On this day in 2001, a team of astronomers announced the discovery of a second Jupiter-size planet orbiting 47 Ursae Majoris, a star in the Big Dipper, heralding the discovery of the first solar system outside our own.
The groundwork for the August 2001 discovery was laid in 1996, when veteran planet hunters Geoffrey Marcy and R. Paul Butler discovered the first extrasolar planet, 47 Ursae Majoris b, orbiting the Big Dipper star known as 47 Ursae Majoris. The discovery was made when the pair observed the Doppler shift of the star’s spectrum, indicating a change in the star’s radial velocity as the planet’s gravity pulled it during orbit. This 1996 discovery was the first long-period, low-eccentricity-orbit extrasolar planet discovered. That is, its orbit is fairly circular and it takes 1,078 days, or 2.95 years to orbit its star.
This discovery set the stage for another groundbreaking one in 2001. On 15 August 2001, a team of astronomers discovered the second planet orbiting 47 Ursae Majoris, indicating the existence of a solar system outside our own—the first one most like our own to be discovered. It was found 51 light years from Earth using telescopes to measure Doppler-shifted light reaching Earth from the stars. (Astronomers believe regular changes in Doppler shift, known colloquially as “wobbles,” signal the presence of a planet periodically pulling a star toward or away from Earth.) This planet, 47 Ursae Majoris c, has a minimum mass three-quarters that of Jupiter in a 7.1-year orbit.
In this case, it was the second discovery of a planet orbiting the same sun-like star that proved critical. “For the first time we have detected two planets in nearly circular orbits around the same star,” said Debra Fischer, a University of California Berkeley astronomer who was among the team to discover the new planet. “Most of the 70 planets people have found to date are in bizarre solar systems, with short periods and eccentric orbits close to the star. As our sensitivity improves we are finally seeing planets with longer orbital period, planetary systems that look more like our solar system.”
Since then, a third planet has been discovered in the first known solar system outside our own: 47 Ursae Majoris d was discovered in 2010. Astronomers are still learning about the characteristics of planetary systems outside our own. “Every new planetary system reveals some new quirk that we didn’t expect,” Geoffrey Marcy, who helped discover both 47 Ursae Majoris b and c, told the press at the latter’s discovery. “With 47 Ursae Majoris, it’s heartwarming to find a planetary system that finally reminds us of our solar system.”