As the first system other than our own to be confirmed as a home to multiple planets (Butler et al. 1999), Upsilon Andromedae is one of our best-studied neighbors. 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 times Solar, a diameter 1.6 times Solar, and a mass 1.3 times Solar (Butler et al. 1999, Takeda et al. 2007). Its age is estimated at about 3 billion years (Takeda et al. 2007), making its planets younger than those in our Solar System but still mature enough to have achieved long-term physical and orbital stability.

The yellow-white star has a binary companion, presumably of similar age: a lightweight red dwarf of spectral type M4 orbiting at a distance of about 750 AU (Desidera & Barbieri 2007). Although only 0.19 MSOL (Desidera & Barbieri 2007), this star’s wide separation from the primary make it a potential exoplanetary host in its own right.

Upsilon Andromedae A is richer in heavy elements than our Sun, with its metallicity calculated at +0.15 (Butler et al. 2006). The same value probably applies to its M dwarf companion. Stellar enrichment in metals is associated with the presence of giant planets at small semimajor axes, as well as the formation of multiple-planet systems (Fischer & Valenti 2005, Greaves et al. 2007). Upsilon Andromedae, in fact, provided one of the earliest clues to this correlation. All three of its detected planets are 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.

system architecture

The ice line of Upsilon Andromedae is wider than our Sun's, so all three planets probably formed beyond 5 AU and migrated inward to their present orbits. As in most known multi-planet systems, the most distant planet is also the most massive. Its heavyweight status means that this outer planet has played a key role the system’s evolution, just as Jupiter did in the Solar System. (In the following discussion, all planet parameters follow Wright et al. 2009.)

Architecture of the Upsilon Andromedae system. Colored circles indicate the relative sizes of the 3 planets, assuming the minimum masses provided by Wright et al. 2009, the mass-radius relationships provided by Fortney et al. 2007, and rock/metal cores. Semimajor axes are indicated in astronomical units (AU) on a logarithmic scale. White dots mark the ice line. Although these exoplanets are quite dissimilar in mass, with planet d at least 6 times heavier than planet b, all gas giants of Saturn mass or more are expected to have very similar radii.

upsilon andromedae b

With a semimajor axis of 0.06 AU, a period of 4.6 days, and an inferred minimum mass of 0.67 MJUP, the innermost planet is a classic Hot Jupiter -- so hot that its deep atmosphere must be an inferno of turbulent gases. Although the planet 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 2007). Otherwise, planet b’s semimajor axis would have enlarged as its mass dissipated, leading to a much wider orbit than we actually detect (Nagasawa & Lin 2005).

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 (HD 189733 b, HD 209458 b) have returned no similar 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 both hemispheres of these planets. This discrepancy suggests that Hot Jupiters come in at least two flavors: one whose atmospheres can effectively transport heat, like these transiting planets, and another whose global circulation patterns are more limited, like Upsilon Andromedae b.

This planet is notable for another reason, insofar as more than 90% of the known Hot Jupiters are the only planets detected in orbit around their host stars. With two gas giant companions, planet b is unique. Nevertheless, the significance of this distinction is unknown.

upsilon andromedae c

The second planet of Upsilon Andromedae is a still heavier gas giant with a minimum mass of 1.92 MJUP. It orbits at 0.83 AU in a period of 241 days. In period and semimajor axis this planet is similar to Venus, but its larger mass and more luminous host star make it much hotter. Sudarsky et al. (2003) classify planet c 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 Upsilon Andromedae c a likely candidate for a family of moons.

• Its super-Jovian mass ensured a substantial accretion disk that would have favored the formation of large satellites.
• Its migration through the inner system’s zone of rocky planetesimals provided ample opportunities for accreting quantities of heavy elements.
• Its final semimajor axis is large enough to prevent satellite loss to the host star’s tidal drag.

Nevertheless, such hypothetical moons would 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).

Like most exoplanets located more than one-tenth of an astronomical unit away from their host stars, Upsilon Andromedae c has an eccentricity higher than any of our Sun's eight planets, with e = 0.22. This value corresponds to an apastron of 1 AU and a periastron of 0.65 AU, as if this giant were to travel from the orbit of Earth to the orbit of Venus and back again with each revolution.

upsilon andromedae d

The third planet is also the heaviest. With a minimum mass of 4.13 MJUP, it lies well above the median for extrasolar gas giants. Its orbit has a period of 3.5 years and a semimajor axis of 2.53 AU, placing it inside the system's ice line. With an eccentricity of 0.27, this planet travels from an apastron of 3.2 AU to a periastron of 1.85 AU, implying a considerable annual change in atmospheric heating.

Like the second planet, the third is likely to sustain a family of moons, some co-formed in its original circumplanetary 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 may 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 Mars-like (or even Earthlike) surface conditions. Unfortunately, there is no consensus on the location of Upsilon Andromedae's habitable zone. Rivera & Haghighipour (2007) calculate its boundaries as 1.68 to 2.00 AU. In their solution, planet d would spent only a small part of its orbit at temperatures conducive to 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 wide habitable zone extending from 2.3 to 4.3 AU. This region overlaps with most of planet d's orbit, suggesting that Earthlike temperatures would be possible on rocky moons with the appropriate atmospheres.

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 global ocean.

secular dynamics

As the first star known to host three planets, with the outer two on notably elliptical orbits, Upsilon Andromedae gave astronomers their first test case in the long-term evolution of an exoplanetary system.

It was immediately clear that planet b, the Hot Jupiter, must have traveled to its present location by Type II migration, and that its tight orbit was stable on billion-year ("secular") time scales. The two outer planets, however, presented problems. Their orbital eccentricities are inconsistent with a history of unperturbed Type II migration through a gas disk, because this process should produce circular orbits like the ones found in the 55 Cancri system. A series of studies based on numerical simulations agreed that Upsilon Andromedae's outer system must have had a violent past (Laughlin & Adams 1999, Ford et al. 2005).

The original protoplanetary nebula around Upsilon Andromedae evidently produced at least one additional gas giant that was probably more massive than Jupiter, traveling on a wider orbit than the present planet d. While the gas nebula remained, its damping influence ensured that all four planets followed circular orbits. After the three inner giants migrated inside the ice line, the primordial gases dispersed. The fourth planet then interacted chaotically with the third until they experienced a close encounter, resulting in the abrupt ejection of the outermost planet and the deformation of the orbit of planet d (Ford et al. 2005).

With these studies, Upsilon Andromedae became the first confirmed instance of planet-planet scattering, a process that is evidently very common in extrasolar systems.

After the scattering event, planet d interacted with planet c, increasing the eccentricity of the inner planet's orbit. Extended numerical integrations demonstrated that the outer planet (d) maintains its present eccentricity over million- to billion-year time scales, while planet c undergoes oscillations over a period of several thousand years, cycling from its current value down to zero and then back up again (Ford et al. 2005).

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 survives within 7.5 AU.

Last update April 2009