HD 209458 b

Artist's conception of HD 209458 b transiting its star, ESA/Hubble.

HD 209458 b is an extrasolar planet that orbits the Sun-like star HD 209458 in the constellation Pegasus, some 150 light-years from Earth's solar system, with evidence of water vapor.[1]

The radius of the planet's orbit is 7 million kilometers, about 0.047 astronomical units, or one eighth the radius of Mercury's orbit. This small radius results in a year that is 3.5 Earth days long and an estimated surface temperature of about 1,000 degrees Celsius or 1,800 degrees Fahrenheit. Its mass is 220 times that of Earth (0.69 Jupiter masses). Its volume is some 146% greater than that of Jupiter. The high mass and great volume of HD 209458 b lead to its classification as a gas giant.

HD 209458 b represents a number of milestones in extraplanetary research. It was the first transiting extrasolar planet discovered, the first extrasolar planet known to have an atmosphere, the first extrasolar planet observed to have an evaporating hydrogen atmosphere, the first extrasolar planet found to have an atmosphere containing oxygen and carbon, and one of the first two extrasolar planets to be directly observed spectroscopically. Based on the application of new, theoretical models, as of April 2007, it is alleged to be the first extrasolar planet found to have water vapor in its atmosphere.[2][3][4]

HD 209458 b has been informally referred to as Osiris, though this name was not given by the astronomers credited with its discovery. The use of the Osiris nickname has faded among astronomers and the media.[5][6] HD 209458 is an 8th magnitude star, visible from Earth with binoculars.

Discovery

Spectroscopic studies first revealed the presence of a planet around HD 209458 on November 5, 1999. Astronomers had made careful photometric measurements of several stars known to be orbited by planets, in the hope that they might observe a dip in brightness caused by the transit of the planet across the star's face. This would require the planet's orbit to be inclined such that it would pass between the Earth and the star, and previously no transits had been detected.

Soon after the discovery, separate teams, one led by David Charbonneau including Timothy Brown and others, and the other by Gregory W. Henry, were able to detect a transit of the planet across the surface of the star making it the first known transiting extrasolar planet. On September 9 and 16, 1999, Charbonneau's team measured a 1.7% drop in HD 209458's brightness, which was attributed to the passage of the planet across the star. On November 8, Henry's team observed a transit ingress.[7] Each transit lasts about three hours, during which the planet covers about 1.5% of the star's face.

The star had been observed many times by the Hipparcos satellite, which allowed astronomers to calculate the orbital period of HD 209458 b very accurately at 3.524736 days.[8]

Physical parameters

Spectroscopic analysis had shown that the planet had a mass about 0.69 times that of Jupiter.[9] The occurrence of transits allowed astronomers to calculate the planet's radius, which had not been possible for any previously known exoplanet, and it turned out to have a radius some 35% larger than Jupiter's. It had been previously hypothesised that hot Jupiters particularly close to their parent star should exhibit this kind of inflation due to intense heating of their outer atmosphere. Tidal heating due to the planet's eccentric orbit may also play a role.[10]

Observations by the orbiting Microvariability and Oscillations of STars telescope has shown that the planet has an albedo (or reflectivity) that is less than 30%, making it a surprisingly dark object. In comparison, Jupiter has a much higher albedo of 52%. This would suggest that HD 209458 b may not be covered with clouds (which are in general quite reflective), against all expectations.[11]

It is assumed that HD 209458 b's days are the same length as its years; that is, it rotates about its axis every time it rotates around its star due to tidal locking.[citation needed]

Detection of the atmosphere

The transit of HD 209458 b.

On November 27, 2001 the Hubble Space Telescope detected sodium in the planet's atmosphere, the first planetary atmosphere outside our solar system to be measured. This detection was predicted by Sara Seager in late 2001.[12] In 2003-4, astronomers used the Hubble Space Telescope Imaging Spectrograph to discover an enormous ellipsoidal envelope of hydrogen, carbon and oxygen around the planet that reaches 10,000 K. At this temperature, the Maxwell-Boltzmann distribution of particle velocities gives rise to a significant 'tail' of atoms moving at speeds greater than the escape velocity, and the planet is estimated to be losing about 100-500 million (1-5×108) kg of hydrogen per second. Analysis of the starlight passing through the envelope shows that the heavier carbon and oxygen atoms are being blown from the planet by the extreme "hydrodynamic drag" created by its evaporating hydrogen atmosphere. The hydrogen tail streaming from the planet is approximately 200,000 kilometers long which is roughly equivalent to its diameter.

It is thought that this type of atmosphere loss may be common to all planets orbiting Sun-like stars closer than around 0.1 AU. HD 209458 b will not evaporate entirely, although it may have lost up to about 7% of its mass over its estimated lifetime of 5 billion years.[13]

Direct detection

On March 22, 2005, NASA released news that infrared light from the planet had been measured by the Spitzer Space Telescope, the first ever direct detection of light from an extrasolar planet. This was done by subtracting the parent star's constant light and noting the difference as the planet transited in front of the star and was eclipsed behind it, providing a measure of the light from the planet itself. New measurements from this observation determined the planet's temperature as at least 750 °C (1300 °F). The circular orbit of HD 209458 b was also confirmed.

Spectral observation

On February 21, 2007, NASA and Nature released news that HD 209458 b was one of the first two extrasolar planets to have their spectra directly observed, the other one being HD 189733b.[14][15] This was long seen as the first mechanism by which extrasolar but non-sentient life forms could be searched for, by way of influence on a planet's atmosphere. A group of investigators led by Dr. Jeremy Richardson of NASA's Goddard Space Flight Center spectrally measured HD 209458 b's atmosphere in the range of 7.5 to 13.2 micrometers. The results defied theoretical expectations in several ways. The spectrum had been predicted to have a peak at 10 micrometers which would have indicated water vapor in the atmosphere, but such a peak was absent, indicating no detectable water vapor. Another, unpredicted peak was observed at 9.65 micrometers, which the investigators attributed to clouds of silicate dust, a phenomenon not previously observed. Another unpredicted peak occurred at 7.78 micrometers, which the investigators did not have an explanation for. A separate team led by Mark Swain of the Jet Propulsion Laboratory also observed HD 209458 b's spectrum, and had not yet published their results when the Richardson et al. article came out, but made similar findings.

Atmospheric water vapor

On April 10, 2007, Travis Barman of the Lowell Observatory announced evidence that the atmosphere of HD 209458 b contained water vapor. Using a combination of previously published Hubble Space Telescope measurements and new theoretical models, Barman found strong evidence for water absorption in the planet's atmosphere.[1][2][16][17] His method modelled light passing directly through the atmosphere from the planet's star as the planet passed in front of it. However, this hypothesis is still being investigated for confirmation.

Barman drew on data and measurements taken by Heather Knutson, a student at Harvard University, from the Hubble Space Telescope, and applied new theoretical models to demonstrate the likelihood of water absorption in the atmosphere of the planet. The planet orbits its parent star every three and a half days, and each time it passes in front of its parent star, the atmospheric contents can be analysed by examining how the atmosphere absorbs light passing from the star directly through the atmosphere in the direction of Earth.

According to a summary of the research, atmospheric water absorption in such an exoplanet renders it larger in appearance across one part of the infrared spectrum, compared to wavelengths in the visible spectrum. Barman took Knutson's Hubble data on HD209458 b, applied to his theoretical model, and allegedly identified water absorption in the planet's atmosphere.

On April 24, 2007, the astronomer David Charbonneau, who led the team that made the Hubble observations, cautioned that the telescope itself may have introduced variations that caused the theoretical model to suggest the presence of water. He hoped that further observations would clear the matter up in the following months.[18] As of April 2007, further investigation is being conducted.

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