| Extrasolar planet | List of extrasolar planets | |
|---|---|---|
| Parent star | ||
| Star | XO-1 | |
| Constellation | Corona Borealis | |
| Right ascension | (α) | 16h 02m 12s |
| Declination | (δ) | +28° 10′ 11″ |
| Distance | 600 ± 30 ly (180 ± 10 pc) |
|
| Spectral type | G1V | |
| Orbital elements | ||
| Semimajor axis | (a) | 0.0488 ± 0.0005 AU |
| Eccentricity | (e) | 0 |
| Orbital period | (P) | 3.941534 ± 2.7e-05 d |
| Inclination | (i) | 87.7 ± 1.2° |
| Time of periastron | (T0) | 2453808.917 ± 0.0011 JD |
| Semi-amplitude | (K) | 116 ± 9 m/s |
| Physical characteristics | ||
| Mass | (m) | 0.9 ± 0.07 MJ |
| Radius | (r) | 1.3 ± 0.11 RJ |
| Density | (ρ) | 543 kg m-3 |
| Surface gravity | (g) | 1.26 g |
| Discovery information | ||
| Discovery date | 17 May 2006 | |
| Discoverer(s) | Peter McCullough et al. | |
| Detection method | Transit and Radial velocity | |
| Discovery site | France | |
| Discovery status | Published | |
| Database references | ||
| Extrasolar Planets Encyclopaedia |
data | |
| SIMBAD | data | |
XO-1b is an extrasolar planet approximately 600 light-years away in the constellation of Corona Borealis. The planet was discovered orbiting the yellow dwarf star now designated XO-1 in 2006.
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In 2006, an international team of professional and amateur astronomers discovered a Jupiter-sized planet, later named XO-1b, orbiting a Sun-like star. The team, led by Peter McCullough of the Space Telescope Science Institute in Baltimore, had four amateur astronomers hailing from North America and Europe.
McCullough and his team employed a relatively inexpensive telescope called an XO Telescope, made from commercial equipment, to search for extrasolar planets. This telescope consists of two 200-millimeter telephoto camera lenses, and resembles binoculars in shape. It stands on the summit of the Haleakalā volcano in Hawaii.
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.
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 have 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.
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.
McCullough believes the newly found planet is a perfect candidate for study by the Hubble and Spitzer Space Telescopes. Hubble can precisely measure the star's distance and the planet's size, while Spitzer can photograph infrared radiation from the planet. By timing the disappearance of the planet behind the star, Spitzer can also measure the eccentricity of the planet's orbit—how much its orbital shape deviates from a circle. A high eccentricity implies a highly elliptical orbit, which would mean that the varying gravitational force from its parent star must heat the planet, expanding its atmosphere and perhaps explaining why the object's diameter exceeds the expectation from its calculated mass.
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.
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