61 Virginis, also known as HD 115617, is a G5 star located at a distance of 8.52 parsecs (28 light years) in the constellation Virgo. Like our Sun, and unlike most yellow stars in the Solar neighborhood, it is single, and it closely resembles our Sun in mass, metallicity, spectral type, and age. 61 Virginis is also one of the very nearest G-type stars, a distinction that has drawn plenty of attention from astronomers and exobiologists. For decades they have placed it high on the list of stars most likely to host planetary systems resembling our own (Porto de Mello et al. 2006, Vogt et al. 2010).

Over the past 15 years, however, as exoplanet discoveries accumulated at an increasing pace, this old favorite remained conspicuous in its absence from the headlines. Its obscurity lifted at the end of 2009, when an astronomical team led by Steven Vogt announced the discovery of 3 planets orbiting 61 Virginis within an astrocentric radius smaller than the orbit of Venus (Vogt et al. 2010).

So our neighbor does host a planetary system, even if it is nothing like home. Instead, its architecture is very similar to those of 2 other nearby planetary systems orbiting reasonably Sun-like stars: HD 69830 and HD 40307.

Architecture of the 61 Virginis system. Colored circles indicate the relative sizes of the 3 planets, assuming the minimum masses provided by Vogt et al. 2010 and the mass-radius relationships provided by Fortney et al. 2007. Semimajor axes are indicated in astronomical units (AU) on a logarithmic scale, where 1 = the distance of Earth from the Sun. White dots mark the ice line.

For the central star, Valenti and Fischer (2005) provide a mass of 0.95 Msol, a radius of 0.96 Rsol, a luminosity of 0.805 Lsol, and a likely age of 6.3 billion years. Stellar age, however, is the most difficult characteristic to determine. Recently proposed ages for 61 Virginis range from a low of 3.1 to a high of 11.72 billion years (Vogt et al. 2010). Baliunas and colleagues (1996) provide a stellar rotational period of 29 days, which is very similar to our Sun's period of about 25 days. This close resemblance suggests that 61 Virginis and our Sun (at 4.6 billion years) are also close in age.

system architecture

Rightly or wrongly, the Solar System shapes our understanding of how exoplanetary systems centered on Sun-like stars (such as 61 Virginis) will be arranged. The Solar System's architecture is characterized by: [1] a group of small rocky planets orbiting inside 2 AU; [2] a belt of small rocky and icy asteroids between 2 and 4 AU; [3] a pair of gas giant planets between 5 and 10 AU; [4] a pair of ice giant planets between 10 and 30 AU; and [5] a large region of rocky and icy objects (the Kuiper Belt) extending from 30 AU to at least 50 AU, with outlying objects scattered as far as 100 AU.

The system of 61 Virginis shares none of these architectural features, although it offers a few analogs. It hosts: [1] a pair of ice giant planets inside 0.50 AU, as well as a third planet in the same region that may be an ice giant or a Super Earth; and [2] a belt of rocky and icy objects, analogous to our Kuiper Belt, but with an estimated inner edge of only 10 AU (Trilling et al. 2008). Given these distinctive elements, the 61 Virginis system is almost a lookalike for HD 69830, which hosts 3 ice giants inside 0.65 AU and a debris ring analogous to our Asteroid Belt centered around 1 AU. The minimum masses and orbital elements of 61 Virginis' 3 planets are also remarkably similar to those of HD 69830.

1. With a minimum mass of 5.1 MEA and an orbital period slightly in excess of 4 days, the inner planet, “b,” qualifies as a Super Earth. It joins a rapidly growing population of low-mass objects orbiting M, K, and G stars with periods that are typically shorter than 10 days (although this boundary doubtless reflects the limitations of current technologies rather than the true orbital distribution of Super Earths). Both theory and observation indicate that objects in this mass range can have a bulk composition dominated by metal and rock, with a relatively thin atmosphere, or one that is half rock and half ice, with a fairly extensive atmosphere of water vapor (Selsis et al. 2007, Miller-Ricci & Fortney 2010).
   
   Planet b's proximity to the star guarantees that it will be tidally locked, so that one hemisphere lies in perpetual darkness while the other is bathed in endless daylight, and therefore extremely hot. If this planet is rocky, it will probably have a magma sea on its day side. If it has a large watery component, on the other hand, it will be a steam planet with scalding temperatures distributed over both hemispheres.
   
   
2. The middle planet, “c,” has a minimum mass of 18.2 MEA, very similar to the actual mass of Neptune. Its period of about 38 days is less than half the period of Mercury, while its semimajor axis of 0.22 AU is a little more than half of Mercury's.
   
   
3. The outer planet, “d,” has a minimum mass of 22.9 MEA, close to the actual mass of GJ 436 b, a transiting Hot Neptune whose mass and radius have been measured with some precision. Its orbital period of about 123 days and semimajor axis of 0.48 AU would place it just outside the orbit of Mercury in our own system.

According to the discovery team, available radial velocity data are insufficient to constrain the eccentricities of these 3 planets (Vogt et al. 2010). Observations are consistent with at least 2 different architectures: one in which all 3 planets travel on circular orbits, and another in which the 2 inner planets have moderate eccentricities in the range of 0.1 to 0.2 (similar to Mercury's) while the outer planet has a substantial eccentricity of about 0.35. In the second model, the distance of planet c from the central star would vary widely over the course of a single orbit, from a periastron of 0.31 AU to an apastron of 0.65 AU, with corresponding changes in atmospheric temperature.

The uncertainty over the shape of the 3 planets' orbits translates into an uncertainty about their potential masses. If the orbits are relatively circular, we may be viewing the system in a pole-on orientation, so that the planets' masses are at their maximum -- ranging from more than 1 Jupiter mass for planet b to at least 4.5 Jupiter masses for planet d (Vogt et al. 2010). In fact, this alternative is just as likely as the case in which we see the planetary orbits in an edge-on orientation, and the planets receive the minimum masses stated above.

Even more likely, however, is an outcome in which the 3 planets are within 50% of their minimum masses. In that case, the 61 Virginis system would still contain 1 Super Earth and 2 ice giants.

debris belts

Two recent studies have found that 61 Virginis harbors an extensive region of cool debris, with an inner edge at about 8 to 10 AU (Trilling et al. 2008) and an outer edge between 100 and 200 AU (Tanner et al. 2009). The radial extent of this debris system, as characterized by the 2 studies, is dramatically larger than our Kuiper Belt. In particular, the inner radius found by Trilling and colleagues indicates that planet formation around 61 Virginis was confined to a much smaller region than in our Solar System. On this evidence we should not expect outer giant planets analogous to Saturn, Uranus, and Neptune in orbit around 61 Virginis.

Of interest, the debris belt around Tau Ceti, another nearby G-star that has long been the target of unsuccessful planet searches, is also thought to have an inner radius around 10 AU (Trilling et al. 2008).

additional planets

Radial velocity observations of 61 Virginis have been conducted for the past 25 years, yet even instruments of the highest precision have revealed only 3 low-mass planets. It therefore seems unlikely that the system contains any gas giants comparable to Jupiter. Nevertheless, Vogt and colleagues consider it possible that additional low-mass planets are present, with that likelihood increasing along with the circularity of the detected planets' orbits. They note that 1 or more terrestrial to Super-Earth mass planets are possible on orbits exterior to planet c, even in the system's habitable zone, which occupies a radius only slightly smaller than in our Solar System (Vogt et al. 2010).

Last revised January 2010