Drake Equation Terms:The Fraction of Stars with Planets
Drake Equation Terms:
As the cloud contracts under the force of gravity, angular momentum causes it to flatten into a disk. The inner core if the disk contracts, increasing the temperature and density of the gas. Eventually the temperature and density reach a point where nuclear fusion begins and a star is born.
Meanwhile, in the disk, dust particles collide and adhere to form larger and larger particles. Over a span of millions of years, these particles collide and grow, perhaps the the point of becoming planets.
The Hubble Space Telescope has also spotted a number of these "protoplanetary disks" around young stars in the orion nebula.
The results of Mayor and Queloz have been independently confirmed by Dr. Geoff Marcy and Dr. Paul Butler at San Francisco State University and the University of California.
The star 51 Pegasi is similar to the Sun in mass, size and temperature. Its planet is unlike any of the planets of our Solar System. The short orbital period, 4 days, indicates that it is very close to the star. (Mercury's orbit is about 66 days.) From the large velocity shift, it can be inferred that the planet is massive, at least half the mass of Jupiter. Because of its proximity to the star, the temperature of the planet is probably greater than 1500 degrees Farenheit and so it is unlikely to have life.
At a meeting of the American Astronomical Society in January, Drs. Marcy and Butler reported the detection of two more planets orbiting solar-type stars. The stars are 70 Virginis and 47 Ursa Majoris. Both planets are large, roughly six times the mass of Jupiter for the 70 Virginis planet. The planet orbiting 47 Ursa Majoris is about twice the mass of Jupiter. Since January, Drs. Marcy and Butler have discovered another Jupiter-sized planet. It orbits the star HR3522. On June 20, 1996, yet another planet discovery was announced. Similar to 51 Peg, the approximately 3.9 Jupiter mass planet orbits the star Tau Bootes (HD120136) with a 3.3 day at a distance of 8.3 stellar radii.
In 1989 Dr. David Latham of Harvard discovered that the star HD114762 had a companion. Until recently, the companion was thought to be a brown dwarf, an object that is too large to be a planet but too small to be a star. Recent analysis of their high precision data has led Marcy and Butler to conclude that the companion is approximately ten Jupiter masses.
More information is available from the San Francisco State University Planet Search Project.
One of these new planets may be in the "habitable zone" of its star (the region where the temperature would allow water to be liquid). Although life as we know it would be difficult on such a massive planet, the planet could have an Earth-like moon. In our own Solar System, Jupiter has two moons that are similar in size to Mercury. Saturn also has a moon of similar size. In any case, these discoveries may have an impact on the next term in the Drake Equation, ne.
Until recently, the existence of extrasolar planets was a theoretical possibility. Now we know of several orbiting Sun-like stars.
At the June meeting of American Astronomical Society in Madison, Wisconsin, Dr. George Gatewood announced the discovery of another planet around a nearby star. Previous discoveries of planets have been made with a technique that measures periodic changes in the radial velocity of the star along the line of sight. This discovery used a different technique, measuring periodic changes in the motion of the star across our line of sight. In both cases, the periodic change is due to the presence of a planet causing the star to orbit about the center of mass of the system.
The newly discovered planet orbits the nearby star Lalande 21185 (BD+362147). It is approxmately the mass of Jupiter and orbits its star at about the distance of Saturn in our Solar System. The star is much smaller than the Sun, spectral type M2, and thus much cooler. Since the star is only 8.1 light years away, it is on the "Nearest 100" list for Project Phoenix.
The data also suggest the presence of another planet closer to the star. If the data prove to be correct, this planet would also be about the mass of Jupiter, but orbiting Lalande 21185 at a distance similar to that of the asteroid belt to the Sun in our Solar System.
