Ross 154

Due to Ross 154's proximity to Sol, the star has been an object of intense interest among astronomers. It has been selected as a "Tier 1" target star for NASA's optical Space Interferometry Mission (SIM) to detect a planet as small as three Earth-masses within two AUs of its host star (and so some summary system information and images of Ross 154 are available from the SIM Teams). Astronomers are also hoping to use the ESA's Darwin group of infrared interferometers to analyze the atmospheres of any rocky planet found in the "habitable zone" (HZ) around Barnard's Star for evidence of Earth-type life (Lisa Kaltenegger, 2005).

The Star

This cool and dim, main sequence red dwarf (M3.5 Ve) has around 17 percent of Sol's mass (RECONS estimate), 24 percent of its diameter (Johnson and Wright, 1983, page 693), and less than 5/10,000th of its luminosity. Ross 154 would be only one of many unremarkable stars except that it appears to be a flare star as well as one of Sol's closest neighbors. Its variable star designation is V1216 Sagittarii. Useful catalogue numbers include: Gl 729, AC-24 2833-183, Hip 92403, and LHS 3414.

Arnold O. Benz, Institute of Astronomy, ETH Zurich

High resolution and jumbo images (Benz et al, 1998).

Ross 154 is a flare star, like UV Ceti (Luyten 726-8 B)
shown flaring at left. UV Ceti is an extreme example
of a flare star that can boost its brightness by five times
in less than a minute, then fall somewhat slower back
down to normal luminosity within two or three minutes
before flaring suddenly again after several hours.

Life Around a Flare Star

Many dim, red (M) dwarf stars exhibit unusually violent flare activity for their size and brightness. These flare stars are actually common because red dwarfs make up more than half of all stars in the galaxy. Although flares do occur on our Sun every so often, the amount of energy released in a solar flare is small compared to the total amount of energy Sol produces. However, a flare the size of a solar flare occurring on a red dwarf star (such as Ross 154) that is more than ten thousand times dimmer than our Sun would emit about as much or more light as the red dwarf itself, doubling its brightness or more.

Flare stars erupt sporadically, with successive flares spaced anywhere from an hour to a few days apart. A flare only takes a a few minutes to reach peak brightness, and more than one flare can occur at a time. Moreover, in addition to bursts of light and radio waves, flares on dim red dwarfs may emit up to 10,000 times as many X-rays as a comparably-sized solar flare on our own Sun, and so flares would be lethal to Earth-type life on planets near the flare star. Hence, Earth-type life around flare stars may be unlikely because planets must be located very close to dim red dwarfs to be warmed sufficiently by star light to have liquid water (about 0.022 AU with an orbital period of about 2.9 days for Ross 154), which makes flares even more dangerous around such stars. In any case, the light emitted by red dwarfs may be too red in color for Earth-type plant life to perform photosynthesis efficiently.

Hunt for Substellar Companions

A recent search for faint companions using the Hubble Space Telescope found no supporting evidence for a large Jupiter or brown dwarf sized object (Schroeder et al, 2000).

Accounting for the great infrared output of M-stars like Ross 154, the equivalent orbital distance for an Earth-type planet be only around 0.19 AUs (Lisa Kaltenegger, 2005, page 48). At slightly under half of Mercury's distance in the Solar System, however, the rotation of the planet could become tidally locked with the star so that one side would have perpetual daylight with the other in darkness. Assuming that Ross 154 has about 17 percent of Sol's mass, a small Earth-type rocky planet would complete its orbit the star in about 73 days.