24 Sextantis

(The Planet Project)

Sextans A cool evolved star, 24 Sextantis has not one, but two Jupiter-like planets in relatively wide resonant orbits that almost touch each other.


The circle shows the location of the evolved class K star 24 Sextantis (in Sextans, the Sextant), about which orbit two planets. The inner, with a mass of at least twice that of Jupiter, takes 1.24 years to make a circuit at an average distance of 1.33 Astronomical Units, a relatively low eccentricity of nine percent taking 24 Sextantis "b" between 1.21 and 1.45 AU from its star. The outer one, 24 Sextantis c, is smaller, with a minimum mass of 0.86 Jupiters, though in a larger orbit with an average radius of 2.08 AU. Two matters then grab further attention. The outer planet has a notably larger eccentricity of 0.29 that moves it as far as 2.68 AU from the star to as close as 1.47 AU. That is, the "apastron" (farthest point in orbit) of 24 Sextantis b almost overlaps with the "periastron" (closest point) of 24 Sextantis c. They are also almost certainly in "resonance" with each other, with the orbits locked at a two-to- one ratio of the periods, which, along with orbital orientation, keep them from crashing into each other. (While the measured ratio of the periods is 1.95, the uncertainties easily allow an exact factor of two.) Such larger planets should have formed farther out where it is colder and they could accumulate their large masses by accreting volatile hydrogen, helium, and water from the primitive circumstellar disk. They, like the great number of close-in "Jupiters," have almost certainly migrated inward through gravitational influences and interactions with the leftover disk, in this case until they became trapped into their resonance.


Originally classified as a simple G star, later work on barely- seventh-magnitude (6.61) 24 Sextantis showed it to be a class K (K1) subgiant, which fits well with its measured temperature of 5100 Kelvin. Such stars have recently ceased core hydrogen fusion and are on their way to becoming true giants with dead helium cores. A distance of 253 light years leads to a luminosity of 13.8 times that of the Sun, a radius of 4.8 times solar, and a mass of 1.6 Suns, the age coming in at just over two billion years. Though planet-holding stars tend to have higher metals than average, 24 Sextantis's iron abundance is near-solar. Stars like 24 Sextantis are important to the study of extrasolar planets in that they started life as hotter, common, class A stars rather like Vega, Fomalhaut, and Sirius. Because of the natures of their absorption line spectra, used to detect the majority of planets, it becomes difficult to know how many class A dwarfs have planets and thus how planet formation is related to stellar mass. Such subgiants (which have been referred to as "retired A stars") then act as proxies for their warmer progenitors. (See J. A. Johnson et al., Astronomical Journal, vol 141, p. 16, 2011.)
Written 2/18/11 by Jim Kaler. Return to The Planet Project or go to STARS.