Red-Hot Planet

Figure 1. Centaur & Centaurette: Pastoral Symphony from Disney’s Fantasia

As the nearest star system, Alpha Centauri has attracted speculation on the plurality of worlds for much of the past century. At last we have good evidence that the system harbors at least one low-mass planet -- Alpha Centauri Bb -- somewhat similar those in the inner Solar System, albeit much hotter than any of them. This provocative discovery was announced last month by a team led by Xavier Dumusque, an astronomer at the Observatoire de Genève who conducts observations with the HARPS spectrograph in La Silla, Chile. Their findings increase the likelihood that most stars in the Milky Way have planetary systems. They also remind us that if we are searching for Earth-like planets around Sun-like stars, our immediate neighborhood is one of the best places to look.

To the naked eye, Alpha Centauri is the brightest star in the classical constellation of the Centaur. Telescopic observations in 1689 revealed that it is actually a binary consisting of two Sun-like stars, one brighter and one dimmer, since designated A and B. Parallactic measurements in the 1830s established that this system is our nearest extrasolar neighbor, at a distance of 4.37 light years (1.34 parsecs).

We now understand that Alpha Centauri A is a yellow star of spectral type G2, identical to our Sun. Its mass is 1.1 Solar, its radius is 1.22 Solar, and its effective temperature is almost exactly Solar, at 5750 K (Pourbaix et al. 2002, Thevenin et al. 2002, Kervella et al. 2003). Because of its larger radius, however, Alpha Centauri A is 50% more luminous than the Sun.

Alpha Centauri B is an amber star with a spectral type of K1, a mass between 0.90 and 0.93 Solar, a radius about 0.86 Solar, and an effective temperature of 5250 K, making it 50% less luminous than our Sun (Thevenin et al. 2002, Kervella et al. 2003).

A third star, designated Proxima Centauri, was discovered in the vicinity of the binary in 1915. Proxima is a very dim red dwarf with a spectral type of M5 and a mass only one-tenth Solar. It is currently separated from stars A and B by about 13,000 AU (430 times the average distance of Neptune from our Sun). Robust proof of Proxima’s association with Alpha Centauri remains elusive (Wertheimer & Laughlin 2006). Conceivably, it could form a moving group with the binary at this stage of Galactic history without sharing a common origin. In any event, Proxima’s wide separation from A and B ensures that it plays no significant role in the immediate environment of either star. For all these reasons, I use the term “Alpha Centauri” to denote the Sun-like binary, and I call the M dwarf “Proxima Centauri” instead of Alpha Centauri C.

No consensus has emerged on the binary’s age, except that all recent estimates make both stars older than our Sun. Various groups have provided ages in the range of 4.85 to 6.52 billion years, compared to 4.6 billion years for the Sun (Thevenin et al. 2002, Guedes et al. 2008, Eggenberger et al. 2004). All analyses agree that Alpha Centauri A and B are mature, main-sequence stars. The two stars’ metallicities (proportion of chemical constituents heavier than helium) have been estimated as +0.19 and +0.23, respectively – a rather puzzling result, given the assumption that close binaries share a common birth environment. In any case, both stars are more metallic than our Sun (metallicity = 0), which itself is slightly richer in heavy elements than the average star in our Galaxy. By now it is well established that more metallic stars are more likely to host planets, especially gas giants, and especially giants on short-period orbits (Marcy et al. 2005, Udry & Santos 2007). 

Stars A and B orbit their common center of mass in a period just under 80 years, with a semimajor axis of 23.4 AU and an eccentricity of 0.52. These values translate into a periastron (closest approach) of 11.2 AU, similar to the distance between Saturn and the Sun, and an apastron (farthest separation) of 35.6 AU, similar to the distance between Pluto and the Sun. This architecture clearly rules out a system of planets resembling our own, which supports gas giants traveling on wide orbits. Nevertheless, many studies over the past two decades have argued that low-mass planets are possible within a few AU of either star.

Dumusque and colleagues have now demonstrated the validity of this hypothesis. They report a planet orbiting Alpha Centauri B with a period of 3.236 days, a semimajor axis of 0.04 AU, and a minimum mass of 1.13 Earth masses (Mea). Their discovery paper and supplemental exhibits include an extensive discussion of the processing and analysis of the radial velocity data. Unfortunately, they provide no more than these three pieces of data about the planet itself, formally designated Alpha Centauri Bb. They say nothing about the implications of their discovery for the history of planet formation in Alpha Centauri, and little about the ways in which the characteristics of planet Bb might constrain our picture of the larger system architecture.

Of all the unanswered questions, these are the ones that I find most tantalizing:

1. What formal constraints can we place on the new planet’s mass and composition?
   
   
2. What, if anything, can we say about the alignment of the planet’s orbit around star B in relation to the binary orbit shared by A and B?
   
   
3. What limits on additional planets around star B can we glean from observations by the HARPS program and other search efforts?
4. Given the known characteristics of planet Bb, what else can we say about the likely system architecture?
5. How does the presence of a very low-mass planet in a 3.4-day orbit comport with theoretical models of planet formation in Alpha Centauri?

I’ll address each point in order, combining 3 and 4 under one heading.

planet characteristics

Dumusque and colleagues offer only a lower limit (m sin i) for planet b’s mass, without commenting on an upper limit. Nevertheless, given a minimum mass of 1.13 Mea, we can be confident that the true mass is less than 10 Mea, and most likely less than 5 Mea (see the discussion of the binary orbit, below). Such a low mass, along with an approximate equilibrium temperature (Teq) of 1500 K, evidently rules out a hydrogen/helium (H/He) atmosphere (Lopez et al. 2012).

Table 1. Physical and orbital characteristics of low-mass, short-period planets

Column 2 shows planet mass in Earth units (Mea); column 3 shows planet radius in Earth units (Rea); column 4 shows orbital period in days; column 5 shows semimajor axis in astronomical units (AU); column 6 shows estimated equilibrium temperature in Kelvin; column 7 shows star mass in Solar units (Msol). With the exception of Mercury, these planets are thought to have orbital eccentricities near zero. Equilibrium temperatures: Kepler Mission Site (Kepler planets), Wikipedia (Mercury, a Centauri Bb), Leger 2009 (CoRoT-7b), Demory 2012 (55 Cancri e), Charbonneau 2009 (GJ 1214 b). All Kepler temperatures are calculated by assuming an albedo of 0.3. Ages: With the possible exception of CoRoT-7b, all the host stars of these planets are older than 2 billion years. Several (55 Cancri, Kepler-10, Kepler-11, Kepler-18) are much older than our Sun.

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Table 1 compares the available data on Alpha Centauri Bb against a sample of low-mass, short-period exoplanets observed in transit (with Mercury added for perspective). All planets with Teq above 1000 K have radii that exclude H/He. Within this subset, the planets below 6 Mea have radii consistent with bare rock, lacking any residue of volatiles. Only the more massive objects – Kepler-18b, 55 Cancri e, and Kepler-20b – may have retained water, possibly in the form of a deep layer of supercritical fluid (Cochran et al. 2012, Gautier et al. 2012, Demory et al. 2012).

Alpha Centauri Bb is therefore likely to be a rocky, airless sphere, possibly resembling Kepler-20e or Kepler-21b. Its estimated dayside temperature of 1500 K (omitted from the discovery paper, but noted by Dumusque in mass media interviews) exceeds the melting point of copper, not to mention the temperature of glowing embers (900 K) or molten lead (600 K). By comparison, Mercury is cool, and Kepler-11f is downright chilly.

Figure 2. Binary orbit of Alpha Centauri viewed against the plane of the sky

Image credit: Wikipedia

orbital alignment

Our single clue to the mass of Bb is the m sin i value of 1.13 Mea. The HARPS spectrograph can detect such a lightweight planet only because its host star is located less than 2 parsecs away, and is therefore optically bright. Capturing the radial velocity signals associated with the planets of Kepler-20 or Kepler-11, located respectively 290 and 613 parsecs away, far exceeds the sensitivity of this instrument. We can estimate the masses of the Kepler planets only because their gravitational interactions cause variations in transit times. Thus we face an observational paradox, such that we know more about the nature of very distant planets like Kepler-11b and Kepler-20f than we do about our nearest neighbor, simply because we can observe the Kepler planets in transit.

The minimum value of 1.13 Mea would represent the true mass of Bb only if we were viewing its orbit edge-on. Such a viewing angle is inherently less likely than one in which the system’s orbital architecture conforms to our general understanding of close binary stars. Both theory and observation indicate that binaries with semimajor axes smaller than 100 AU, and especially smaller than 30 AU (such as Alpha Centauri), have well-aligned spin axes (Hale 1994, Bate et al. 2000). Their primordial planet-forming discs would also be aligned with the plane of the binary orbit, as would the orbits of any resulting planets.

As Figure 2 illustrates, the shared orbit of Alpha Centauri A and B is steeply inclined to the plane of the sky, with an inclination measured at 79.23 degrees (Hale & Doyle 1994). If, as seems likely, the orbit of Bb is coplanar with the binary orbit, then its true mass will be very close to its minimum mass – about 1.15 Mea. As we saw in the previous section, the combination of low mass and high temperature predicts a red-hot rocky sphere.  

system architecture

Beyond observing that lightweight planets often belong to multi-planet systems, Dumusque and colleagues say nothing about the potential architecture of a putative planetary system around Alpha Centauri B. Currently, we know of more than 40 extrasolar systems that contain two or more planets in the Earth-to-Uranus range, without any gas giants in evidence. (This sample will be discussed in an upcoming post.) Well-known examples around Sun-like stars include 61 Virginis, HD 40307, HD 69830, and Kepler-11. The growing body of data on these systems provides an excellent context for understanding planet Bb.

In two-thirds of the low-mass systems, the innermost planet is the least massive, and in slightly less than two-thirds, the outermost planet is the most massive. Most of these systems are also extremely compact, such that the median semimajor axis for all low-mass multi-planet systems is about 0.11 AU, with 95% of their planets orbiting inside 0.8 AU. (By comparison, the semimajor axis of Venus is 0.72 AU.) Any additional planets on wider orbits around Alpha Centauri B are therefore likely to be more massive than Bb.

On the other hand, the examples of Kepler-11 and Kepler-20 demonstrate that mid-system planets can be less massive than inner planets. Kepler-11f, the fifth planet around Kepler-11, has only half the mass of Kepler-11b, the innermost planet, while both Kepler-20e and Kepler-20f, respectively the second and fourth planets around Kepler-20, are less than 20% as massive as Kepler-20b, the innermost planet. So we shouldn’t be too surprised if a planet analogous in mass to Mercury or Mars is eventually reported on a wider orbit around Alpha Centauri B (although by the time planet searches reach that sensitivity I will probably be dead). Even so, I’ll still bet my nickel on sibling spheres in the mass range of Super Earths – or no siblings at all.

Because Alpha Centauri has been a target of radial velocity searches since the 1990s, previous research has already ruled out massive planets around either star. Michael Endl and colleagues reported detection limits of 1.5 Mjup within 2 AU of Alpha Centauri B, and of 0.5 Mjup within 0.4 AU (Endl et al. 2001). Javiera Guedes and colleagues subsequently revisited these limits, arguing that planets of Saturn mass (0.3 Mjup) or more are unlikely within 1 AU (Guedes et al. 2008).

Since we now know of many planetary systems that lack planets even half as massive as Saturn, such constraints no longer seem terribly constraining. Given the sensitivity of the HARPS spectrograph, and the three-year duration of their observations, we can only wish that Dumusque and colleagues had provided their own perspective on detection limits for Alpha Centauri B. The closest they come is this puzzling statement: “the observed radial-velocity semi-amplitude [of Alpha Centauri Bb] is equivalent to that induced by a planet of minimum mass four times that of Earth in the habitable zone of the star (P = 200 d).”

Does this mean that they have already ruled out the presence of planets of 4 Mea or more at semimajor axes of 0.8 AU or less? Impossible for me to say. Nevertheless, I have to suspect that a planet of 10 Mea – the approximate boundary between objects like Earth and objects like Uranus – would already have been announced if it were present anywhere within 1 AU. That suspicion feeds my hunch that Alpha Centauri B supports either a single planet or a multi-planet system in which none of the companions are more massive than ~3 Mea.  

theoretical context

Since 1997, numerous theoretical studies have investigated the potential for planet formation in Alpha Centauri. Until 2008, these studies tended to reach optimistic conclusions (Wiegert & Holman 1997, Marzari & Scholl 2000, Barbieri et al. 2002, Quintana et al. 2002, Quintana 2003, Guedes et al. 2008). Most concurred that the periastron separation of stars A and B would truncate their protoplanetary disks at radii of about 2 to 3 AU, approximately equivalent to their primordial ice lines. Since massive planets of Sun-like stars are thought to assemble outside the ice line, this truncation would strip volatiles from the disk surrounding star A and thereby eliminate its “sweet spot” or gas giant nursery. The effects on star B might not be so extreme, but they would still disfavor the formation of giant planets (although to my knowledge the survival of volatiles around B has not been rigorously investigated). Fortunately, according to this generation of studies, the formation of rocky planets in each star’s habitable zone would not be curtailed.

Indeed, various numerical simulations (Barbieri et al. 2002, Quintana et al. 2002, Quintana 2003, Guedes et al. 2008) found that systems containing anywhere from one to five planets, with masses ranging from half Mercury to twice Earth, might have formed around either star, all within semimajor axes of 2 AU. These studies typically assumed that the respective protoplanetary disks were coplanar with the binary orbit, but some also included simulations with disks at various inclinations. Notably, they rarely predicted planets inside 0.5 AU, and never inside 0.15 AU, even with inclined disks.

Although the simulations readily produced Earth-mass planets with habitable temperatures, Elisa Quintana and colleagues argued that these planets would be completely lacking in volatiles, and so would be “devoid of the C/H₂O-based life that thrives on Earth” (2002).

The theoretical tide turned in 2008 when F. Marzari and H. Scholl, who had previously published optimistic models of planet formation in Alpha Centauri, collaborated with Philippe Thébault on the first of two studies demonstrating that planetesimal accretion would be severely curtailed around both stars (Thebault et al. 2008, 2009). This team found that for each star, during the early phases of protoplanetary disk evolution, strong gravitational perturbations by the close binary companion would lead to differential orbital phasing of planetesimals. This phasing would sort the planetesimals by size and induce such high collisional velocities that the result would be shattering rather than sticking. The only place where planetesimals might coalesce into planets would be the orbital space inside 0.5 AU around either star.

The habitable zone of Alpha Centauri A includes the region between 1.1 and 1.3 AU, while the corresponding orbital space for the cooler star B is 0.5-0.9 AU. Thebault and colleagues therefore concluded that in situ formation of habitable zone planets was impossible for star A, and at best marginal for star B. Since it is well known that the search for exoplanets depends heavily on public interest in habitable extrasolar worlds, this finding threatened both stars with demotion from Cinderellas-in-waiting to ugly stepsisters.

Thebault followed up his sobering analyses with a slightly more hopeful study in collaboration with lead author Matthew Payne and co-author Mark Wyatt (best known for his work on debris disks). They argued that infant planets born in the accretion-friendly zone of Alpha Centauri B would most likely experience outward migration, potentially reaching semimajor axes as large as 1.2 AU (Payne et al. 2009). Thus, planets with habitable temperatures might be present in this system. However, despite their use of the term “habitable zone,” the authors had no comment on the likelihood that such planets might actually support bodies of liquid water – an outcome that Quintana et al. (2002) had already discounted.

The characteristics of Alpha Centauri Bb, as announced by Dumusque et al. (2012), are completely consistent with the recent theoretical models of Thebault and collaborators. On the other hand, none of the optimistic studies conducted before 2008 predicted the existence of such an object.

what lies ahead

Some extrasolar destinations are far more popular than others. The cumulative bibliography on such systems as 55 Cancri and HD 189733 is large and growing. Based on a search limited to 21st century publications in the SAO/NASA astrophysics database, Alpha Centauri was already more than twice as popular as either of these systems as a target for astronomical research, even without an exoplanet detection. We can now expect many more studies of this binary to appear over the next few years.

Whether we should also anticipate more detections for Alpha Centauri is a different story. Maybe both stars have planets. Maybe star B has additional planets. But as Dumusque et al. emphasize, finding Bb stretched current methods to the limit. So for more planets, we may need better hardware, better search methods, and better analytic tools.

There remains the problem of confirming our favorite Centaur’s red-hot planet. In the opinion of astronomer Artie Hatzes, himself a longtime observer of Alpha Centauri, the existence of this object is “still open to debate,” given the low ratio of signal to noise in the data (Hatzes 2012). As Hatzes argues, “Only if other analyses come to the same conclusion can we be sure that this planet exists.”

His caution is admirable. Our excitement, even if Bb remains for now a candidate rather than a certified winner, is inevitable.

Figure 3. Venus with two Centaurides. Mosaic, Tunisia, second century AD

 Credit: Wikimedia

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