A White Dwarf is the final stage in the life of most stars, a star that has used up all its Hydrogen then undergone a phase of expansion as a Red Giant, but which lacks the mass to undergo a supernova type explosion at the end of this stage of its cycle will end its life as a White Dwarf a small, dense star radiating residual thermal energy. White Dwarfs are typically very dense, with masses similar to that of the sun, but volume's similar to that of the Earth.
The size of a typical White Dwarf compared to the Earth. BBC.
GD66 is a White Dwarf star roughly 183 light years from Earth. It has a mass 66% of that of the Sun, which suggests that it is the remnant of a star originally 2.2-2.6 × the mass of our Sun, and a surface temperature of 11 980 K (compared to 5778 K for our Sun). It has been shown to undergo periodic dimming, that can be best explained by an object orbiting it at a distance of 2.4 AU (i.e. 2.4 × the distance at which the Earth orbits the Sun). It was thought that a planet-sized object would be unable to have survived the evolution of the star at a distance of less than 3.6 AU, leading to speculation that the object orbiting GD66 could be a small star or a Brown Dwarf (an object intermediate in size between a planet and a star).
In a paper published on the arXiv database at Cornell University Library on 7 May 2012, and accepted for publication in the Monthly Notices of the Royal Astronomical Society, a team of scientists lead by Jay Farihi of the Department of Physics & Astronomy at the University of Leicester discuss the results of a study of GD66 using the Hubble Space Telescope and the U.S. Naval Observatory.
Based upon this study Farihi et al. concluded that the object, dubbed GD66b, had a maximum size of 11 times the mass of Jupiter, and is probably less than 9 times Jupiter's mass. This is big, but still planet-sized, suggesting that planets can survive, or form, closer to White Dwarf's than had previously been realized. This has implications for the number of planets in the Galaxy, as White Dwarfs are very common objects.
See also The origin of the cthonian planets orbiting KIC 05807616, The impossible planets of HU Aquarii, KOI-13b, a big, hot planet not a Brown Dwarf, Planets in the NY Virginis system and Exoplanets on Sciency Thoughts YouTube.
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