiter — it blocks only 2.5 percent of the star’s light, along with another 0.3 percent that is accounted for by the planet’s atmosphere. Exoplanetary atmosphere studies using transits are built around the slight differences that are revealed when the planet passes in front of the star. It’s worth reflecting on all this as we wait for tomorrow’s transit of Venus and ponder how far transit studies have taken us since the first scientific expeditions set out to study them.
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk. — Garrison Keillor
News Release Number: STScI-2012-23
Hubble, Swift Detect First-Ever Changes in an Exoplanet Atmosphere
Image: Exoplanet HD 189733b (Artist's Illustration)
STScI-PRC2012-23a
ABOUT THIS IMAGE:
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.
The international team of astronomers in this study consists of A. Lecavelier des Etangs (Institut d'astrophysique de Paris, CNRS, UPMC, France), V. Bourrier (Institut d'astrophysique de Paris, CNRS, UPMC, France), P. J. Wheatley (Department of Physics, University of Warwick, UK), H. Dupuy (Institut d'astrophysique de Paris, CNRS, UPMC, France), D. Ehrenreich (Institut de Planetologie et d'Astrophysique de Grenoble, UJF/CNRS, Grenoble, France), A. Vidal-Madjar (Institut d'astrophysique de Paris, CNRS, UPMC, France), G. Hebrard (Institut d'astrophysique de Paris, CNRS, UPMC, France), G. E. Ballester (Lunar and Planetary Laboratory, University of Arizona, USA), J.-M. Desert (Harvard-Smithsonian Center for Astrophysics, USA), R. Ferlet (Institut d'astrophysique de Paris, CNRS, UPMC, France), and D. K. Sing (Astrophysics Group, School of Physics, University of Exeter, UK).
Object Name: HD 189733b
Image Type: Artwork
Illustration Credit: NASA Goddard Space Flight Center
Science Credit: NASA, ESA, A. Lecavelier des Etangs (CNRS-UMPC, France), and P. Wheatley (University of Warwick)
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 scientists conclude the atmospheric variations occurred in response to a powerful eruption on the planet's host star, an event observed by NASA's Swift satellite.
"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 degrees Celsius).
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.
When HD 189733b transits its star, some of the star's light passes through the planet's atmosphere. This interaction imprints information on the composition and motion of the planet's atmosphere into the star's light.
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 STIS observations in September 2011 showed a surprising reversal, with striking evidence that a plume of gas was streaming away from the exoplanet.
The researchers determined that at least 1,000 tons of gas was leaving the planet's atmosphere every second. The hydrogen atoms were racing away at speeds greater than 300,000 miles per hour. The findings will appear in an upcoming issue of the journal Astronomy & Astrophysics.
Because X-rays and extreme ultraviolet starlight heat the planet's atmosphere and likely drive its escape, the team also monitored the star with Swift's X-ray Telescope (XRT). 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. It brightened by 3.6 times in X-rays, a spike occurring atop emission levels that already were greater than the Sun's.
"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.
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.
Astronomer Alain Lecavelier des Etangs (CNRS-UPMC, France) and his team used Hubble to observe the atmosphere of exoplanet HD 189733b [1] during two periods in early 2010 and late 2011, as it was silhouetted against its parent star [2]. 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 (heic0403).
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.”
The next question was: why the change?
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 [3], 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. [4]
Notes:
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.
This method of observing exoplanets is known as the transit method, as it takes advantage of the planet transiting across the face of its parent star. 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.
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.
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.
Temporal variations in the evaporating atmosphere of the exoplanet HD 189733b - A. Lecavelier des Etangs et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk. — Garrison Keillor
