X-Rays Determine Planet Is Burning Out Its Atmosphere



Planet HD 189733b is deep blue in color.  The silicates in its atmosphere can make it rain glass.

HD 189733b is 63 million light years from Earth and orbits its parent in the HD 189733 system. And it is rapidly burning out its atmosphere.

HD 189733b is about the size of Jupiter, and is in fact called “hot Jupiter.” It is 30 times closer to its star than the Earth is to the sun.  Its solar year lasts about two days.  Surface temperatures on the side facing its star reach 2,000 degrees.  Its winds travel at up to 4,500 miles per hour.

Earlier this month, astronomers studied it through the Hubble Space Telescope.

Now astronomers at NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM Newton Observatory have used X-rays to watch the planet as it eclipsed its sun.  For the first time since exoplanets, or planets around stars other than the sun, were discovered, almost 20 years ago, scientists have used X-ray observations to detect an exoplanet transiting its parent star. Six transits have been observed and data collected on HD 189733b.

Most planets outside of our solar system have been discovered when they pass in front of stars.

The transiting causes part of the star’s brightness to dim and the effect can be observed using our eyes’ normal optical wavelengths.  In 2005, French astronomers discovered HD 189733b in that manner as it eclipsed its sun.  By utilizing X-ray wavelengths, astronomers at Chandra and Newton could detect that the decrease in X-ray light was three times greater than the corresponding decrease in optical light.

X-rays, light waves and other waves are all electromagnetic fields differing only in the frequency of their oscillations.  Visible light waves oscillate between 1012 to 1016 cycles per second.  X-rays oscillate between 1016 and 1019 cycles per second.  By way of comparison, TV waves oscillate at 107  to 108.  For cosmic rays the variance is between 1026 and 1029 cycles per second.  (See Fritjof Capra, The Tao of Physics.)

X-rays allowed astronomers to learn more about the composition for the planet.  The atmosphere is more expansive than originally estimated. Because a planet’s atmosphere obstructs X-rays, but not optical light, scientists could compare the relative obstruction.

Katja Poppenhaeger, the lead author of the paper, published in the current issue of the Astrophysical Journal, said that the atmosphere of HD 189733b blocks X-rays at altitudes of more than 60,000 kilometers above the corresponding optical radius, an area 75% larger.  The atmosphere is calculated to be heated to 20,000° kelvin (K) in order to sustain itself at such high altitudes. The larger X-ray radius indicates that the planet is losing its atmosphere about 40% faster than previously thought.

 Astronomers have known for about a decade that ultraviolet and X-ray radiation from the main star is evaporating the atmosphere of HD 189733b over time. The planet is losing 100 million to 600 million kilograms of mass per second. Its atmosphere is thinning 25 to 65 percent faster than it would be if the planet’s atmosphere were less expansive.

The X-rays have detected a faint red companion of the main star in HD 189733.  This peer orbits the main star about once every 3,200 years.  Although both stars were likely formed at the same time, the main star appears to be 3 billion to 3 ½ billion years younger than its companion, because it rotates faster, displays higher levels of magnetic activity and is about 30 times brighter.  Having an orbiting planet as big as HD 189733b may be keeping the star’s rotation and magnetic activity high because of tidal forces, which makes the star appear to be much younger.

Poppenhaeger said that if other hot exoplanets have an expanded X-ray radius, this may be due to the nearness of the planets to their solar hosts, to their overall density, or to other factors yet to be determined.   In any event, HD 189733b is rapidly burning up and losing its atmosphere.

 By:  Tom Ukinski



Chandra X-Ray