The HAT-P-13 planetary system was discovered in 2009, by the Hungarian-made Automated Telescope Network, a network of six small telescopes located at Konkoly Observatory near Budapest, Steward Observatory in Arizona, the Wise Observatory in the Negev Desert, Israel, Las Campanas Observatory in Chile, Siding Spring Observatory in Australia and the International Amateur Observatory near Gamsberg in Namibia. The HATNet project aims to detect and study extrasolar planets transiting their host stars. The star-system is in the constellation Ursa Major, roughly 697 light years from the Earth. It is also sometimes referred to as GSC 03416-00543, which is General Star Catalogue followed by a set of co-ordinates.
The star in the system HAT-P-13A ('A' for the first object discovered in the system, capitalized because it is a star) is a G-type main sequence star, the same type of star as our sun, though it is slightly larger, older and cooler. It is 1.2 times as massive as the sun, has a radius 1.5 times as great, and has a much higher metal content, a sign it is reaching the end of its life on the main sequence of stars.
This star is circled by at least two major planets, with the possibility of a third.
HAT-P-13b ('b' for the second body in the system, not capitalized because it is not a star) is a transiting Hot-Jupiter type planet. It orbits the star every 70 hours at 4% of the distance between the Earth and the sun, and has a mass 85% of that of Jupiter. Despite its lower mass it has a considerably larger volume due to its high temperature, giving it a radius of 1.3 times that of Jupiter.
HAT-P-13c is a much has not been directly observed, but its presence is known with a considerable degree of confidence, due to its strong gravitational effect on HAT-P-13b. It has a mass of at least 15 times that of Jupiter, making it a borderline contender for Brown Dwarf status; an object large enough to undergo convection in the interior like a star, rather than chemical stratification like a planet, but not massive enough to fuse hydrogen like a star. Brown Dwarfs may fuse deuterium and possibly lithium, and generate a considerable amount of heat due to gravitational effects. HAT-P-13c orbits at approximately 1.86 times the distance at which Earth orbits the sun (slightly further out than Mars) every 428 days. It has not been seen to transit (pass in front of) HAT-P-16A, but may do so.
The orbit of HAT-P-13b and HAT-P-13c. It is not clear if HAT-P-13c passes in front of the star when seen from Earth.
Despite the confidence with which astronomers view the existence of HAT-P-13c, attempts at building a model of the system have consistently failed to predict accurately the transits of HAT-P-13b. This has led to speculation that the system may contain a fourth body, HAT-P-13d, though this has not been detected, nor have scientists been able to develop a reliable model of the system using a four-body scenario.
On 24 November 2011 a paper was posted on the online arXiv database at Cornell University Library, by a team lead by John Southworth of the Astrophysics Group at Keele University, describing a new study on the HAT-P-13 system and the conclusions drawn from it. This paper is due to be published in print form in the Monthly Notices of the Royal Astronomical Society.
Southworth et al. reviewed a number of studies made of the HAT-P-13 system using Cassini Telescope at Loiano Observatory in Italy, a Schmidt-Cassegrain telescope at Portalegre in Portugal, a 1.2 m telescope at the Fred L Whipple Observatory in Arizona, two telescopes at Konkoly Observatory, and the Faulkes Telescope in Hawaii, and attempted to build a model of the system based upon this.
Like previous studies, Southworth et al. were unable to satisfactorily resolve the known data about the HAT-P-13 system and come up with a working model that could predict the transits of HAT-P-13b, either with or without HAT-P-13d. They were, however, able to build a model of the system without a forth body if they assumed that both HAT-P-13A and HAT-P-13b were considerably more massive than previously assumed, leading the team to conclude that this was in error in earlier studies, rather than a missing body from the system.
See also The Kepler-18 planetary system, Just how big is Iota Draconis b? and Exoplanets on Sciency Thoughts YouTube.