No Kuiper Belt Jupiters
October 11, 2005 | Astronomers have long wondered whether the solar system might have an unseen giant planet far out in the darkness beyond Pluto and the Kuiper Belt. If so, it must be very far out indeed. Nadia Zakamska and Scott Tremaine (Princeton University) have set new limits on the existence of any such massive object, based on timing data from spinning pulsars and other very precise natural timekeepers around the sky. The two astronomers determined, to a new level of precision, that there is no unexplained gravitational acceleration of the Sun and solar system thereby ruling out any Jupiter-mass object within 200 astronomical units. In addition, any unnoticed brown or red dwarf must be much farther out than that. Their paper appears in the October 2005 Astronomical Journal.
Gravity Probe B Calls It a Wrap
October 11, 2005 | After 50 weeks of taking data, NASA's Gravity Probe B satellite has run out of liquid helium, ending its mission to determine the behavior of spacetime in Earth's vicinity more precisely than has ever been attempted before. The mission was designed to measure two effects predicted to arise from Einstein's general theory of relativity. One is the well-known "geodetic effect," the amount by which Earth warps the local spacetime in which it resides. The other is the more exotic Lense-Thirring effect, also called "frame-dragging": the amount by which the rotating Earth drags local spacetime around with it as it turns. Other methods have found only indirect evidence that frame-dragging happens at all, but Gravity Probe B was designed to measure it to 1 percent precision. (See "Earth's Twisty Spacewarp" and "Gravity Probe B Puts General Relativity to the Test".)
Analysis of Gravity Probe B's data will be no small task; it's expected to take about a year. More information can be found in a NASA press release and the links within it.