A-Type Stars with Planets and Debris Disks

e x t r a s o l a r &nbsp &nbsp p l a n e t s

planets & debris around A-type stars

Disks of bright nearby stars

Beta Pictoris debris disk

Hot white stars of spectral class A are among the brightest stars in the night sky. Sirius, Altair, and Vega are familiar examples of this type, lending their names to consumer products and often appearing in science fiction narratives. Despite their visibility, however, stars of this spectral class may represent less than 1% of the stellar population of the Milky Way Galaxy (Hillenbrand 2004, RECONS).

A-type stars range in mass from 1.5 MSOL to 3.0 MSOL (Johnson et al. 2010), and in main sequence lifetime from 600 to 850 million years (Su et al. 2006, Sato et al. 2007). Thus they are considerably more massive than Sun-like stars, and their life spans are considerably shorter.

Both theory and observation indicate that these stars are at least as likely as Sun-like stars to harbor planetary systems. In fact, Kennedy and Kenyon argue that “the probability that a star has at least one gas giant increases linearly with stellar mass” up to a peak at 3 MSOL, the upper limit for A-type stars. They predict that more than 10% of stars in the mass range for A-type stars will host at least one giant planet (Kennedy & Kenyon 2008).

Unfortunately, the method that has proven most successful in detecting extrasolar planets – high-precision radial velocity measurements of stellar spectra – has been ineffective for spectral class A. Key characteristics of these stars, including their high temperatures and rapid rotation, make it very difficult to determine their radial velocities (Setiawan et al. 2005, Galland et al. 2005). Another successful method, photometric transit observation, has yielded limited results, but for a different reason. The large diameters and high, sometimes variable luminosities of A-type stars make it difficult to detect the periodic reduction in brightness caused by a Jupiter-sized planet in transit. So far, one transiting object consistent with an inflated gas giant has been reported in a very short-period orbit around the A5-type star HD 15082, also known as WASP-33 (Collier Cameron et al. 2010).

More promising is direct photographic imaging, a method that has been applied to young Sun-like stars for more than a decade, with ambiguous results. It turns out that the youth and mass of A-type stars make direct imaging far more effective for them than for older, smaller stars like our Sun. Young planets are hotter and thus brighter than older planets, and massive stars are expected to host planets on wide orbits where they are easier to find.

Direct imaging had its first success in 2008, with the photographic detection of candidate gas giant planets orbiting 3 nearby A-type stars: Fomalhaut, Beta Pictoris, and HR 8799 (Kalas et al. 2008, Lagrange et al. 2009, Marois et al. 2008). At last we have good evidence that A stars can host planetary systems comparable to those of dimmer stars. In all cases, the imaged planets orbit their stars at much greater distances than the planets detected by radial velocity measurements – at least 100 AU in the case of Fomalhaut b, between 15 and 68 AU for the 4 proposed planets of HR 8799, and about 8 AU for the candidate around Beta Pictoris. These discoveries hint at a large population of gas giants hiding in the vicinity of the Milky Way’s bright stars.

Less direct methods have also captured a great deal of data on two phenomena closely related to the likelihood of A-star planets:

  • Debris disks similar to the Asteroid and Kuiper Belts orbiting existing A stars

  • Planetary systems of subgiant and giant stars whose progenitors were A stars

Much research has focused on debris disks, as discussed in the next several paragraphs (see also Debris Disk Systems). Indeed, all the A stars whose planets have been imaged originally attracted attention because they support such disks. Much less attention has been directed to the planetary systems of giant stars, since they are far less common than debris disk systems and have only recently been detected. Such “retired A stars” and their companions are discussed at the end of this chapter.

The study of A-star planets has a long way to go before it rivals the extensive data on Sun-like stars and M dwarfs. The evidence so far suggests that A-star planets will be massive, and their orbits will be wider (perhaps much wider) than those of exoplanets around Sun-like stars. Hot Jupiters will be absent or rare (Currie 2009, Kennedy & Kenyon 2009). Comets and asteroids will be plentiful, and their collisions with orbiting giants will be frequent. Whether rocky planets like Earth can form in these environments remains unknown, and life-bearing worlds are not widely anticipated.

A stars with debris

Recent surveys have found that about one-third of A stars within a few hundred parsecs currently possess debris disks that are dusty enough to be detected by infrared observations (Su et al. 2006, Trilling et al. 2008). The investigators attribute this frequency to two factors. First, given their short lifetimes, A stars are younger on average than F, G, K, or M stars, and thus more likely to preserve large populations of colliding planetesimals. Second, A stars are brighter on average than stars of the more numerous spectral classes, providing the intense luminosity that enables their disks to be detected. Nevertheless, even this high rate of detections represents only a lower limit on the presence of debris around A stars. Many systems with quantities of dust comparable to those in the Solar System’s debris belts remain beyond the reach of current observing techniques.

By far the best-studied debris disks of any kind are those of three nearby A stars: Beta Pictoris, Fomalhaut, and Vega. As a group they have been called “Vega-type stars,” since Vega’s disk was the first to be discovered (Aumann et al. 1984). Although all are much younger than our Sun, these stars have already completed a substantial portion of their time on the main sequence.

Table 1. Nearby A-type stars with debris disks

Disk Boundaries

Fomalhaut A3 2.3 200 18 7.7 130-160

Vega A0 2.2 455 37 7.8 86-200

Denebola A3 2.3 50 17 11 20-100

Iota Centauri A2 2.5 350 26 18 6-100

Beta Pictoris A5 2.0 12 13 19 6-800

Zeta Leporis A2 2.0 230 15 22 2-4, 4-8

Gamma Ophiuchi A0 2.3 180 31 29 13-520

HR 8799 A5 1.5 60 5 39 75-170

Sources: Akeson et al. 2009 (“Dust in the inner regions of debris disks around A stars”); Aufdenberg et al. 2006 (“First results from the CHARA Array VII: Long-baseline interferometric measurements of Vega”); Chiang et al. 2009 (“Fomalhaut’s debris disk and planet: Constraining the mass of Fomalhaut b from disk morphology”); Kaler 2009 (“Muliphen“); Marois et al. 2008 (“Direct imaging of multiple planets orbiting the star HR 8799”); Moerchen et al. 2008 (“Mid-infrared resolution of a 3 AU radius debris disk around Zeta Leporis”); Su et al. 2005 (“The Vega debris disk – a surprise from Spitzer”); Su et al. 2006 (“Debris disk evolution around A stars”); Su et al. 2008 (“The exceptionally large debris disk around Gamma Ophiuchi”); Wyatt et al. 2007b (“Steady-state evolution of debris disks around A stars”); Yoon et al. 2010 (“A new view of Vega’s composition, mass, and age”).

Planetesimals in Beta Pictoris

Located at a distance of 19.3 parsecs (63 light years), Beta Pictoris has barely emerged from stellar infancy, with an estimated age between 10 and 20 million years (Okamoto et al. 2004, Su et al. 2006). Published values for its mass range from 1.75 to 2.0 times Solar, and for its luminosity from 8 to 13 times Solar (Heap et al. 2000, Freistetter et al. 2007, Wyatt et al. 2007b). Also known as HD 39060, this star’s more familiar name designates the Beta Pictoris Moving Group, an association of young nearby stars of various spectral types that formed together in the same molecular cloud about 12 million years ago (Zuckerman & Song 2004). Another prominent member of this group is AU Microscopii, an M-type star that is also surrounded by a dusty debris disk (Kalas et al. 2004).

Despite its youth, the Beta Pictoris disk has already lost its primordial gases, and any of its original fine-grained dust has been blasted to extreme distances. The heavier dust that we now observe results from the disintegration of comets and from collisions between planetesimals or asteroid-like bodies. Collisions are occurring throughout the disk, but most of the dust is produced in orbits beyond 100 AU.

Beta Pictoris is positioned so that we see its disk almost edge-on. Several studies are in broad agreement as to its morphology, which is quite complex (e.g., Heap et al. 2000, Wahhaj et al. 2003, Okamoto et al. 2004, Golimowski et al. 2006).

  • Dust is relatively depleted within an astrocentric radius of 80 AU, although it displays a pattern of clumps and gaps throughout this region.

  • Dust density increases from 80 AU until it reaches a plateau at about 100 AU (Golimowski et al. 2006).

  • At about 135 AU, the disk flares to a much greater thickness, extending radially to distances in excess of 1000 AU while slowly attenuating in density (Golimowski et al. 2006).

  • Calculations by Alice Quillen and colleagues indicate that the average diameter of colliding planetesimals at the “break radius” – the radius containing half of the debris belt’s mass – is about 180 km (110 mi). Their collisional cascade has been triggered by perturbations from still larger planetoids measuring at least 1000 km (620 mi) in diameter (Quillen et al. 2007).

Rather than a continuous field of dust and random debris, the Beta Pictoris disk is more accurately described as a series of rings separated by gaps, forming a composite structure. Within the inner, dust-depleted region of the system, Wahhaj and colleagues discern four more or less evenly spaced rings, with dust concentrations at radii of 14, 28, 52, and 82 AU. They further note that these four rings are non-coplanar, with some of them inclined at an angle of about 5 degrees to the rest (Wahhaj et al. 2003). Okamoto and colleagues note slightly different but analogous peaks in dust emissions: at radii of 6 and 16 AU, flanking the “A ring” of Wahhaj and colleagues, and at 30 AU, corresponding to their “B ring” (Okamoto et al. 2004).

The ring-like morphology of the Beta Pictoris disk has frequently been explained as an effect of recent planet formation around the star. A seris of studies have hypothesized the existence of one or more gas giant planets orbiting between 12 and 44 AU (Okamoto et al. 2004, Freistetter et al. 2007). The ring structure would then be the result of mean motion resonances with these objects, on analogy with the mechanism by which Jupiter maintains the structure of the Asteroid Belt.

A team led by Anne-Marie Lagrange has repeatedly imaged a planetary candidate within the Beta Pictoris disk (Lagrange et al. 2009, 2010). They identify it with the hypothetical gas giant at about 12 AU proposed by Okamoto, Freistetter, and colleagues. The candidate’s semimajor axis is estimated as 8 to 15 AU, on the basis of its observed separation from the central star. This implies an orbital period in the range of 17 to 35 years, similar to the period of Saturn. According to theoretical models, the object’s brightness corresponds to a mass of about 9 MJUP. Such an object is fully consistent with the accretion model of planetary evolution, which argues that the most favorable region for gas giant formation lies just beyond a star system’s ice line. For a star as hot as Beta Pictoris, this region coincides with the estimates for planet b’s semimajor axis (Lagrange et al. 2010).

Fomalhaut debris ring

Fomalhaut (also Alpha Piscis Austrini and HD 216956) is one of the nearest A stars, located only 7.7 parsecs (25 light years) away. It seems a fairly ordinary example of its spectral class, with a mass about 2 to 2.3 times Solar, a luminosity 16 times Solar, and an age of about 200 million years (Chiang et al. 2008). Two features make Fomalhaut remarkable: its well-imaged debris disk, which is “arguably [the] most spectacular example” of an extrasolar Kuiper Belt (Absil & Mawet 2009), and its Jupiter-mass planetary candidate, which has been imaged at multiple epochs and is known by convention as Fomalhaut b.

Some sources argue that Fomalhaut is a sibling to Vega, both stars having formed in the molecular cloud that produced the Castor Moving Group, whose members include many other nearby stars (Barrado y Navascues 1998, Su et al. 2006). However, the most recent investigations into Vega’s age indicate that it is considerably older than Fomalhaut, making such a kinship impossible (Yoon et al. 2010).

The star’s debris disk is actually a well-defined ring extending from 133 AU to about 158 AU, for an approximate width of 25 AU. These modest dimensions contrast strongly with the extensive debris systems proposed for Beta Pictoris and Vega. Alice Quillen and colleagues (2007) calculate that the observed dust has been generated by collisions among planetesimals averaging 80 km (50 mi) in diameter, stirred by perturbations from still larger objects whose average size is 1000 km (620 mi). Both Quillen and Paul Kalas have argued for the presence of at least one planet to maintain the dust ring’s sharp and eccentric inner edge (Kalas et al. 2005).

These predictions were eventually borne out by Hubble Space Telescope images of a co-moving object in just the right place (Kalas et al. 2008). Kalas’ team conclude that this object is a gas giant with a minimum mass of 0.1 MJUP and a maximum mass of 3 MJUP, following a mildly eccentric orbit at a semimajor axis of 100-120 AU (Chiang et al. 2008). This remarkably wide radius is about 4 times larger than the orbit of Neptune, the outermost planet of the Solar System. Kalas and colleagues consider it likely that additional planets exist on smaller orbits; the more inner planets, the smaller the maximum mass of Fomalhaut b must be (Chiang et al. 2008).

They conclude that Fomalhaut b formed through accretion of planetesimals (see Evolution of Planetary Systems) and suggest a variety of mechanisms to explain its present wide orbit: the planet may have formed in situ; it may have migrated outward from a smaller orbit through interactions with the system’s primordial nebula; or it may have been scattered outward by perturbations from a more massive planet located closer to the star (Kalas et al. 2008).

Kalas and colleagues caution that their conclusions are still preliminary, pending further high-precision observations. One of their most intriguing speculations is that certain brightness anomalies in the data may be explained by the existence of an extensive and still-evolving system of rings and moons in orbit around Fomalhaut b, with a radius almost 10 times larger than the optically visible rings of Saturn (Chiang et al. 2008). Although this attractive picture remains completely hypothetical, its plausibility has been enhanced by the recent discovery that Saturn itself has an outer dust ring whose radius is 20 times larger than its classically known ring system (Verbiscer et al. 2009).

Vega debris disk

Vega is the brightest of the nearby group of A stars with debris disks. Located 7.8 parsecs (25 light years) from our Sun, it is also known as Alpha Lyrae and HD 172167. Its complement of dusty debris was the first to be detected outside our Solar System (Aumann et al. 1984). Despite Vega’s ease of observation from northern latitudes, wide disagreement persists regarding its basic physical characteristics. Estimates of the star’s age range from 200 to 455 million years (Wyatt et al. 2007b, Su et al. 2005, Yoon et al. 2010); estimates of its mass range from 2.1 to 2.9 times Solar, and of its luminosity from 37 to 58 times Solar (Aufdenberg et al. 2006, Isbell & Allen 2006, Su et al. 2005, Wyatt et al. 2007b, Yoon et al. 2010). Over the past decade, published values have trended toward lower mass and luminosity, and older age.

Thus Yoon and colleagues (2010) find a stellar mass of only 2.157 MSOL, an age of 455 million years, and a strikingly low metallicity of about -0.50. Their results are in good agreement with a previous study by Aufdenberg and colleagues (2006), who established that we observe Vega in a pole-on orientation, rotating in a period of only 12.5 hours. Such a rapid spin results in an equatorial diameter that is 20% wider than the polar diameter, creating a dimmer, cooler belt around the star’s midsection (Aufdenberg et al. 2006). The same study concluded that Vega’s overall luminosity is 37 LSOL (+/- 3).

Observations by the Spitzer Space Telescope indicate that Vega’s debris fields have an inner edge at 86 AU. The particle density peaks at about 100 AU and diminishes outward beyond 200 AU. Fine dust has been blown by stellar radiation to distances as great as 815 AU (all values Su et al. 2005). This formation is explained as a ring of silicate and carbonaceous bodies rather like the asteroids in our own system, centered around 100 AU, with ever finer particles ranging steadily outward. On the basis of high-resolution imaging, Su and colleagues find that the debris is evenly distributed rather than exhibiting clumpiness, and that the belt itself is centered on the star, rather than offset. It is widely agreed that the source of the extreme dustiness now observed in Vega’s debris field is the result of an astronomically recent collisional cascade involving large planetesimals or protoplanets, probably hundreds or even thousands of kilometers in diameter (Su et al. 2005, Wyatt et al. 2007b). These objects are very similar to those hypothesized by Quillen and colleagues to explain the debris around Fomalhaut (Quillen et al. 2007).

As with other dusty A stars, the inner clearance in Vega’s debris disk is regarded as an indication that planets have formed within a few dozen AU of the star (Aumann et al. 1984, Wyatt et al. 2007b). Given the discovery of gas giant shepherds for the dusty rings around Fomalhaut and Beta Pictoris, the odds of a planetary system around Vega have become much more favorable.

retired A stars & their companions

Radial velocity searches have begun to detect planets around relatively massive subgiant and giant stars. By the middle of 2010, more than 2 dozen of these systems had been reported around host stars in the range of 1.5-3.0 MSOL. Because the transition to the giant phase involves little or no mass loss, the present values for these stars are an excellent indication that all of them once belonged to spectral class A (Johnson et al. 2007). As a result, their companions provide our best available evidence on the configurations of planetary systems centered on A-type stars. Data for the most reliably constrained systems are summarized in Table 2.

Table 2. Planets of evolved A stars



HD 180902 K0 IV 110 1.52 4.1 9.4 +0.04 1.6 1.39 0.09 479
24 Sextantis G5 IV 74.8 1.54 4.9 14.6 -0.03 1.99 1.33 0.09 453

0.86 2.08 0.29 883
HD 206610 K0 IV 194 1.56 6.1 18.9 +0.14 2.2 1.68 0.23 610
HD 95089 K0 IV 139 1.58 4.9 13.5 +0.05 1.2 1.51 0.16 507
HD 175541 G8 IV 128 1.65 3.8 9.6 -0.07 0.7 1.03 0.08 297
HD 102956 K0 IV 126 1.68 4.4 11.6 +0.19 0.96 0.08 0.05 6.5
HD 192699 G8 IV 67 1.69 3.9 11.5 -0.15 2.4 1.15 0.13 352
HD 4313 G5 IV 137 1.72 4.9 14.1 +0.14 2.3 1.19 0.04 356
HD 167042 K1 IV 50 1.72 4.3 10.5 +0.05 1.7 1.32 0.09 413
11 Ursae Minoris K4 III 120 1.80 24 n/a +0.04 10.5 1.54 0.08 516
6 Lyncis K0 IV 56.9 1.82 5.2 15 -0.13 2.2 2.18 0.06 899
Kappa Coronae Borealis K0 IV 31 1.84 4.71 12.3 +0.14 2.0 2.80 0.04 1208
HD 181342 K0 IV 111 1.84 4.6 12 +0.26 3.3 1.78 0.18 663
HD 210702 K1 IV 56 1.85 4.72 13.1 +0.12 2.0 1.17 0.15 341
HD 11977 G5 III 66.5 1.90 10 n/a -0.21 6.5 1.93 0.40 711
HD 102272 K2 III 362 1.90 10 n/a -0.26 5.9 0.61 0.05 128

2.6 1.57 0.68 520
Pollux K0 III 10.3 2.0 8.9 n/a +0.19 2.6 1.6 0.02 589
HD 173416 G8 III 135 2.0 13.5 78 -0.22 2.7 1.16 0.21 324
HD 81688 K0 III 88.3 2.1 13 72 -0.36 2.7 0.81 0 184
Xi Aquilae K0 III 62.7 2.2 12 69 -0.21 2.8 0.68 0 137
14 Andromedae K0 III 76.4 2.2 11 58 -0.24 4.8 0.83 0 186
18 Delphini G6 III 73 2.3 8.5 40 -0.05 10.3 2.6 0.08 993
HD 17092 K0 III 109 2.3 10 n/a +0.18 4.6 1.29 0.17 360
HD 104985 G9 III 102 2.3 11 59 -0.35 8.3 0.95 0.08 199
HD 110014 K2 III 90 2.4 21 154 +0.19 9.5 2.15 0.46 835
81 Ceti G5 III 97.2 2.4 11 60 -0.06 5.3 2.5 0.21 953
Epsilon Tauri K0 III 47.5 2.7 13.7 97 +0.17 7.6 1.93 0.15 595

Abbreviations: Dist = distance in parsecs; Msol = star mass in Solar units; Rsol = radius in Solar units; Lsol = luminosity in Solar units; [Fe/H] = metallicity; Mjup = planet mass in Jupiter units; a = semimajor axis in astronomical units (AU); e = eccentricity; p = period in days.
Sources: Johnson et al. 2010a (HD 180902, HD 206610, HD 95089, HD 4313, HD 181342); Johnson et al. 2011 (24 Sextantis); Johnson et al. 2010b (HD 102956); Johnson et al. 2007, 2008; Bowler et al. 2010 (HD 167042, HD 175541, HD 192699, Kappa Coronae Borealis, HD 210702); Sato et al. 2003, 2007, 2008a, 2008b (6 Lyncis, HD 81688, Xi Aquilae, 14 Andromedae, 18 Delphini, HD 104985, 81 Ceti, Epsilon Tauri); Dollinger et al. 2009 (11 Ursae Minoris); Setiawan et al. 2005 (HD 11977); Niedzielski et al. 2009, Wright et al. 2009, Berdyugin 2002 (HD 102272); Hatzes et al. 2006, Hatzes & Zechmeister 2007 (Pollux); Liu et al. 2009 (HD 173416); Niedzielski et al. 2007 (HD 17092); de Medeiros et al. 2009 (HD 110014).
Alternate Names: 6 Lyncis is HD 45410; Kappa Coronae Borealis is HD 142091; 11 Ursae Minoris is HD 136726; Pollux is Beta Geminorum and HD 62509; Xi Aquilae is HD 188310; 14 Andromedae is HD 221345; 18 Delphini is HD 199665; 81 Ceti is HD 16400; Epsilon Tauri is HD 28305.


Immediately evident is a trend toward massive gas giant planets in Mars-like orbits. The median mass of this sample is 2.6 MJUP (compared to 1.7 MJUP for gas giants orbiting Sun-like stars), while 30% have minimum masses in excess of 5 MJUP. The median semimajor axis is 1.41 AU (compared to 1.52 AU for Mars), with orbital periods ranging from 6.5 days to 3.3 years. More planets on longer-period orbits will doubtless emerge as observations continue. Already clear is a paucity of elliptical orbits, since 75% of the planets in this group have eccentricities smaller than 0.2, unlike the vast majority of exoplanets orbiting Sun-like stars at similar semimajor axes.

a unique population

Bowler and colleagues recently studied a population of retired A stars in the subgiant phase, whose membership overlaps with the sample characterized in Table 2. Their robust conclusion is that A-star systems are “qualitatively quite different” from those of Sun-like stars – especially with regard to the frequency of planet-hosting systems and the distribution of planetary masses and orbits (Bowler et al. 2010). Notably, they found that about 25% of A stars in the subgiant phase host at least one gas giant planet. This is much higher than the frequency of gas giants around Sun-like stars, and even higher than the proportion suggested by Kennedy & Kenyon (2008). As Bowler’s group argues, subgiants are excellent proxies for main sequence stars, so their conclusions most likely encompass current as well as retired A stars.

The analyses of Bowler et al. confirm the trends noted above regarding masses and orbits. The planets of A-type stars are substantially more massive than those of later spectral types, with a dearth of sub-Jovian objects and an abundance of Super Jupiters. A-star planets also tend to follow more circular orbits than those of comparably massive planets around Sun-like stars. Finally, they have much wider semimajor axes and longer orbital periods than the planets of Sun-like stars, since more than 75% of retired A-star planets have semimajor axes wider than 1 AU.

For several years, Hot Jupiters were unknown in this sample, prompting considerable discussion. The dearth of hot planets was clearly not the result of detection biases, since radial velocity observations are sensitive to all Jupiter-mass planets orbiting within a few AU of retired A stars, especially at the smallest semimajor axes. Indeed, for host stars between 1.5 and 1.9 Msol, observations can detect even Saturn-mass planets orbiting within 0.3 AU (Bowler et al. 2010). One possibility is that Hot Jupiters were originally present in orbit around the main-sequence progenitors of these stars, but were engulfed or otherwise annihilated by expanding stellar radii as the stars evolved into the giant phase. Sato and colleagues note that even if a red giant does not expand enough to physically absorb its short-period companions, its enlarged radius will produce tidal forces that can destabilize the orbits of adjacent planets and cause them to fall into the central star (Sato et al. 2008b). Thus we should not expect any planets to survive within about 0.5 AU of an A star that has evolved into the giant phase.

However, as other studies have found, this argument does not apply to A stars in the subgiant phase, which have not expanded enough to affect short-period orbits (Johnson et al. 2008, Bowler et al. 2010). The scarcity of Hot Jupiters and Hot Saturns around subgiants, at least, must be primordial.

Both theory and observation predict a rapid depletion of protoplanetary disks around massive stars (Haisch et al. 2001, Ida & Lin 2005, Mandell et al. 2007, Johnson et al. 2007, Currie et al. 2009, Bowler et al. 2010). Fast depletion means that circumstellar gases will dissipate before any forming gas giant planets have time to migrate to small semimajor axes by Type II migration, a process that can occur only in the presence of the nebula. As a result, few detectable planets are likely to reach short orbital periods around stars in this mass range.

So far, 2 Hot Jupiters have been reported in the population of interest: HD 15082 b (AKA WASP-33b), orbiting an A5 star of 1.5 Msol, and HD 102956 b, orbiting a K-type subgiant of 1.7 Msol.

planet formation around A stars

Until recently, the question of planet formation around A-type stars was largely ignored by astronomical researchers. The most widely endorsed theory of planetary evolution, based on studies of Sun-like stars, holds that planets of all kinds are born from the collision and mutual sticking of primordial dust grains spinning in the nebula around a newly-ignited star (Pollack 1996, Ida & Lin 2004). This iterative process is known as accretion. Dust accretes into pebbles, pebbles grow into planetesimals, planetesimals accrete into protoplanets, and protoplanets build planets. Since the accretion process varies according to the mass, density, temperature, chemical composition, and overall dimensions of the primordial nebula, this scenario can be scaled to fit any given population of stars (Ida & Lin 2005, Kennedy & Kenyon 2008, Thommes et al. 2008).

The same theory suggests that nebular mass scales with star mass (Andrews & Williams 2005, Raymond et al. 2007), that more massive nebulae produce more massive planets (Thommes et al. 2008), and that the density of solids in the nebula increases abruptly outside the radius at which volatiles such as water condense into ice – i.e., the ice line (Ida & Lin 2004, Kennedy & Kenyon 2008). Further, the location of the ice line varies according to stellar luminosity (Kennedy & Kenyon 2008), and the location of the inner edge of the nebula varies according to the stellar magnetosphere (Terquem & Papaloizou 2007), such that their radial distance tends to increase along with stellar mass.

Ida and Lin suggest about 11 AU for the ice line of an A star of 2 MSOL (e.g., Fomalhaut), while Kennedy & Kenyon argue for an initial ice line at 8 AU, moving inward to about 2.7 AU as primordial gases dissipate (Ida & Lin 2004, Kennedy & Kenyon 2008). Several lines of evidence indicate that the optimal planet-forming region around an A star will be much more extensive than that of a Sun-like star. Dodson-Robinson and colleagues (2009) conclude that, given a sufficiently massive protoplanetary disk, gas giants can form between 1 AU and 35 AU even around a low-mass A star of 1.5 MSOL.

These basic principles present a low-resolution picture of potential planetary systems orbiting A stars. Their massive, short-lived nebulae will tend to produce planets that have larger masses and wider orbits than those around Sun-like stars. Both the truncation of the inner nebula through magnetospheric activity and the rapid dissipation of nebular gas will tend to limit the effects of migratory processes that produce short-period planets (e.g., Hot Jupiters, Hot Neptunes, and Hot Super-Earths) around F, G, and K-type stars. Yet many questions remain. For example, what is the typical range of semimajor axes for giant planets orbiting A stars? Are such objects likely to undergo planet-planet scattering, as so commonly occurs around Sun-like stars (Chatterjee et al. 2008, Ford & Rasio 2008)? Are Earth-like planets possible in these extreme environments?

The recent recognition that planetary systems around massive subgiant and giant stars are actually the successors of A star systems (e.g., Johnson et al. 2007, 2008, 2010a, 2010b, 2011), coupled with the recent imaging of giant planets orbiting nearby A stars that still reside on the main sequence (Kalas et al. 2008, Marois et al. 2008, Lagrange et al. 2010), has inspired a growing number of theoretical attempts to explain their formation. As with other kinds of exoplanetary systems, however, each study tends to focus on a highly specific type of planet – sometimes in just a single system – rather than trying to characterize a wide range of potential A-star systems.

One study by Katharine Kretke and colleagues (2009) addresses gas giants orbiting retired A stars at semimajor axes of a few AU. Although their investigation uses the theoretical apparatus of planetesimal accretion, they never appeal to such basic concepts as the ice line and Type II migration. Instead, they focus on the potential presence of “dead zones” in the massive protoplanetary disks of A stars, caused by turbulence-induced variations in the pressure and surface density of the dusty nebula. Such dead zones would trap solid particles at about 1 AU around a star of 2 Solar masses. The accumulation of metals and silicates at this radius encourages the rapid formation of rocky protoplanets, which grow to several times the mass of Earth before accreting deep hydrogen atmospheres and evolving into gas giant planets.

According to this scenario, the recently detected gas giants orbiting retired A stars have formed more or less in situ, without any substantial inward migration (Kretke et al. 2009). The potential for gas giants to form within a few AU of A-type stars, having been explored in several studies, now seems a robust finding (Ida & Lin 2004, 2005; Kennedy & Kenyon 2008; Dodson-Robinson et al. 2010).

The directly imaged companions of nearby A stars burning on the main sequence have received far more attention than the better characterized systems of retired A stars. For 2 nearby stars – Fomalhaut and Beta Pictoris – astronomers have had little difficulty accounting for the imaged planets in terms of formation by accretion (Kalas et al. 2008, Crida et al. 2009, Lagrange et al. 2010).

However, the proposed four-planet system of HR 8799 (spectral type A5) presents more formidable challenges to accretion theory, given the large masses and wide separations implied by existing evidence (Reidemeister et al. 2009, Dodson-Robinson et al. 2010). Some studies conclude that one or more of the imaged planets must be the product of gravitational instability (Dodson-Robinson et al. 2009, Helled & Bodenheimer 2010) , a formation mechanism with relatively little support in the community of exoplanet researchers. Others point out that currently available data on this system are highly imprecise, and that more than one alternative scenario might account for the observed configuration, without recourse to the theory of gravitational instability (Gozdziewski & Migaszewski 2009, Fabrycky & Murray-Clay 2010, Kratter et al. 2010).

In any event, current understanding argues in favor of “a prolific production of multiple planetary systems” around A stars (Kretke et al. 2009). Solid evidence now supports this theoretical prediction, with multi-planet systems detected by radial velocity observations of the evolved stars HD 102272 and 24 Sextantis (Wright et al. 2009, Johnson et al. 2010b) and by direct imaging of the main-sequence star HR 8799 (Marois et al. 2008).

Last update January 2011

Index of exoplanetary topics
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This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France.

All text is copyright Raymond Harris 2006-2010