For the first time, a giant exoplanet has been discovered orbiting a dead star (which in this case happens to a white dwarf) every 1.4 days. The planet is roughly the same size as Jupiter and is no more than 14 times as massive and has an equilibrium temperature of 165 K. The mass of the white dwarf is approximately 0.5 solar masses and with the radius roughly 10% that of the Sun. The groundbreaking discovery demonstrated that giant planets can be scattered into tight orbits without being tidally disrupted, and motivates searches for smaller transiting planets around white dwarfs.
NGC 2392: A planetary nebula, a phase that results when a star like the Sun becomes a red giant and sheds its outer layers. Source: Chandra |
Exoplanet research is clearly amongst the most interesting research topics in astronomy. With the advancement of high-quality telescopes and detectors, thousands of exoplanets have been discovered and the numbers are only projected to surge in the coming years. Normally, exoplanets orbiting a star would eventually find itself in danger of being engulfed when the host star evolves into a phase called "the red giant". Our Sun will enter this phase in about 5 billion years and will engulf Earth in the process. However, the outer gas giants such as Jupiter might survive this phase but may find themselves orbiting a much smaller object known as a white dwarf.
Although astronomers previously have detected the presence of rocky debris and gaseous discs orbiting white dwarfs, no conclusive evidence of intact planets orbiting white dwarfs have been reported until the new study made the discovery of a massive planet orbiting the white dwarf 1856+534 every 1.4 days. WD 1856+534 is located at a distance of 81 light-years and has an effective temperature of roughly 4700 K.
The authors note that when the WD 1856 evolved into a red giant, the planet must probably be very far away (more than 1 astronomical unit) to avoid being engulfed or stripped. This however then raises the question of how the planet arrived in the close orbit around the white dwarf we observe today. The existence of a planet so close to a white dwarf is intriguing and cannot be explained by a popular theory known as common envelope evolution. In this theory, once a star hits the red giant stage, the expanding outer envelopes of the star engulfs the companion. Friction from the envelope causes the companion to rapidly spiral inward towards the giant star and results in a close orbit ranging from hours to days. Nonetheless, the inward spiral could be halted if the system has very high gravitational potential energy to unbind the envelope. The authors note that the existence of the planet close to WD 1856 cannot be explained by this theory because of the combination of the lowest mass and the longest orbital period of any similar system. This implies that the gravitational potential energy released during the common envelope phase is very small, which in turn makes it difficult to successfully eject the envelope of the WD progenitor. Therefore, the authors conclude that there may be some other mechanism by which such stable configuration has been attained by the system.
An X-ray image of the Sirius star system located 8.6 light-years from Earth. Source: Chandra |
Although astronomers previously have detected the presence of rocky debris and gaseous discs orbiting white dwarfs, no conclusive evidence of intact planets orbiting white dwarfs have been reported until the new study made the discovery of a massive planet orbiting the white dwarf 1856+534 every 1.4 days. WD 1856+534 is located at a distance of 81 light-years and has an effective temperature of roughly 4700 K.
Artist's illustration of an exoplanet near a white dwarf. Source: NASA |
The authors note that when the WD 1856 evolved into a red giant, the planet must probably be very far away (more than 1 astronomical unit) to avoid being engulfed or stripped. This however then raises the question of how the planet arrived in the close orbit around the white dwarf we observe today. The existence of a planet so close to a white dwarf is intriguing and cannot be explained by a popular theory known as common envelope evolution. In this theory, once a star hits the red giant stage, the expanding outer envelopes of the star engulfs the companion. Friction from the envelope causes the companion to rapidly spiral inward towards the giant star and results in a close orbit ranging from hours to days. Nonetheless, the inward spiral could be halted if the system has very high gravitational potential energy to unbind the envelope. The authors note that the existence of the planet close to WD 1856 cannot be explained by this theory because of the combination of the lowest mass and the longest orbital period of any similar system. This implies that the gravitational potential energy released during the common envelope phase is very small, which in turn makes it difficult to successfully eject the envelope of the WD progenitor. Therefore, the authors conclude that there may be some other mechanism by which such stable configuration has been attained by the system.
Left image: The X-ray image of the planetary nebula known as the Cat's Eye. Right image: The composite image of Chandra and Hubble Space Telescope of the same object. Source: Chandra The authors conclude by noting that future observations with the help of Spitzer or JWST should be able to verify the nature of the exoplanet. The exoplanet is also amongst the coolest planet detected so far with an equilibrium temperature of only 165 K and therefore, transmission spectroscopy observations could probe for species like methane and ammonia in the atmosphere which could shed light on the nature of such exoplanets. The study has been published in the journal Nature. Article Information: A. Vanderburg et al., "A Giant Planet Candidate Transiting a White Dwarf", Nature, 585, 363–367 (2020). |
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