29 November 1999
For immediate release
The world's most prolific team of planet hunters has found six new planets orbiting nearby stars, bringing the total number of planets that astronomers have detected outside the solar system to 28. The researchers also found evidence suggesting that two previously discovered planets have additional companion planets.
Kevin Apps, an undergraduate at the University of Sussex, has been credited with the co-discovery of the six new planets. Mr Apps and his American colleagues, Steven Vogt, Geoffrey Marcy and Paul Butler, made the discoveries using the "HIRES" spectrometer on the Keck I Telescope in Hawaii. Their findings will be published in the Astrophysical Journal.
The researchers have been using the facilities at the Keck Observatory to conduct a survey of 500 nearby Sun-like stars in search of planets. The project is supported by the NASA Origins programme and the National Science Foundation.
The six new planets increase by about 25 per cent the number of known "extrasolar" planets, giving astronomers a substantial amount of additional information about planetary systems. The new planets orbit stars that are similar in size, age, and brightness to the Sun and are at distances ranging from 65 to 192 light years from Earth. The planets themselves range in mass from slightly smaller to several times larger than the planet Jupiter (0.8 to 6.5 times the mass of Jupiter). They are probably also similar to Jupiter in their compositions - basically giant balls of hydrogen and helium gas.
The presence of a planet around a star is indicated by a telltale "wobble" in the motion of the star as a result of the gravitational force exerted by the orbiting planet. The research team recently achieved independent confirmation of this method for detecting planets when they were able to measure the dimming of a star as a planet passed in front of it.
The orbits of the six new planets, like those of most of the extrasolar planets discovered so far, tend to be quite eccentric, tracing paths that are oval rather than circular. The researchers conclude from this fact that neatly stacked, circular orbits such as we see in our own solar system are relatively rare.
One of the planets, around a star called HD 222582, has the most wildly eccentric orbit yet known, carrying it from as close as 0.39 astronomical units (AU: the distance from Earth to the Sun) to as far as 2.31 AU from its parent star in the course of its 576-day orbit.
Interestingly, five of the six planets are located within the so-called "habitable zones" of their stars. This is the region where temperatures would allow water to exist in liquid form. Most of the extrasolar planets the researchers have studied have turned out to be outside the habitable zone, either too close to their star or too far away, and therefore too hot or too cold. These planets, on the other hand, are at just the right distance, with temperatures in one case around 108 degrees Fahrenheit.
Planetary systems with Jupiter-sized planets in oval-shaped orbits are not expected to harbour Earth-like planets. In fact, if an Earth-like planet were put into such a system, it would be quickly ejected by the gravitational influence of the Jupiter-mass planet. However, if these Jupiter-sized planets are similar to those in our own solar system, they probably have numerous moons associated with them. For a planet in the habitable zone of its star, such moons offer the possibility of liquid water and the eventual emergence of life.
In addition to the discovery of six new planets, the researchers gathered new data on four previously known planets. Two of them, around the stars HD 217107 and HD 187123, showed long-term trends in their orbits, indicating the presence of an additional companion. These companion planets appear to be orbiting their host stars in a long period, taking at least two to three years to complete an orbit. These findings are significant because previously only one other system of multiple planets, around the star Upsilon Andromedae, had been identified. It will take years of additional observations to work out the orbits of these companion planets, say the research team, but the evidence suggests that a fair number of multiple planet systems exist.
Specific details about the new planets and their host stars are given below:
HD 10697 is a G5IV star, slightly cooler and a bit larger than the Sun. It lies 106 light years away in the constellation Pisces. Its planet has a minimum mass of 6.35 Jupiter masses and a 1072-day orbit. The radius of this orbit is about 2.13 AU, but the orbit is somewhat eccentric, so the planet's distance from its star ranges from 1.87 AU to 2.39 AU. At its average orbital distance, it lies just at the outside edge of the habitable zone of its star, and is expected to have an equilibrium temperature (due to receiving energy from its parent star) of about 15 degrees Fahrenheit.
HD 37124 is a G4V star, slightly cooler than the Sun. It lies 108 light years away in the constellation Taurus. Its planet has a minimum mass of 1.04 Jupiter masses and a 155.7-day orbit. This orbit is also quite eccentric. At its average orbital distance of 0.55 AU, it sits just within the inner edge of the habitable zone of its star, and is expected to have an equilibrium temperature of about 130 degrees Fahrenheit. This is the lowest metallicity star known to have a planet.
HD 134987 is a G5V star, 83 light years away in the constellation Libra. Its planet orbits in a 260-day eccentric orbit. This planet has a minimum mass of 1.58 Jupiter masses. At its average orbital distance of 0.81 AU, its expected equilibrium temperature is a balmy 108 degrees Fahrenheit. It lies well within the habitable zone of its star.
HD 177830 is a K2IV star, about 1,000 degrees Kelvin cooler than the Sun, lying about 192 light years away in the constellation Vulpecula. It harbours a 1.22 Jupiter-mass planet in a 392-day, highly eccentric orbit. This orbit carries the planet from as close as 0.63 AU from its star to as far as 1.57 AU. At its mean orbital distance of 1.10 AU its expected temperature is about 192 degrees Fahrenheit. The planet is probably within the habitable zone of its star.
HD 192263 is a K2V star lying 65 light years away in the constellation Aquila. A planet around this star was first reported by Nuno Santos and collaborators at the University of Geneva while Mr Apps and his colleagues were preparing their paper. The research team has obtained essentially the same results as Santos: a 0.78 Jupiter mass planet orbiting in a 24.36-day orbit. This orbit has a radius of only 0.15 AU, with little or no eccentricity. It orbits well outside the habitable zone of its star.
HD 222582, a G3V star, is a near solar twin, 137 light years away in the constellation Aquarius. Its planet orbits in a wildly eccentric 576-day orbit, which carries the planet from 0.39 AU to 2.31 AU from the parent star in the course of its oval orbit. This is the most eccentric extrasolar planet orbit yet known. The planet's expected temperature is about -38 degrees Fahrenheit. Its mean orbital distance places it squarely in the habitable zone of its star.
Further information about the planet search is available on the Web at www.physics.sfsu.edu/~gmarcy/planetsearch/planetsearch.html.
For further information, please contact Alison Field, Communications Officer, University of Sussex Tel: 01273 678888, Fax: 01273 678335, email: A.Field@sussex.ac.uk
Kevin Apps can be contacted by email at firstname.lastname@example.org
or by telephone on 01293 535904 (home) and 0780123 1649 (mobile)
Photographs of Kevin Apps are available