New Planet Found, Big Dog Constellation

A planetary companion has been detected circling the star HD 73256, bringing to 106 the total number of planets thus far discovered orbiting stars other than our Sun. The planet is one of a class known as "hot jupiters" — gas giants located in tight orbits extremely close to their parent stars.

Henry Draper (HD) catalog for planet HD73256, located in the southern sky near Canis Major (the Big Dog).
Credit: Digital Sky Survey

The discovery was made by a team of European astronomers carrying out precise Doppler observations at the La Silla Observatory in Chile. Their method of detection, called radial velocity, or Doppler spectroscopy, infers the presence of an unseen companion because of the back-and-forth movement it induces in the host star. A planet exerts a small gravitational pull on its parent star, causing the star to wobble. The motion amplitude depends on the orbital distance and mass of the planet. This movement is detectable as a periodic red shift and blue shift in the star’s spectral lines.

The Atacama Desert is in the north of Chile, about 1300km (800mi) from Santiago.[There are parts of Atacama where rain has never been recorded and the precious little precipitation (1cm/0.3in per year) that does fall comes from fog.]
Credit: ESO

With a magnitude of 8.08, the host star can be viewed with binoculars or a low-powered telescope in the southern sky. The star is slightly larger than our Sun and is located 119 light-years away in the constellation Canis Major (the Big Dog). The new planet is the second one found in the constellation.

The planetary companion is a gas giant with a derived mass nearly twice (1.85) that of Jupiter. Its orbit is nearly circular and lasts just over two-and-half days. Many of these huge planets orbit closer to their parent star than our own innermost planet, Mercury.

Recent surveys of some 1,000 stars reveal about 10 percent have gas giants orbiting them, generally ranging in size from about the mass of Jupiter to 10 times that large. Among these large planets, a handful of "hot Jupiters" have been found in tight orbits, the closest taking only 3 days to revolve around its parent star. Theorists have explained the existence of "hot Jupiters" by hypothesizing that the planet forms farther out in the disk of primordial material surrounding a newborn star. The gas giant then migrates inward, pulled by disk matter closer to the star and pushed by disk matter farther out. Any planet that moved too far inward was expected to be pushed completely into the star, where it would be swallowed up and destroyed.

The discovery was made as part of the Coralie Planet Search Program, an ongoing survey that has discovered eight extrasolar planets to date. The European Southern Observatory (ESO) at La Silla (the ‘saddle’) borders the southern extremity of the driest place on Earth , the Atacama desert in Chile.

Previous planets have been found using a host of inferred techniques that don’t directly image the planet, but do measure its effects on stars that can be seen. These include the periodic red- and blue-shifts in a star (Doppler technique) as it wobbles, the eclipsing of the star (transit search), and bending of the star’s light (microlensing). Astronomers using NASA’s Hubble Space Telescope have observed just recently for the first time the atmosphere of a planet beyond our solar system evaporating into space.

What’s Next

During the next 15 years or so, American and European scientists hope to launch more than half a dozen missions to search our corner of the Milky Way galaxy for terrestrial planets.

The CORALIE spectrometer housed at the Geneva Observatory was used in the discovery of previous exoplanets.
Credit: The Geneva Extrasolar Planet Search Programmes

To search for Earth-like planets around stars beyond our solar system, the Kepler Mission , scheduled for launch in 2006, will use a space-borne telescope. Kepler will simultaneously observe 100,000 stars in our galactic "neighborhood," looking for Earth-sized or larger planets within the "habitable zone" around each star – the not-too-hot, not-too-cold zone where liquid water might exist on a planet. One NASA estimate says Kepler should discover 50 terrestrial planets if most of those found are about Earth’s size, 185 planets if most are 30 percent larger than Earth and 640 if most are 2.2 times Earth’s size. In addition, Kepler is expected to find almost 900 giant planets close to their stars and about 30 giants orbiting at Jupiter-like distances from their parent stars. A key criterion for such suitable planets would be whether they reside in habitable zones, or regions sometimes protected by gas giants but with temperate climates and liquid water.

After Kepler, NASA is considering a 2009 launch for SIM, the Space Interferometry Mission . SIM’s primary mission will be to measure distances to stars with 100 times greater precision than now is possible. This will improve estimates of the size of the universe and help astronomers determine the true brightness of stars, and thus learn more about their chemical composition and evolution. SIM also will look for Earth-sized planets in the habitable zones around some 200 stars. SIM will be an interferometer, which means it will combine interacting light waves from its three component telescopes. This interaction, called interference, makes the individual telescopes, which are separated from each other on the spacecraft, act as though they were a single, larger telescope with greater light-gathering ability.

Future missions, such as ESA’s Herschel mission will search for many more and take detailed pictures of stars that might harbor dusty remnants of entire solar systems. As these images become available, astronomers will be able to predict the sizes and orbits of giant planets within the distant solar system.

Related Web Pages

European Southern Observatory
Astrobiology Magazine New Planets
Transit Search
Extrasolar Planets Encyclopedia
Planet Quest (JPL)
Kepler Mission
Eddington Mission
Darwin Mission
Herschel Mission
Space Interferometry Mission
Geneva Observatory JPL PlanetQuest for HD 73256