Upsilon Andromedae

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Upsilon Andromedae at
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Multi-planet systems compared

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 most intensively studied neighbors. Located in the constellation Andromeda at a distance of 13.5 parsecs (44 light years), 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 substantial distance. Its orbital elements remain poorly constrained, with estimates of the 2 stars’ current separation ranging from about 700 AU to more than 10,000 AU (Desidera & Barbieri 2007, McArthur et al. 2010). Although only 0.19 MSOL (Desidera & Barbieri 2007), the M dwarf’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 with 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 3 of its detected planets are likely gas giants, ranging from a Hot Jupiter orbiting at less than 6 million miles to a pair of super-Jovian worlds on moderately eccentric orbits between 0.65 and 3 AU.

system architecture

Until 2010, all studies of the Upsilon Andromedae system used the minimum planet masses derived from radial velocity observations, which assume that we view the planetary orbits edge-on. This assumption presented 3 extrasolar gas giants of more or less average mass. However, new research by Barbara McArthur and colleagues provides a determination of the actual inclination of the orbits of the 2 outer planets against the plane of the sky, enabling their true masses to be calculated (McArthur et al. 2010). The resulting values place them in the realm of the Super Jupiters, on the uncertain border between gas giant planets and brown dwarf stars.

The resulting system architecture is interesting in several ways. First, the fact that we observe both a Hot Jupiter and 2 additional planets makes this system unusual — more than 95% of the known Hot Jupiters are the only planets detected in their home systems, and only one other system (HIP 14810) includes 2 gas giants in addition to a Hot Jupiter. Second, the revised data on the 2 outer planets presents a non-hierarchical distribution of masses, with the second planet more massive than the third. In three-quarters of the systems with exactly 2 planets, the outer planet is more massive than the inner, and in more than half of the multi-planet systems, the outermost is the most massive. Finally, the orbits of the 2 outer planets have a significant mutual inclination, meaning that they do not orbit in the same spatial plane. Their mutual tilt is calculated at about 30 degrees. Although such a configuration may be common in systems of 2 or more exoplanets, Upsilon Andromedae is the first one in which such a relationship has been demonstrated.

Residual data in the radial velocity observations suggest the presence of a fourth gas giant traveling on a wider orbit, with a period that probably exceeds the 14-year time span sampled to date (McArthur et al. 2010).

Mass distribution in the Upsilon Andromedae system. Colored circles indicate the relative sizes of the 3 planets, assuming the masses provided by McArthur et al. 2010, 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. Paradoxically, the least massive of the 3 (planet b) has the largest radius.

Comparative sizes of nearby stars

System architectures

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 atmosphere must be an inferno of turbulent gases. Although the planet has been subject to substantial irradiation 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).

Recent work suggests that Upsilon Andromedae b is considerably more massive than the minimum value derived from radial velocity data, perhaps in the vicinity of 1.4 MJUP (McArthur et al. 2010). Among transiting Hot Jupiters (the only distinct population of gas giants for which true masses are known), more than two-thirds are less massive than this value.

Infrared observations reveal a large temperature difference between the day and night sides of planet b, although even the night side remains hotter than Venus (Harrington et al. 2006). These observations confirm that the planet is tidally locked, always turning the same hemisphere to the primary star. Its stalled rotation evidently results in 2 meteorological poles defined by heat: a hot pole on the illuminated side, where temperatures are highest and gases boil upward, and a cooler pole on the dark side, where temperatures are lower and gases flow downward. Recent data demonstrate an unexpected “phase offset” in the location of the hot pole (Crossfield et al. 2010). Instead of occupying the substellar point (high noon on the planet’s bright hemisphere), this hot spot is displaced by 80 degrees toward the horizon. The source and significance of the offset is unknown.

Notably, spectrographic studies of 2 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 2 flavors: one whose atmospheres can effectively transport heat, like HD 189733 b, and another whose global circulation patterns are more limited, like Upsilon Andromedae b.

upsilon andromedae c

The second planet of Upsilon Andromedae is a much heavier object orbiting at a semimajor axis of 0.83 AU in a period of 241 days. In period and orbital radius this planet is similar to Venus, but its larger mass and more luminous host star make it considerably 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 depicted it as a sky-blue globe with tenuous bands of cirrus-like clouds.

The minimum mass (m sin i) of Upsilon Andromedae c is calculated as 1.96 MJUP, less than half the m sin i of planet d but still above the median for extrasolar gas giants (~1.7 MJUP). However, McArthur and colleagues estimate its true mass at a whopping 13.98 MJUP, placing this object in the mass range for brown dwarfs (McArthur et al. 2010).

At least 3 factors make Upsilon Andromedae c a potential candidate for a family of moons.

  • Its super-Jovian mass ensured a substantial accretion disk that would favor the formation of large satellites, including objects as massive as Mars or Venus.
  • Its migration through the inner system’s zone of rocky planetesimals enabled the disk to accrete quantities of heavy elements.
  • Its final semimajor axis is probably wide 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). In addition, the evolutionary history of planet c probably includes episodes of orbit crossing and scattering – violent events whose effects on planets and their satellite systems remain largely unexplored.

Like most exoplanets located more than 0.1 AU away from their host stars, Upsilon Andromedae c has an eccentricity higher than any of our Sun’s eight planets, with e = 0.245. This value corresponds to an apastron of 1.03 AU and a periastron of 0.63 AU, as if this giant were to make a round trip from Earth to Venus with each revolution.

upsilon andromedae d

The third planet has a minimum mass of 4.33 MJUP, higher than the value for planet c and well above the median for extrasolar gas giants. However, McArthur and colleagues place its true mass still higher, at 10.25 MJUP. Thus this planet is a bona fide Super Jupiter, although it is less massive than its inner companion. Its orbit has a period of about 3.5 years and a semimajor axis of 2.53 AU, placing it comfortably inside the system’s ice line. With an eccentricity of 0.316, nevertheless, this planet travels from an apastron of 3.33 AU to a periastron of 1.73 AU, implying a considerable annual change in atmospheric heating.

Like the second planet, Upsilon Andromedae d 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 Earthlike moons orbiting within a few AU of a nearby Sun-like star leads to the question of whether they might sustain Earthlike (or at least Mars-like) 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 spend only a small part of its orbit at temperatures warm enough to permit 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 atmospheric composition.

As we speculate on the likelihood of extrasolar habitats, however, we should bear in mind that when our own planet was the same age as the Upsilon Andromedae system (during the Palaeoproterozoic Era), its most advanced inhabitants were single-celled organisms swimming in the global ocean. Nevertheless, if future observational methods can establish the existence of Earthlike moons orbiting an exoplanet in the immediate Solar neighborhood, their intrinsic interest would be substantial, regardless of the presence of highly evolved life forms.

Orbital configuration of the Upsilon Andromedae system. This figure shows the orbits of Upsilon Andromedae c and d at their approximate relative scale, with the central star indicated by a purple dot. The grid marks off units of 1 AU. Panel 1: Overhead view, with the orbits projected on a single flat plane. Panel 2: Perspective view, showing the relative inclination of the 2 orbits in three-dimensional space. Panel 2 is based on the left panel of Figure 8 in McArthur et al. 2010.

secular dynamics

As the first star known to host 3 planets, with the outer 2 on notably eccentric orbits, Upsilon Andromedae gave astronomers their first test case in the long-term (“secular”) evolution of an exoplanetary system.

It was immediately clear that planet b, the Hot Jupiter, must have formed on a much wider orbit than its present location. The same case can be made, albeit less forcefully, for planets c and d. Assuming that all 3 objects assembled through the accretion of planetesimals within a dusty protoplanetary nebula, their original nursery must have been located outside the system’s ice line. Given the host star’s mass and luminosity, this implies a region beyond 4 AU. According to the standard model of planet formation by accretion, the 3 growing gas giants would have cleared gaps in the primordial nebula, one by one, and then spiraled into the inner system by Type II migration. As the gaseous nebula dissipated, they would also be stranded, one by one, in the vicinity of their present orbits.

The 2 outer planets, however, present problems with this neat evolutionary scenario. Their orbital eccentricities are inconsistent with a history of unperturbed migration through a gas disk, because this process should produce circular orbits like the ones found in the 55 Cancri system, rather than the elliptical, starkly non-coplanar orbits that we observe. 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, Barnes & Greenberg 2007). Planets c and d, and perhaps an additional planet, evidently experienced close approaches and perturbed each other’s orbits gravitationally, resulting in the skewed architecture now deduced from astrometric analysis (McArthur et al. 2010). This chaotic process, typically known as planet-planet scattering, seems common in extrasolar systems.

A recent study by Rory Barnes and colleagues (in press 2010) used the latest system data to find a plausible formation history for Upsilon Andromedae. This study confirmed previous analyses concluding that the system’s dynamical architecture lies on the brink of chaos, with the eccentricities and inclinations of planets c and d oscillating on secular time scales. Like earlier investigators, Barnes and colleagues also determined that this configuration must have resulted from a violent instability in the system’s infancy. The sparse data on Ups And B, the binary companion, prevented them from ascertaining its role in the instability; it remains possible that this M dwarf played no part at all. Perhaps a more likely culprit is simply the highly “planetic” environment provided by the F star’s protoplanetary nebula – in effect, an embarrassment of riches, like a kingdom that produces too many royal heirs and then suffers a civil war while the princelings fight it out to determine their precedence.

According to Barnes and colleagues, at least 3 super-Jovian planets (mass > 5 MJUP) formed in close proximity to one another, in addition to the smaller gas giant that ended up as planet b, the Hot Jupiter. After an episode of planet-planet scattering, one of the Super Jupiters was either ejected from the system or accreted by a companion. The end result was the complex system we now observe, with its non-hierarchical distribution of masses.

This study could not decide whether ejection or collisional accretion was a more likely fate for the rogue planet. An argument in favor of accretion would hinge on the huge mass in orbiting bodies still retained by Upsilon Andromedae. Whereas our Sun has conserved less than 1.5 Jupiter masses of its original nebula in the form of planets and moons, Upsilon Andromedae retains more than 25 Jupiter masses – potential evidence of thrift (collision) rather than extravagance (ejection).

Last update November 2010

Index of exoplanetary topics

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Barnes R, Greenberg R, Quinn TR, McArthur B, Benedict GF. (2010) Origin and dynamics of the mutually inclined orbits of Upsilon Andromedae c and d. In press.
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Wright JT, Upadhyay S, Marcy GW, Fischer DA, Ford EB, Johnson JA. (2009) Ten New and Updated Multi-planet Systems, and a Survey of Exoplanetary Systems. Astrophysical Journal, 693: 1084-1099. Abstract.

All text is copyright Raymond Harris 2006-2011