61 Ursae Majoris

JPL, CalTech, NASA


Larger illustration


Astronomers have identified 61
Ursae Majoris as a prime target
for the Terrestrial Planet Finder
(TPF), now planned for launch
between 2014 and 2020.

The Star

61 Ursae Majoris is a yellow-orange main sequence dwarf star of spectral and luminosity type G8 Ve, with about 81 percent of Sol's mass, 84 to 89 percent of its diameter (Morossi and Malagnini, 1985, page 369; and Johnson and Wright, 1983, page 673), and around 57 percent of its luminosity. The star may be only 40 to 105 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Cayrel de Strobel et al, 1991, page 292). It is a New Suspected Variable star designated as NSV 5291. While chromospheric activity measurements suggest that the star may be as young as 500 million years (see HD 101501 in Table 2 from Don Barry, 1988), the lack of an easily observable dust disk -- as is found at Epsilon Eridani -- suggests that the stars could be well over a billion years old. Useful star catalogue numbers for the star include: 61 UMa, HR 4496, Gl 434, Hip 56997, HD 101501, BD+35 2270, SAO 62655, FK5 1300, and LTT 13200.

Hunt for Substellar Companions

The failure, thus far, to find large substellar objects like brown dwarfs or a Jupiter- or Saturn-class planet in a "torch" orbit (closer than the Mercury to Sun distance) around 61 Ursae Majoris -- with even the highly sensitive radial-velocity methods of Geoffrey W. Marcy and R. Paul Butler -- bodes well for the possibility of Earth-type terrestrial planets around this star (Cumming et al, 1999). The orbit of an Earth-like planet (with liquid water) around this star would be centered around 0.77 AU -- somewhat farther than the orbital distance of Venus in the Solar System -- with an orbital period under 273 days or more than two thirds of an Earth year. However, if this star is as young as chromospheric activity alone would suggest, then it is likely that only primitive organisms like bacteria that can survive heavy meteorite or cometary bombardment would be likely to survive on any Earth-type planet that has cooled sufficiently to allow carbon-based lifeforms to develop.

Astronomers are hoping to use NASA's Terrestrial Planet Finder (TPF) and the ESA's Darwin planned groups of observatories to search for a rocky inner planet in the so-called "habitable zone" (HZ) around 61 Ursae Majoris. As currently planned, the TPF will include two complementary observatory groups: a visible-light coronagraph to launch around 2014; and a "formation-flying" infrared interferometer to launch before 2020, while Darwin will launch a flotilla of three mid-infrared telescopes and a fourth communications hub beginning in 2015.