The debris disk around HD 4796 A.

The term debris disk is applied to any ring of sub-planetary sized objects around a star. In our Solar System both the Main Asteroid Belt and the Kuiper Belt would qualify as debris disks, though in practice neither of these belts would be detectable around another star with current technology. However younger star systems often have much more detectable debris disks, containing extensive fields of dust from frequent collisions from asteroid or comet sized bodies (such collisions probably petered out when our Solar System was about 40 million years old). To date over three dozen such debris disks have been discovered around young stars, and are taken as signs of planetary formation occurring or having occurred in the very recent past, and planets have been subsequently discovered in a number of systems with debris disks.

HD 4796 A is an A0 type white dwarf star with a mass 2.2-2.4 times the mass of our Sun around 237 light years from Earth in the constellation of Centaurus, which is thought to be 8-10 million years old. This star has been known to have a debris disk since 1998. This is viewed at an inclination of 75.88˚ when viewed from Earth, and is brighter in the northwest than in the southeast. In centre of the ring was found to be slightly offset from the star. It has been suggested that this asymmetry may be due to the ring receiving a variation in stellar radiation around its circumference, or possibly due the presence of a M type red dwarf companion star. The ring is apparently unusually narrow, with an inner edge 76.4 AU from the star (i.e. 76.4 times as far from the star as the Earth is from out Sun, and an outer edge that appears to become rapidly more tenuous as it gets further from the star. It has been suggested that the dust is being created by constant collisions between small planetesimals within a narrow ring, and the tailing off on the outer edge is caused by the dust being blown away by the pressure of radiation from the star.

In a paper published in the journal Astronomy & Astrophysics on 29 April 2014, and on the arXiv database at Cornell University Library on 25 April 2014, a team of scientists led by Zahed Wahhaj of the European Southern Observatory describe the results of a new study of HD 4796 A using the Near Infrared Coronagraphic Imager on the Gemini-South 8.1 meter Telescope, and discuss the results of this study.

Hahaj et al. observed HD 4796 A on 14 January 2009 and 6 & 7 April 2012, at a total of five different wavelengths (different materials radiate and reflect light at different wavelengths – giving them distinctive colours – as well as using this to understand the physical properties of observed objects, astronomers can use observations at different wavelengths to filter out background interference), thereby producing a better model of the system than had previously been the case.

A JHKs false-color image of the HR 4796 A ring, showing its relative reflectivity, as estimated in section 4.4. North is up and east is left (the directions east and west are reversed in astronomical images as the observer is looking up). The J, H and Ks-bands are coloured blue, green and red, respectively. The unsaturated star is normalized to one in all the bands and appears white in the image. The ring appears yellow because it reflects light more efficiently in the H and Ks-bands than in the J-band. The noise-dominated regions with 0.2′′ to 0.5′′ separation from the star are not shown. Regions away from the ring are coloured white and given the median intensity of the three bands. Wahaj et al. (2014).

Based upon these observations Wahaj et al. were able to construct a better model of the HD 4796 A system. This confirmed that the ring is offset compared to the star, and also showed the ring to be narrower and more confined than previously thought, restricted to between 71.7 and 86.9 AU from the star.

Wahaj et al. conclude that the most likely explanation for these observations is an undetected planet in the HD 4796 A system. Planets are often used to explain sharp edges to debris disks, but this does not happen simply by the planet hovering up material gravitationally, but rather by the action of tidal forces. 

Objects orbiting at certain distances from larger bodies are in stable areas and free from the tidal influences of the larger body. However objects close to these distances are subject to much stronger tidal forces, either locking them into the stable point or pushing them away from it. Some of these stable points are found at orbital resonances with the larger object; thus a small body in a 2:1 or 3:2 resonance with a large body is in a stable orbit (an object in a 2:1 resonance with another completes 2 orbits for every one of the other’s), but one in a 2:1.05 or 2.95:3 resonance will be ejected from the system by tidal forces. Since getting into an exact resonance with a large body with a slightly eccentric orbit is in practice very hard to do, bodies are effectively herded away from these resonances, and trapped into safe zone between them. In our Solar System there are several instances of this, for example the Kuiper Belt has sharply delaminated inner and outer limits, which coincide with the 2:1 and 3:2 orbital resonances of Neptune.

Wahaj et al. calculate that a planet orbiting at a distance of 54.7 AU from HD 4796 A would produce a 2:1 orbital resonance at 71.7 AU from the star and a 3:2 orbital resonance at 86.9 AU from the star, and therefore conclude that the presence of such a planet is the best explanation for the observed properties of the debris disk. In order to produce the observed properties the planet would have to have a mass of less than 1.6 times that of Jupiter (and could potentially be as small as three Earth masses), considerably smaller than would have been detected directly with the instruments that have been directed at the system so far. The offset observed in the orbit of the debris disk almost certainly reflects the eccentricity of the planet’s orbit.

See also…

 Searching for circumplanetary disks around LkCa 15.

 Discovering new debris disks in old Hubble images.

 The debris disk of 49 Ceti.

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The planets of NN Serpentis.

The NN Serpentis system comprises a young binary pair of stars 1670 light years from Earth in the constellation of Serpens. The pair comprises a White Dwarf star with 53.5% of the Sun's mass and a Red Dwarf star with 11% of the Sun's mass orbiting at a distance of 1.5 AU (1.5 times the distance at which the Earth orbit's the Sun, or slightly under the radius of Mars's orbit). The pair form an eclipsing binary; they orbit one-another edge on when seen from Earth, so that they regularly eclipse one another every 3.12 hours. The system is probably only about a million years old, and it is likely that the main star was originally considerably larger, and shed matter under the smaller stars influence. This would have formed a common envelope around the two stars, with some of the mass eventually falling back, some settling on the smaller star and some being lost into space.

In 2009 a team of scientists led by Shengbang Qian of the Yunnan Astronomical Observatory and the Graduate University of the Chinese Academy of Sciences proposed that the system was orbited every 7.56 year, by a large planet (or possibly small Brown Dwarf), with a mass 1.4% of the Sun's at a distance of under 3.29 AU, in a paper published in The Astrophysical Journal.

A follow up study led by Klaus Beuermann of the Institut für Astrophysik at Georg-August-Universität published in the journal Astronomy & Astrophysics in 2010, based upon data from the MONET/North 1.2-m telescope at McDonald Observatory suggested that this was incorrect, and that the system was in fact orbited by two smaller (though still substantial) planets, but was not able to provide a confident estimate of the size and orbits of these. 

In a paper published on the online arXiv database at Cornell University Library on 28 May 2013, and in the journal Astronomy and Astrophysics on 29 May, Klaus Beuermann, along with Stefan Dreizler and Frederic Hessman, also of the Institut für Astrophysik at Georg-August-Universität, present the results of a more long term study with the MONET/North telescope, which resolves the masses and orbits of the planets of NN Serpentis.

Beuermann et al. conclude that the inner planet has a mass 1.74 times that of Jupiter, and orbits the primary star at a distance of 3.358 AU every 7.647 years, while the outer planet has a mass 6.96 times that of Jupiter, and orbits at a distance of 5.389 AU every 15.482 years. Furthermore the two planets appear to be locked into an orbital resonance, so that the inner planet completes two orbits for every single orbit of the outer planet.

The orbits of the planets of NN Serpentis. The locations of the periapses (point at which the planets are closes to the center of the system) are marked ‘P’, the solid dots indicate conjugation, and the open circles opposition. Orbital motion is counter-clockwise. Beuemann et al. (2013).

See also Citizen scientists discover a sub-Jovian planet in a quaternary star system, Two new more planets discovered in the Gliese 676 system, Two new views of τ Boötis b, The formation of a Keplerian Disk in the L1551 NE Protostellar System and Thermal imaging 55 Cancri e.

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The object orbiting GD66 is probably a planet, not a Brown Dwarf.

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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Giant planet in the RR Caeli System.

RR Caeli is an eclipsing binary star system in the constellation of Caelum, 36 light years from Earth. The name implies the tenth variable star in the constellation of Caelum, but the variability is in fact the two stars passing in front of one-another in our line of sight (eclipsing), rather than any variation in their output. The system is referred to as a pre-cataclysmic binary, as the two stars are thought to be in a decaying orbit, that will eventually cause them to collide, but this will not happen for 9-20 billion years, so we are unlikely to be around to see it (our own sun will probably swell into a Red Giant and swallow the Earth in about 5.4 billion years, so the 'pre-cataclysmic' RR Caeli system should be safe for longer than our own solar system).

The position of RR Caeli (RR Cae). The position of East (bottom right)is reversed as this is a map of the sky. The scale bar is 5 arc-minutes; the sky is divided into 360 degrees (assuming the Earth is invisible and you can see underneath you) and each degree into 60 arc-minutes. From Bruch and Diaz (1998).

The two stars of the RR Caeli system are a cool White Dwarf star 44% of the mass of the Sun and a smaller M4-type Red Dwarf with 18.2% of the Sun's mass. These two orbit about one-another every 7.3 hours.

On 2 January 2012 a team lead by S.B. Qian of the Yunnan Observatory and the Chinese Academy of Sciences published a paper on arXiv online database at Cornell University Library, detailing the discovery a planet in the RR Caeli system, using data collected by the Jorge Sahade Telescope at the Complejo Astronómico El Leoncito in San Juan, Argentina.

Qian et al. were able to detect minute variations in the period of RR Caili, from which they were able to infer the presence of a planet with a mass of 4.2 times that of Jupiter, orbiting RR Caeli at a distance of 5.3 AU (5.3 times the distance between the Earth and the Sun, or 2% more than the distance at which Jupiter orbits the Sun) every 11.9 years, with an apparent eccentricity of zero (i.e. in a completely circular path, rather than an elliptical one like most planets). The orbital period of 11.9 years is so close to the orbital period of Jupiter (11.86 years) that the team were concerned that the gravitation influence of Jupiter might be somehow influencing their findings, however they were eventually able to rule this out.

See also Could there be a habitable planet in the Kepler-16 system? The strange debris disk of 99 Herculis, Third body in the AV Canis Minoris system and Exoplanets on Sciency Thoughts YouTube.