National Aeronautics and Space Administration Two Dissipating Exoplanet Atmospheres Taken from: Hubble 2010: Science Year in Review Produced by NASA Goddard Space Flight Center and the Space Telescope Science Institute. The full contents of this book include Hubble science articles, an overview of the telescope, and more. The complete volume and its component sections are available for download online at: www.hubblesite.org/hubble_discoveries/science_year_in_review
Two Dissipating Exoplanet Atmospheres Many hundreds of large Jupiter-like planets are known to orbit other stars. These extrasolar planets, or exoplanets, are almost always invisible, even to Hubble. This is because their immense distances from Earth make them extremely faint and easily lost in the glow of the much brighter stars they circle. Exoplanets were first detected by the small but recognizably cyclic gravitational tug they exert on their stars. Sensitive spectroscopy can measure the tiny changes such a planet induces in its star s velocity. Once astronomers have established a repeatable pattern in these changes they can calculate various orbital parameters of the exoplanet and estimate its mass. A number of these bodies are called Hot Jupiters. These are giant gas planets that orbit exceedingly close to their parent stars. In contrast to Jupiter, which circles the Sun in approximately twelve (Earth) years, these exoplanets revolve about their stars in only days or weeks. Being so close to their parent stars, these bodies are intensely hot as much as thousands of degrees Fahrenheit. For comparison, the maximum temperature on the sunlit side of the Sun s closest planet (Mercury) is approximately 800 F (427 C). When viewed from Earth, less than 10 percent of these Hot Jupiters orbit so that they pass in front of, or transit, their stars. During transit they measurably dim the brightness of the stars they cross. This data can be used to determine an exoplanet s size. Astronomers can also study the chemical composition of a transiting exoplanet s atmosphere by comparing the star s spectrum during transit with that of its normal, unblocked state. The Cosmic Origins Spectrograph (COS), installed into Hubble by astronauts in May 2009, is both uniquely situated in orbit above Earth s light-absorbing atmosphere and sensitive enough to probe the atmospheres of these transiting Hot Jupiters. Using COS, astronomers recently observed two of these exoplanets. The data gathered clearly indicates that both these worlds are in the process of losing their atmospheres. This artist s illustration shows a view of HD 209458b with its comet-like tail as seen from the surface of an imaginary nearby companion object. 91
HD 209458b The giant gas planet HD 209458b is slightly less massive than Jupiter but orbits 100 times closer to its star than Jupiter does to the Sun. Located 153 light-years from Earth, the planet circles the star HD 209458 in a mere 3½ Earth days. In contrast, Mercury, the innermost planet of our solar system, takes 88 days to orbit the Sun. HD 209458b was co-discovered in 1999 by two separate teams using ground-based telescopes. It has the distinction of being the first exoplanet found to transit its parent star. Colorado astronomer Jeffrey Linsky led the team that used COS to study HD 209458b. When they observed the transit in visible-light wavelengths, they saw the brightness of the star decrease by a small, 1.46 percent. The opaque body of the planet itself caused this dimming, as the planet s atmosphere passed through all of the starlight. Viewed in a specific ultraviolet wavelength, however, the exoplanet blocked an additional 6.5 percent of the star s light. The atmosphere of WASP-12b is being accreted by its parent star according to observations made by Hubble s COS instrument. This artist s concept shows the exoplanet so close to its Sun-like star that it is superheated and stretched into a football shape by enormous tidal forces. 92
Hydrogen atoms in its atmosphere absorbed the light at this wavelength, effectively increasing the diameter of the exoplanet s light-blocking disk. This phenomenon can only be seen from space, since Earth s own atmosphere also blocks this particular ultraviolet wavelength. The observation revealed the existence of a greatly swollen atmosphere surrounding HD 209458b. The team also found the spectral signatures of the elements carbon and silicon in the planet s super-hot 2,000 F (1093 C) atmosphere. Their work revealed that the parent star is heating HD 209458b s entire atmosphere, dredging up these heavier elements from below. The data also showed that not all of the material was traveling at the same speed as the planet (which is orbiting at 326,000 miles per hour around its star). The team found that gas escapes from the planet at very high velocities; a large amount flows outward at 22,000 miles per hour. This large flow forms a comet-like tail trailing the planet that is likely caused by stellar winds sweeping the gas away. Before astronauts installed COS in Hubble, astronomers first used the Space Telescope Imaging Spectrograph (STIS) to observe HD 209458b in 2003. The STIS data also showed an active, evaporating atmosphere, which the team envisioned as a comet-like tail. However, STIS was unable to achieve the spectroscopic detail necessary to show an actual outflow of the gas. In contrast, the COS instrument s combination of very high ultraviolet sensitivity and good spectral resolution enabled Linsky s team to detect the suspected tail directly. From the rate-loss data obtained, they calculated that despite the harsh conditions inflicted on this exoplanet by its star, the gas on HD 209458b will take about a trillion years to evaporate completely. WASP-12b A team formed by English astronomer Carole Haswell also used COS to examine the atmosphere of transiting exoplanet WASP-12b. This body was discovered by the United Kingdom s Wide Angle Search for Planets (WASP) in 2008. During the course of its operation this sky-survey program identified WASP-12b, an exoplanet orbiting a yellow dwarf star named WASP- 12. The star is located approximately 600 light-years away in the winter constellation Auriga. The hot planet circling WASP-12 contains 1.4 times the mass of Jupiter, but orbits so close to the star that it completes a revolution in a mere 1.1 days. Haswell s team also imaged the transit in various wavelengths of light. They found that the planet s ultraviolet-absorbing exosphere is much more extended than that of a normal planet of its size. Forty percent more massive than Jupiter, WASP-12b has an atmosphere that has ballooned to nearly three times Jupiter s radius. WASP-12b is so extended and in such proximity to its parent star that the planet s radius exceeds its Roche lobe the gravitational boundary beyond which material is pulled away from the planet and lost forever into its star. 93
The graphic on the left illustrates the dip in light as exoplanet HD 209458b passes in front of its parent star. As the transit occurs, 1.46 percent of the star s light is blocked. A similar graphic on the right shows the light curve for WASP-12b. Unlike the planet HD 209458b, WASP-12b is essentially being accreted onto its stellar neighbor. It is superheated to 2,800 F (1,538 C) and stretched into a football shape by the enormous gravitational tidal forces induced in it by its nearby star. Its outer material is being stripped away, spiraling downward onto the star in what is known as an accretion disk. This feature is much different from the comet-like tail of HD 209458b because WASP-12b is so much closer to its parent star. This type of matter exchange is more commonly seen in close binary star systems. This is the first time it has been detected so clearly for a planet-star system. Shu-lin Li of Peking University, Beijing, first predicted that the planet s surface would be distorted by its star s gravity, and that gravitational tidal forces would heat WASP-12b s interior to the point that it greatly expands the planet s outer atmosphere. These recent Hubble observations have confirmed her predictions. Haskell s team also identified aluminum, tin, manganese, and other elements never before seen on planets outside our own solar system. The fact the COS could even detect these atomic species offers additional strong evidence that the planet s atmosphere is dredged up from below and greatly extended. Due to its unfortunate location so close to WASP-12, WASP-12b is in a most precarious situation. Unlike HD 209458b, this exoplanet may only have another 10 million years left before it is completely gone. 94
Non-transit Transit Non-transit Transit Carbon Silicon Chlorine Oxygen Oxygen 1206.1 1206.3 1206.5 1206.7 1206.9 1340 1360 1380 1400 Wavelength (Angstroms) Left: This panel shows a (magnified) silicon spectral emission line from HD 209458 during transit (red) and out of transit (yellow). It is typical of the various emission lines associated with chemical elements that can be seen in the star s spectrum (right; partial spectrum shown). HD 209458 is found in the constellation Pegasus. At 2,800 F (1,538 C), its planet, HD 209458b, is the hottest planet known in the galaxy. (Photo credit: Akira Fujii) Alpheratz a (alpha) And Scheat b (beta) Peg Matar h (eta) Peg i (iota) Peg Pegasus HD 209458 Algenib g (gamma) Peg Markab a (alpha) Peg Enif e (epsilon) Peg 95
WASP-12, a star located in the center of this image, is located about 600 light-years away from the constellation Auriga. (Photo Credit: A. Fujii) Lynx Menkalinan b (beta) Aur Capella a (alpha) Aur Perseus q (theta) Aur Auriga Hassaleh i (iota) Aur Caster a (alpha) Gem Pollux b (beta) Gem WASP-12 Elnath b (beta) Tau Gemini NGC 2174 Taurus Alhena g (gamma) Gem Canis Minor Cone Nebula NGC 2264 Rosette Nebula NGC 2238 Monoceros Betelgeuse a (alpha) Ori Bellatrix g (gamma) Ori Orion 96
Further Reading Andrews, B. It s a Planet! It s a Comet! It s Both? Astronomy 38, no. 11 (November 2010): 18. Andrews, B. Pulling Apart a Stellar WASP. Astronomy 38, no. 6 (June 2010): 21. Cruz, M. Too Close for Comfort? Science 328, no. 5981 (May 21, 2010): 955. Haswell, C. Transiting Exoplanets. Cambridge: Cambridge University Press, 2010. Li, S., et al. WASP-12b as a Prolate, Inflated and Disrupting Planet from Tidal Dissipation. Nature 463, no. 7284 (February 25, 2010): 1054 1056. Minogue, K. ScienceShot: A Jupiter-Sized Comet. Science Now, July 15, 2010. http://news.sciencemag.org/sciencenow/2010/07/scienceshot-a-jupiter-sized-come.html Dr. Jeffrey L. Linsky is a fellow of JILA and a research professor in the Department of Astrophysical and Planetary Sciences at the University of Colorado, Boulder. Originally from Buffalo, New York, he now considers Boulder his hometown. He holds a BS in physics from MIT and a PhD in astronomy from Harvard University. Dr. Linsky leads a research group that analyzes high-resolution ultraviolet and X-ray spectra to study a broad range of topics. These include stellar atmospheres, circumstellar disks, the local interstellar medium, transiting planets, and the deuterium abundance of the universe. He was on the science teams that developed the Goddard High Resolution Spectrograph, the Space Telescope Imaging Spectrograph, and the Cosmic Origins Spectrograph for the Hubble as well as earlier space-based spectrographs. He was also an interdisciplinary scientist for the Chandra X-ray Observatory. Dr. Carole Haswell is a senior lecturer in the Department of Physics and Astronomy within the Faculty of Science at the Open University in the United Kingdom. She is a member of the Centre for Earth, Planetary, Space & Astronomical Research. The main focus of her work is exoplanets with a concentration on transiting exoplanets. Dr. Haswell has worked on the SuperWASP (Wide Angle Search for Planets) project, and has recently written a book entitled Transiting Exoplanets. Born in Middlesbrough, England, she studied physics at Oxford University as an undergraduate and earned her PhD in astronomy from the University of Texas, Austin. She spent two years at the Space Telescope Science Institute, worked as a post-doc and associate research scientist at Columbia University, and lectured at Barnard College. She taught at the Astronomy Centre at the University of Sussex before joining the Open University in 1999. (Photo credit: Joanne J. Haswell) 97