83 Leonis is a nearby binary system containing two Sun-like stars, located about 18 parsecs (59 light years) away from the Solar System. The brighter and more massive member of the pair, HD 99491 or 83 Leonis A, is a G8 star of 1.01 MSOL (Desidera & Barbieri 2007). The dimmer member, HD 99492 or 83 Leonis B, is a K2 star of 0.86 MSOL (Butler et al. 2006).

Although the two stars' present separation is about 515 AU (Desidera & Barbieri 2007), the binary orbit is not well determined. Most likely, they are closer to their maximum separation (apastron) than to their minimum separation (periastron), since a binary system spends a larger portion of its orbital period near apastron. Assuming a typical orbital eccentricity of 0.5, the binary semimajor axis would be about 343 AU and the periastron about 172 AU. Such a configuration would not present any major obstacles to planet formation around either star (Desidera & Barbieri 2007), but higher eccentricities would imply smaller periastra and stricter constraints on system architectures.

No consensus has emerged on the system's age. Geoff Marcy and colleagues suggested a range of 2 to 6 billion years (Marcy et al. 2005), whereas Brian Jackson and colleagues favor a maximum of only 1.8 billion years (Jackson et al. 2009). The heavy element content of the two stars is much higher than average, with their metallicity estimated at +0.36 (Butler et al. 2006). Given the well-established link between high stellar metallicity and massive planets on small orbits, both stars have a similar a priori likelihood of hosting planetary systems.

In 2005, Marcy and colleagues reported on 7 years of radial velocity observations of the 83 Leonis system (Marcy et al. 2005). Remarkably, they found evidence for a planet of sub-Saturn mass orbiting the dimmer of the two stars, but no sign of any planets around the brighter, more Sun-like star. The detected planet, HD 99492 b, has a minimum mass of 0.109 Mjup (35 Mea), placing it below the threshold for gas giant planets (~0.20 Mjup/65 Mea) and within the range of ice giants like Neptune and GJ 436 b. Of course, the planet's true mass could be substantially higher, so it may turn out to resemble Saturn rather than Neptune in mass and bulk composition. Its period is relatively short, at 17 days; its semimajor axis is tight, at 0.12 AU; and its eccentricity is moderate, at 0.25. This value implies a periastron of 0.09 AU and an apastron of 0.15 AU.

The discovery team noted a trend in the radial velocity data for HD 99492 b (Butler et al. 2006), but they did not comment on its likely source -- i.e., whether it might be due to another planet on a wider orbit or to the star's binary companion.

The orbital configuration of the known Hot Neptune poses intriguing questions about the system's evolutionary history.

Little scientific interest has been stimulated by this nearby planetary system, and no substantial follow-up studies have appeared to date. The discovery team estimated a transit probability of about 3%, but no transits or secondary eclipses have been reported (Kane & von Braun 2009)

Last update August 2009