Ross 154

Medialab, © ESA 2002



Larger illustration of
the Darwin Mission.



Astronomers have identified
Ross 154 as a prime target
for NASA's optical SIM and the
ESA's infrared Darwin missions,
now both indefinitely postponed.

The Star

This cool and dim, main sequence red dwarf (M3.5 Ve) has around 17 percent of Sol's mass (RECONS estimate and NASA Star and Exoplanet Database, derived from Henry and McCarthy, 1993), 19-24 percent of Sol's diameter (Turon et al, 1992; Johnson and Wright, 1983, page 693; and NASA Star and Exoplanet Database, derived from Kenneth R. Lang, 1980), less than 5/10,000th of Sol's visual luminosity, and 0.4 percent of its bolometric luminosity (NASA Star and Exoplanet Database, derived from Kenneth R. Lang, 1980). With less than 20 percent of Sol's mass, Ross 154 is so small that it can transport core heat to its surface only through convection, unlike larger red dwarf stars like Gliese 752 A -- also known as Wolf 1055 A or Van Biesbroeck's Star (more). The star has only around 56 percent of Sol's metallicity based on the concentration of iron relative to hygrogen (Wargelin et al, 2008; and Olin J. Eggen, 1996).

Jeffrey L. Linsky, JILA, STScI, NASA




Larger image.




Like Gliese 752 B, Ross 154 is so small,
with less than 20 percent of Sol's mass,
that it can transport core heat only through
convection, unlike larger larger red dwarf
stars like Gliese 752 A (more).

Ross 154 would be only one of many unremarkable stars except that it appears to be a flare star, with X-ray emission comparable or greater than that of Sol (Wargelin et al, 2008), as well as one of Sol's closest neighbors. This is not so surprising, as the star is a "moderating fast" rotator and relatively young at less a billion years old (Wargelin et al, 2008, from Johns-Krull and Valenti, 1996). Its variable star designation is V1216 Sagittarii. Useful catalogue numbers include: V1216 Sgr, Gl 729, Hip 92403, AC-24 2833-183, CD-23 14742, Hip 92403, LPM 688, LFT 1437, LTT 7462, LHS 3414, and 2MASS J18494929-2350101.

Habitable Zone

With a spectral type of M3.5, Ross 154 can be used as a rough proxy for Wolf 1055 A (M3.5 V). According to one type of model calculations performed for the NASA Star and Exoplanet Database, the inner edge of Ross 154's habitable zone should be located very close to the star, at around 0.065 AU from the star, while the outer edge lies around 0.126 AUs. Accounting for the great infrared output of M-stars like Ross 154, the equivalent orbital distance for an Earth-type planet be only around 0.096 AUs. At slightly a fourth of Mercury's orbital 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 26 days.

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 and Dust Disks

------------------------------------- [Guide] -- [Full Near Star Map] -------------------------------------