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Lunar and Planetary Laboratory
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Astronomers using the NASA/ESA Hubble Space Telescope have seen dramatic changes in the upper atmosphere of a faraway planet, giving a tantalizing glimpse of the changing climates and weather on planets outside our solar system.
HD 189733b has a blue sky, but that's where the similarities with Earth end. The planet is a huge gas giant similar to Jupiter, but it lies extremely close to its star, more than 10 times closer than Mercury orbits the sun on average.
Even though its star is slightly smaller and cooler than the sun, this makes the planet's climate exceptionally hot, at above 1,800 degrees Fahrenheit, and its upper atmosphere is battered by energetic extreme-ultraviolet and X-ray radiation, or XEUV radiation.
As such, this "Hot Jupiter" is an excellent candidate to study the effects of a star on a planetary atmosphere.
An international team including Gilda Ballester, an associate staff scientist at the University of Arizona's Lunar and Planetary Lab, used the Hubble Space Telescope 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 using Hubble's Space Telescope Imaging Spectrograph, an instrument that, much like a prism, splits light into its constituent colors.
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 was looking for hydrogen gas, the predominant component of HD 189733b's atmosphere being blown off the atmosphere.
"The star is very active," Ballester said. "We had some indication of changes in the planet before, but this is the first time anyone has definitely observed changes similar to space weather in another solar system."
Stellar wind describes streams of charged particles, mostly electrons and protons that a star hurls out into space. Interactions between the solar wind and the Earth's magnetic field light up the sky as the Aurora Borealis, and, in extreme cases, can affect the function of satellites and electrical grids.
"In the case of HD 189733b, the weather in the exoplanet's atmosphere is driven by variations in the strong stellar XEUV radiation and also likely in the wind streaming from its parent star," Ballester said.
The study is published (pdf) in the June 28 issue of the journal Astronomy and Astrophysics.
The observations made in 2010 showed no trace of the planet's atmosphere, said astronomer Alain Lecavelier des Etangs from the Institut d' Astrophysique in Paris, France, who led the study. In contrast, the team's follow-up observations, made in 2011, showed a dramatic change, with clear signs of a cloud of gas being blown from the planet at a rate of at least 1,000 metric tons per second.
Ballester said it is likely that the planet was losing atmosphere in 2010 that went undetected by the team's instruments, which only pick up signatures from certain particles.
"However, we know for sure that the rate of atmospheric loss was much stronger the following year."
Despite the extreme temperature of the planet, the lower atmosphere is not hot enough to evaporate gases. 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.
X-rays and extreme-ultraviolet radiation 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.
Evidence to support enhanced evaporation driven by XEUV radiation observed in 2011 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 four times brighter in X-rays.
"The planet's close proximity to the star means it was struck by a blast of X-rays tens of thousands of times stronger that the Earth suffers even during an X-class solar flare, the strongest category," said co-author Peter Wheatley from the University of Warwick in the U.K. After accounting for the planet's enormous size, the team notes that the planet encountered about 3 million times as many X-rays as Earth receives from a solar flare at the threshold of the X class.
While the team believes that the stellar flare 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 XEUV 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.
"Whether the cause was the stellar flare, an increase in stellar activity or some other process, the large spike in the planet's shedding of hydrogen that we observed is a finding that will boost new research in the understanding of Hot Jupiters and how they interact with their close-by parent stars," Ballester said.
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.
Lunar and Planetary Laboratory
UA University Communications