As the first system other than our own to be confirmed as a home to multiple planets, Upsilon Andromedae is among our best-studied neighbors. Nevertheless, key system parameters remain a matter of disagreement, reminding us of the uncertainty surrounding most available exoplanetary data.

Located at a distance of 13.5 parsecs (44 light years) in the constellation Andromeda, the system’s primary is a bright F8 star, formally known as HD 9826 or Upsilon Andromedae A and often abbreviated "u And A". This star is hotter, bluer, and more massive than our Sun, with a luminosity 3.4 times Solar and a diameter 1.6 times Solar (Kaler 2007, Rivera & Haghighipour 2007). Its age is reasonably well determined at 3.12 billion years (Takeda et al. 2007). The binary companion, presumably of similar age, is a lightweight M4 star orbiting at a distance of about 750 AU (Desidera & Barbieri 2006). Although only 0.19 MSOL (Desidera & Barbieri 2006), this star’s age, favorable metallicity, and wide separation from the primary make it a potential exoplanetary host in its own right.

The three detected planets of the F8 star are all gas giants, ranging from a classic Hot Jupiter orbiting at less than 6 million miles to a super-Jovian world in a moderately eccentric orbit at about 3 AU. The ice line of Upsilon Andromedae is wider than Sol’s, so all three planets probably formed beyond 5 AU and migrated inward to their present orbits. As the most massive of the three, the outermost planet has undoubtedly played a key role this system’s evolution, as Jupiter has done in the Solar System.

upsilon andromedae b

With a semimajor axis of less than 0.06 AU, a period of 4.6 days, and an inferred minimum mass of 0.65 MJUP (Rivera & Haghighipour 2007), the innermost planet must be an inferno of turbulent gases. Although it has been subject to substantial heating over the 3 billion years of its evolution, this process has not led to significant mass loss (Lecavelier des Etangs 2006). Otherwise, the planet’s semimajor axis would have enlarged as its mass dissipated, leading to a much wider orbit than we actually detect (Nagasawa & Lin 2005). Like Hot Jupiters that have been directly observed through transits of their host stars, the first planet of Upsilon Andromedae most likely has a puffed-up radius that may be 30% larger than Jupiter’s (Nagasawa & Lin 2005).

Recent infrared observations reveal a large temperature difference between the day and night sides, although even the night side remains hotter than Venus (Harrington et al. 2006). These observations confirm that the innermost planet is tidally locked, always turning the same hemisphere to the primary star. Its stalled rotation may result in two meteorological poles defined by heat: the starward pole, where temperatures are highest and gases boil upward, and the anti-starward pole, where temperatures are lower and gases flow downward.

It is noteworthy, however, that spectrographic studies of two transiting Hot Jupiters have returned no converging evidence of day side/night side temperature variation (Grillmair et al. 2007, Richardson et al. 2007, Swain et al. 2007). Instead, the transit spectra indicate that heat is distributed evenly across the two hemispheres of the planets studied, HD 189733 b and HD 209458 b. At minimum these additional findings suggest that the atmospheres of Hot Jupiters vary considerably despite their similar orbits. They may also indicate that the temperature profile of Upsilon Andromedae b remains poorly understood.

upsilon andromedae c

The second planet of Upsilon Andromedae is a still heavier gas giant with a minimum mass of 1.8 MJUP (Rivera & Haghighipour 2007). It orbits at about 0.8 AU in a period of 241 days. In period and semimajor axis it is similar to Venus, but its larger mass and more luminous host star make it much hotter. Sudarsky et al. (2000) classify this planet as a Class III or “clear” giant, meaning that its atmosphere is too hot to permit substantial cloud formation. Thus the artist John Whatmough has depicted it as a sky-blue globe with tenuous bands of cirrus-like clouds. At least three factors make this planet a likely candidate for a family of moons.

• Its super-Jovian mass ensured that its accretion disk favored the formation of large satellites.
• Its migration through the inner system’s zone of rocky planetesimals provided ample opportunities for capturing massive moons.
• Its final semimajor axis is large enough to prevent satellite loss to the host star’s tidal drag.

Nevertheless, any such hypothetical moons will be rocky deserts, as this second planet orbits well starward even of the most generous estimate of Upsilon Andromedae’s habitable zone (Rivera & Haghighipour 2007).

upsilon andromedae d

Despite careful observations and analyses, both before and after its public announcement in 1999, the third planet of Upsilon Andromedae remains poorly constrained. The Catalog of Nearby Exoplanets (2008) provides a minimum mass of 3.97 MJUP and a semimajor axis and eccentricity of 2.55 AU and 0.27, respectively. These values imply a periastron of 1.86 AU and an apastron of 3.24 AU. From their own solution to the radial velocity data, Rivera & Haghighipour (2007) have provided a slightly lighter mass of 3.59 MJUP, a wider semimajor axis of 3.43 AU, and an identical eccentricity. Their solution implies a periastron of 2.5 AU and an apastron of 4.35 AU. The planet's period is at least 1275 days (about 3.5 years).

Like the second planet, the third is likely to sustain a large family of moons, some co-formed in its original circumplanetary accretion disk and others captured. Such moons might be as massive as Mars or even Earth. Given their formation inside the system's ice line, at their host planet's final semimajor axis, they will probably be rocky like the terrestrial planets rather than volatile-heavy like the moons of Jupiter and Saturn.

The possibility of Mars-like moons orbiting within a few AU of a nearby Sun-like star leads to the question of whether they might sustain Earthlike (or even Mars-like) surface conditions. Unfortunately, there is no consensus regarding the location of the habitable zone around Upsilon Andromedae. Rivera & Haghighipour (2007) calculate its boundaries as 1.68 to 2.00 AU. If we also accept their orbital solution for planet d, then any of its moons would probably be too cold to sustain bodies of liquid water. Using a different methodology, however, Mandell et al. (2007) suggest a more favorable outlook. For a star of Upsilon Andromedae's mass and spectral class, they define a very wide habitable zone extending from 2.3 to 4.3 AU. This region overlaps with both of the competing orbital solutions for the third planet, suggesting that Earthlike temperatures are possible.

As we speculate on the likelihood of extrasolar habitats, however, we must bear in mind that when our own planet was the same age as the Upsilon Andromedae system, its most advanced inhabitants were single-celled organisms swimming in the primordial ocean.

Rivera & Haghighipour (2007) conclude that additional stable orbits cannot be maintained within 7.5 AU of Upsilon Andromedae, except for an extremely narrow region just beyond the orbit of the Hot Jupiter. We can thus be reasonably sure that this system is complete out to the third planet, and that no appreciable debris rings survive within 7.5 AU.

Last update April 2008