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Exoplanet Comparison XO-1 b.png
Size comparison of XO-1b with Jupiter.
Discovered byPeter McCullough et al.[1]
Discovery siteHaleakala Observatory, Hawaii[1]
Discovery date18 May 2006[2]
Transit and Radial velocity[1]
Orbital characteristics
3.94150685±0.00000091[4] d
Inclination88.8 ± 0.2[4]
Semi-amplitude116 ± 9[1]
Physical characteristics
Mean radius
1.21 ± 0.03[4] RJ
Mass0.913±0.038[3] MJ
Mean density
0.64 ± 0.05 g/cm3 (0.0231 ± 0.0018 lb/cu in)[4]
15.8 ± 1.5 m/s2 (51.8 ± 4.9 ft/s2)[5]

XO-1b is an extrasolar planet approximately 536 light-years away.

In 2006, the XO Project an international team of professional and amateur astronomers discovered a Jupiter-sized planet, later named XO-1b, orbiting a Sun-like star.[1] The team, led by Peter McCullough of the Space Telescope Science Institute in Baltimore, had four amateur astronomers hailing from North America and Europe.[6] An independent confirmation of the planet was made by the Wide Angle Search for Planets project later that same year.[7]

The XO Project team employed the relatively inexpensive XO Telescope, made from commercial equipment, to search for extrasolar planets.[2] This telescope is on the Hawaiian Island of Maui.[8]

From September 2003 to September 2005, the XO Telescope detected tens of thousands of bright stars. In that time, McCullough's team of amateur astronomers studied a few dozen stars they had previously identified as promising candidates for extrasolar planets. The star XO-1, in particular, was marked as a promising candidate in June 2005. The amateur astronomers observed it from June to July 2005, eventually confirming that a planet-sized object was eclipsing it. McCullough's team then turned to the McDonald Observatory in Texas for information on the object's mass and to confirm it was a planet.[1]


McCullough's team found the planet by detecting slight reductions in the star's intensity as the planet moved into transit of the star. The light from the star reduces by approximately two percent when XO-1b is in transit. Their observation revealed that XO-1b is in a tight, four-day orbit around its parent star.

While astronomers had detected more than 180 extrasolar planets, XO-1b is only the tenth planet discovered using the transit method. It is only the second planet found using telephoto lenses. The first, TrES-1, in the constellation Lyra, was reported in 2004. The transit method allows astronomers to determine a planet's mass and size. Astronomers use this information to deduce the planet's characteristics, such as density.

Secondary transits and phase curve of this planet have been observed.[9]

Radial velocity[edit]

The team confirmed the planet's existence by using the Harlan J. Smith Telescope and the Hobby-Eberly Telescope at the University of Texas's McDonald Observatory to measure slight perturbations induced by the planet on its parent star. The radial velocity method allowed the team to calculate a precise mass of the planet, which is slightly less than Jupiter's. This planet is much larger than its mass would suggest. McCullough has said, "Of the planets that pass in front of their stars, XO-1b is the most similar to Jupiter yet known, and the star XO-1 is the most similar to the Sun, but XO-1b is much, much closer to its star than Jupiter is to the Sun."

The technique used by the team to find XO-1b is an innovative method in that it uses a relatively inexpensive telescope to hunt for extrasolar planets. It, however, is limited primarily to planets orbiting close to their parent stars, and only finds planets large enough to cause a measurable depression in starlight.

Physical characteristics[edit]

As a planet with a mass comparable to that of Jupiter in a close-orbit around its star, this planet falls into the category of hot Jupiters. Like other known transiting hot Jupiters such as HD 209458 b and TrES-1, the low density of XO-1b indicates that this planet is a gas giant composed mainly of hydrogen and helium.

Observations with the NICMOS instrument on board the Hubble Space Telescope detected the presence of water vapor, methane, carbon dioxide, and possibly carbon monoxide in the atmosphere of XO-1b.[9] However, an independent reinvestigation of the same data was unable to reproduce these results.[10] Later studies by the Hubble Space Telescope detected water in the atmosphere of the exoplanet.[11][12]

See also[edit]


  1. ^ a b c d e f McCullough, P. R.; et al. (2006). "A Transiting Planet of a Sun-like Star". The Astrophysical Journal. 648 (2): 1228–1238. arXiv:astro-ph/0605414. Bibcode:2006ApJ...648.1228M. doi:10.1086/505651.
  2. ^ a b Donna Weaver (18 May 2006). "Astronomers Use Innovative Technique to Find Extrasolar Planet". STScI-2006-22. Space Telescope Science Institute. Retrieved 7 January 2012.
  3. ^ a b Bonomo, A. S.; et al. (2017). "The GAPS Programme with HARPS-N at TNG . XIV. Investigating giant planet migration history via improved eccentricity and mass determination for 231 transiting planets". Astronomy and Astrophysics. 602. A107. arXiv:1704.00373. Bibcode:2017A&A...602A.107B. doi:10.1051/0004-6361/201629882.
  4. ^ a b c d Burke, Christopher J.; et al. (2010). "NICMOS Observations of the Transiting Hot Jupiter XO-1b". The Astrophysical Journal. 719 (2): 1796–1806. arXiv:1006.1953. Bibcode:2010ApJ...719.1796B. doi:10.1088/0004-637X/719/2/1796.
  5. ^ Southworth, John (2010). "Homogeneous studies of transiting extrasolar planets – III. Additional planets and stellar models". Monthly Notices of the Royal Astronomical Society. 408 (3): 1689–1713. arXiv:1006.4443. Bibcode:2010MNRAS.408.1689S. doi:10.1111/j.1365-2966.2010.17231.x.
  6. ^ Robert Naeye (23 May 2006). "Amateurs Help Discover Transiting Exoplanet". sky and telescope. Sky Publishing. Retrieved 8 January 2012.
  7. ^ Wilson, D. M.; et al. (2006). "SuperWASP Observations of the Transiting Extrasolar Planet XO-1b". The Publications of the Astronomical Society of the Pacific. 118 (847): 1245–1251. arXiv:astro-ph/0607591. Bibcode:2006PASP..118.1245W. doi:10.1086/507957.
  8. ^ McCullough, P. R.; et al. (2005). "The XO Project: Searching for Transiting Extrasolar Planet Candidates". Publications of the Astronomical Society of the Pacific. 117 (834): 783–795. arXiv:astro-ph/0505560. Bibcode:2005PASP..117..783M. doi:10.1086/432024.
  9. ^ a b Tinetti, G.; et al. (2010). "Probing the Terminator Region Atmosphere of the Hot-Jupiter XO-1b with Transmission Spectroscopy". The Astrophysical Journal Letters. 712 (2): L139–L142. arXiv:1002.2434. Bibcode:2010ApJ...712L.139T. doi:10.1088/2041-8205/712/2/L139.
  10. ^ Gibson, N. P.; et al. (2011). "A new look at NICMOS transmission spectroscopy of HD 189733, GJ-436 and XO-1: no conclusive evidence for molecular features". Monthly Notices of the Royal Astronomical Society. 411 (4): 2199–2213. arXiv:1010.1753. Bibcode:2011MNRAS.411.2199G. doi:10.1111/j.1365-2966.2010.17837.x.
  11. ^ Staff (3 December 2013). "Hubble Traces Subtle Signals of Water on Hazy Worlds". NASA. Retrieved 4 December 2013.CS1 maint: Uses authors parameter (link)
  12. ^ Deming, Drake; et al. (2013). "Infrared Transmission Spectroscopy of the Exoplanets HD 209458b and XO-1b Using the Wide Field Camera-3 on the Hubble Space Telescope". The Astrophysical Journal. 774 (2). 95. arXiv:1302.1141. Bibcode:2013ApJ...774...95D. doi:10.1088/0004-637X/774/2/95.

External links[edit]

Media related to XO-1b at Wikimedia Commons

Coordinates: Sky map 16h 02m 12s, +28° 10′ 11″