Monday, May 20, 2013

The Sun's Back Yard



Figure 1. Notable stars in the Sun’s back yard, inside a radius of 10 parsecs (32.6 light years). Stars whose names appear in light blue font are reported to host planetary systems.

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Statistical data on exoplanets are subject to many kinds of bias, among which distance is paramount. We are best informed about nearby star systems, meaning those located in the volume of space within 50 parsecs (160 light years). Two-thirds of all exoplanets detected by radial velocity searches belong to this small sphere. A few numbers put its size in context: Kepler-47, the first known circumbinary system with multiple planets, is 1500 parsecs (5000 light years) away, while the titanic black hole churning at the Galactic Core is about 8500 parsecs (28,000 light years) away. On this scale, the Solar neighborhood is barely a bubble adrift in a cosmic whirlpool.

The volume where bias can be most effectively minimized is even smaller, corresponding to the space within 10 parsecs (32.6 light years). Thanks to long observation and dedicated surveys, this region is the only place where we have a precise census of the stellar population, as well as reliable data on the brightness, temperature, mass, chemical composition, and other key parameters of each star. Exoplanet searches are also most complete here, since several of the very nearest Sun-like stars have been monitored since the 1980s.

The Sun’s back yard seems typical of the territory extending outward for a few hundred parsecs (i.e., at least 500 light years). We know from long observation that our extended neighborhood is empty of star-forming clouds like the Orion Nebula. The closest such region is probably the isolated Rho Ophiuchi complex, about 145 parsecs (475 light years) away (Makarov 2007), whereas the extensive nurseries of Orion are much farther, at 415 parsecs (1350 light years). Our cloud-free neighborhood, where star populations are well-mixed, provides a good proxy for the Galactic Disk in general. Thus, our nearest neighbors can offer valuable insight into the architecture of run-of-the-mill planetary systems – something our home system certainly is not.

nearby star populations

The Solar neighborhood is devoid of the brightest, hottest stars, represented by spectral class O. The lifetimes of these dazzling objects are so brief that they are found only in the vicinity of their native clouds. Our neighborhood also lacks stars of spectral class B, which like O stars are extremely bright, short-lived, and rare. No B stars exist within 20 parsecs (65 light years), although one or more nearby white dwarfs (e.g., Sirius B) are evidently their evolved remnants. Collectively, O and B stars represent much less than 1% of the Galactic census.

Within 10 parsecs of the Sun, we know of 260 gravitationally bound star systems comprising 359 stars and brown dwarfs, with the following distribution by spectral type:

Table 1. Stellar, substellar, and stellar remnant populations within 10 parsecs

D = white dwarfs, L + T = brown dwarfs. Data from the RECONS Survey as of 2012, with the addition of WISE 1049-5319, a brown dwarf binary announced in 2013.

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Bright white stars of spectral class A are the least numerous, and spectral class F is only slightly better represented. Populations increase as mass and luminosity decrease, such that lightweight M dwarfs outnumber Sun-like G stars by a ratio of 12 to 1. This trend does not continue at substellar masses, however, even if we assume that the true population of brown dwarfs is undercounted by an order of magnitude. Red dwarfs are far more common than brown dwarfs.

RECONS data also indicate that most systems (71%) within 10 parsecs contain only one star or brown dwarf, 21% of systems contain 2 stars, 6% contain 3 stars, and 2% contain 4 or more. Stellar multiplicity correlates closely with spectral type, such that about 60% of G and K stars, 30% of M dwarfs, and 20% of brown dwarfs occur in binary or multiple systems (Lada 2006, Allen 2007). Accordingly, most of the single stars in our neighborhood are red dwarfs of class M, while more than half of all nearby Sun-like stars (spectral classes F, G, and K) are found in binary or multiple systems.

These distributions by stellar multiplicity and spectral type may be typical of the entire Milky Way, although we can expect considerable local variation. In any case, the neighborhood population evidently represents stars born in many different parts of the Galaxy, whose Galactic orbits have migrated over hundreds of millions of years and fortuitously coincide at this epoch (Famaey et al. 2007, Ecuvillon et al. 2007).

backyard exoplanets

In addition to our Solar System, 15 planetary systems have been characterized to date within 10 parsecs. Six are centered on G or K type stars, eight accompany M dwarfs, and one orbits a bright, hot star of spectral type A. Some of them also harbor debris belts that are denser and more extensive than the ones in our own system (i.e., the Asteroid and Kuiper Belts).

Compared to the general stellar population in our back yard, the Sun is massive and metal-rich. Both qualities, and especially the latter (Fischer & Valenti 2005), are associated with planet formation. Because astronomers use our Sun to set the standard for chemical abundances in other stars, its metallicity is defined as zero, even though it turns out to be more enriched than most. The average metallicity for G dwarfs in our neighborhood is –0.20, while the median for all exoplanet host stars is +0.10 (Adams 2010). From this perspective our Solar System stands out as a likely haven for planets.

Although the metallicities of the 15 nearest host stars span a broad range, approximately from –0.5 to +0.3, the mean is about –0.07 and the median is about –0.10. This is much lower than the median of the full sample of exoplanetary hosts, suggesting that our population data are biased in favor of metallicity as well as proximity. It’s a safe bet that most stars have planets, both within 30-odd light years and across the Milky Way.

With regard to system architecture, nine systems (60%) within 10 parsecs have only one detected planet, while two have two each, two have three each, and two have four each. Just five out of 15 harbor a gas giant planet. Four of these five are among the single-planet systems, while the fifth, GJ 876, harbors four known planets, including two gas giants and two low-mass planets. The local shortage of gas giants is predicted both by microlensing statistics (Mann et al. 2010) and by the results of the Kepler Mission: in other words, giant-free systems are the majority both in our back yard and throughout the Galaxy.

Among the five backyard systems with giants, three are centered on M dwarfs, one on an A star (Fomalhaut), and one on a K star (Epsilon Eridani). In all but one of these systems, the gas giant travels on a wide orbit beyond the local ice line – the region beyond which free-floating water will stay frozen indefinitely. The exception is the M dwarf GJ 876, whose pair of giants straddles the system habitable zone, where liquid water could persist on the surface of an Earthlike planet. Unfortunately, no such planet exists in that system.

None of our neighbors support Hot Jupiters, defined as gas giants with orbital periods of 10 days or less – a reminder of the absolute rarity of this planetary species, despite its relative ease of detection.

In fact, a two-thirds majority of local systems contain only objects in the mass range of telluric or gas dwarf planets (i.e., less than about 25 Earth masses or Mea), and they are largely confined to circumstellar radii smaller than 0.5 AU. These planets have been called Hot Neptunes, exo-Neptunes, Mini Neptunes, and Super Earths. Compact, low-mass architectures are also common in systems discovered by the Kepler Mission, supporting the hypothesis that our back yard satisfies the Copernican principle of mediocrity. Among the sample of nearby low-mass planets, just one has a semimajor axis larger than the Earth’s. This is GJ 785 c, the cool outer companion of an amber star. Its minimum mass is 24 Mea, compared to 17.2 Mea for Neptune, and its orbital period is 526 days. Only one low-mass planet in the full extrasolar catalog has a longer period: HD 10180 g, with a minimum mass of 21 Mea, which orbits a G-type star in a period of 601 days. The apparent rarity of such objects is almost certainly an illusion resulting from the limitations of current detection methods.

Counting our Sun, the planet-hosting rate among Sun-like stars (types F, G, and K) within 10 parsecs now stands at 10% (7/70). The rate for M dwarfs within the same volume is substantially lower, at 3% (8/248). This deficit of planets around M dwarfs supports theoretical predictions about planet formation in such environments (Laughlin et al. 2004). Nevertheless, exoplanet searches remain incomplete even in the Sun’s back yard, so these numbers and percentages are bound to increase.

backyard debris

In addition to gas giants, gas dwarfs, and terrestrial-mass planets, several nearby stars also harbor debris disks analogous to the Asteroid and Kuiper Belts of the Solar System (Wyatt et al. 2012). They range from hot, massive stars of spectral class A (Vega, Fomalhaut), through Sun-like stars of spectral classes F, G, and K (Zeta Tucanae, Tau Ceti, Epsilon Eridani, 61 Virginis, and 82 Eridani), to cool M dwarfs (GJ 581, AU Microscopii). Leading researchers on debris disks have called these structures “signposts of planetary system formation” (Trilling et al. 2007), implying that where there is debris, there will also be planets. Out of the neighborhood population, five systems – 61 Virginis, 82 Eridani, Epsilon Eridani, Fomalhaut, and GJ 581 – show persuasive evidence of planetary companions. Continuing observations with increasingly sensitive methods should clarify the status of the other nearby debris disk systems – and possibly discover debris belts in systems already known to host planets, as happened with 82 Eridani and GJ 581.

backyard controversies

Every year since 2005, at least one new exoplanetary system has been identified within our local 10-parsec sphere, testimony to the increasing sensitivity of detection methods. Without doubt, many more backyard systems will be reported in the fullness of time. Yet as the numbers increase, so do the questions and controversies. Counterintuitively, then, our back yard is at once the best-known and the most disputed region of exoplanetary space. The next several paragraphs look at the most salient revisions, doubts, and controversies to emerge over the past few years:

GJ 317 is an M dwarf with one confirmed and one candidate gas giant planet, both orbiting outside the system ice line. When the confirmed giant was announced in 2007, the host star was believed to lie at a distance of 9.2 parsecs (30 light years). At that time, only three other M dwarf systems were known inside 10 parsecs. In 2011, a team led by Guillem Anglada-Escude demonstrated that GJ 317 is actually more than 60% farther away, at a distance of 15.3 parsecs (50 light years). This surprising revision implies that the star is brighter, more massive, and more metallic than previously estimated, leading to upward revisions in the minimum masses of its proposed companions. The fact that such a nearby system was initially mismeasured offers a cautionary tale for all exoplanetary enthusiasts, whether amateur or professional.

VB 10 is an “ultracool” M dwarf of spectral type M8, only 8% as massive as our Sun and just 6 parsecs (19.6 light years) away. In 2009, two astronomers announced the detection of a gas giant at least six times as massive as Jupiter in orbit around this star (Pravdo & Shaklan 2009). Their detection method was astrometry: photographic measurement of apparent changes in the position of the star against the Galactic background, a method that had never previously returned convincing results. However, this detection was immediately contested by another team who had been conducting high-precision radial velocity observations of the same star (Bean et al. 2010). The outcome: no planet after all, and no vindication of astrometry.

GJ 667C, a red dwarf of spectral type M1.5 and mass 0.31 Solar, is the third member of a triple star system located about 7 parsecs (23 light years) away. The other two stars are late K dwarfs that share a close binary orbit, separated from star C by about 300 AU. As I discussed back in January, three different studies have returned three different interpretations of the proposed planetary system orbiting this “red dwarf next door.” The first, which was circulated early in 2012, describes a compact system of two low-mass planets: GJ 667C b with a period of 7 days and a minimum mass of 5.46 Mea, and GJ 667C c with a period of 28 days and a minimum mass of 4.25 Mea (Delfosse et al. 2012). Notably, planet c orbits in the system’s habitable zone.

The next two studies agree on planets b and c, but offer different snapshots of the rest of the system. Anglada-Escude and colleagues note a potential signal with a period of 75 days, corresponding to a possible third planet similar in mass to planets b and c (Anglada-Escude et al. 2012), while Gregory finds evidence for not one but three more planets, two of them in the habitable zone along with planet b, and all of them in the mass range of Super Earths (Gregory ). This little-noted backyard controversy remains unresolved, but given the stakes (a nearby possibly potentially habitable Super Earth!) we are likely to hear more about it. For now, let me go on the record in opposition to the habitability hypothesis: any cool planet of 4 Mea or more is probably too massive to sustain plate tectonics, and most likely has a substantial hydrogen atmosphere anyhow.

GJ 581, a run-of-the-mill M dwarf less than one-third as massive as our Sun, is located at a distance of 6.2 parsecs (20 light years). Since 2009 this star has been known as the host of a system of four low-mass planets inside a semimajor axis of about 0.25 AU. Two of the four planets (GJ 581 c and d) have been proposed to support habitable temperatures. In 2010, Steve Vogt and colleagues (2010b) reported the discovery of two additional low-mass planets, one of which – “GJ 581 g” – they characterized as a Super Earth of only 3 Mea orbiting squarely in the system habitable zone. Controversy ensued (Mullen 2010), as I’ve discussed in a previous post. The upshot is that both of the new planet candidates were declared spurious. GJ 581 still has just four planets (Tuomi & Jenkins 2012).

To everybody’s surprise, and so far without any controversy at all, the same ordinary red dwarf was found in 2012 to support a massive debris disk: a larger analog of our own Kuiper Belt (Lestrade et al. 2012). The debris around GJ 581 may extend from about 25 AU to about 60 AU (similar to the inner and outer radii of the Kuiper Belt, which extends from 30 AU to 50 AU). To produce the dust that renders this belt visible, GJ 581 must harbor at least one more planet or dwarf planet on an orbit wide enough to perturb the orbiting debris into collisions. As Lestrade et al. note, orbits wider than 6 AU around GJ 581 may still support quite massive planets, given the limits of radial velocity observations. Therefore, objects analogous to Pluto, Neptune, or even Saturn cannot be ruled out.

Lestrade and colleagues also report a rough estimate of the inclination of the GJ 581 debris belt: at least 30 degrees but no more than 70 degrees, where zero represents a face-on ring and 90 degrees an edge-on ring. Their finding supports earlier estimates of the inclination of the system’s orbital plane (Mayor et al. 2009), such that the likely masses of the reported planets are 1.6 times their minimum masses. Taking the proposed “habitable Super Earth” GJ 581 d as an example, this increment would transform its currently reported minimum mass of 6 Mea into an actual mass of 9.67 Mea. Given planet d’s thermal environment, and considering it in the context of Kepler Mission data, such a mass defines a gas dwarf comparable to Uranus, not a telluric planet like Earth or Venus.


Figure 2. Fomalhaut and its debris ring, which has an outer diameter of about 300-400 astronomical units. Left: Composite of Hubble and ALMA images. Right: Herschel image.

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Fomalhaut is one of just four A type stars in our immediate back yard, at a distance of 7.7 parsecs (25 light years). In the 1980s it became one of the first stars other than our Sun to be identified as the host of a massive field of debris. The Fomalhaut debris ring has since been the object of successive photographic imaging at increasing resolution, placing Fomalhaut among the best characterized debris disk systems. Because the ring has a sharp inner edge and is offset from the central star, astronomers have long suspected that an eccentric giant planet might be orbiting immediately inside the ring to shepherd its constituent particles. In 2008, imaging by the Hubble Space Telescope revealed an object interpreted as a gas giant (Fomalhaut b) in the approximate mass range of Jupiter, separated from the host star by about 120 astronomical units (AU), equivalent to four times the semimajor axis of Neptune in our Solar System (Kalas et al. 2008, Chiang et al. 2009).

Unfortunately, a follow-up study by another group a few years later failed to confirm the initial detection (Janson et al. 2012). The reality of the proposed planet became controversial, even as other directly imaged exoplanet candidates around A stars (Beta Pictoris b, HR 8799 b through d) benefited from additional follow-up that further clarified their parameters.

But when all hope seemed lost, Fomalhaut b came roaring back. A series of studies began appearing in late 2012, re-examining old data and bringing in new results, all with the effect of reviving the planet’s candidacy (Currie et al. 2012, Galicher et al. 2013, Kalas et al. 2013). Because the first revivifying manuscript became public around Halloween, one blogger was moved to christen Fomalhaut b a zombie planet. The most recent study, headed by the same astronomer who led the original discovery team, significantly expands our understanding of what’s going on around Fomalhaut, including revised parameters for the debris as well as the planet.

Paul Kalas and colleagues now strengthen an earlier argument (Chiang et al. 2009) that Fomalhaut b is a Neptune-to-Jupiter mass planet surrounded by an extended system of evolving rings and moons that create a dust cloud visible in telescopic imaging. The planet appears to follow a longer and much more eccentric orbit than previously suspected (semimajor axis ~177 AU, eccentricity 0.8), meaning that it cannot be responsible for sculpting the debris ring, whose eccentricity is much smaller. Fomalhaut b may have been launched on its present orbit by an episode of planet scattering (implying that another, undetected massive planet lurks nearby) and it may even be on course to cross the inner edge of the debris ring within 20 years – an especially exciting and eminently falsifiable prediction (Kalas et al. 2013). Other models are also possible; for these I refer you to the preprint.


Figure 3. A new model of the Fomalhaut system by Kalas et al. 2013. The white dot in the middle of the green ring is Fomalhaut itself; the white circle inside the ring represents the orbit of a hypothetical “nested Jupiter” (“Fomalhaut c”) that shepherds the ring; while the ellipse with the green cross represents the orbit of the “rogue Saturn” Fomalhaut b, which is now traveling inside the ring but may cross it in 20 years.

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Epsilon Eridani, at a distance of only 3.2 parsecs (10.5 light years), has been near the top of the list of nearby stars likely to harbor life-bearing planets for as long as people have made those lists. In 2000 this young K2 star became the second exoplanetary host (after GJ 876) to be reported within 10 parsecs (Hatzes et al. 2000). Epsilon Eridani b is characterized as a gas giant more massive than Jupiter on an eccentric orbit with a period of 6.85 years. In addition to this candidate planet, a substantial debris disk has been observed around Epsilon Eridani. Backman et al. (2009) define three distinct regions of debris, consisting of a narrow inner ring similar to our Asteroid Belt at 3 AU, another narrow ring at about 20 AU, and a large outer field beginning around 35 AU and extending beyond 90 AU.

Even though Epsilon Eridani is very well-studied, the existence of its proposed gas giant companion is not universally accepted. The star is less than one billion years old (compared to 4.6 billion for our Sun), so it supports a high level of magnetic activity that makes radial velocity observations challenging. As a result, different observers have presented quite different parameters for the candidate planet. Notably, three studies over the past decade have described this object as “controversial,” “tentative,” “suspicious,” and “suspected but still unconfirmed” (Moran et al. 2004, Backman et al. 2009, Anglada-Escude & Butler 2012). The authors of the most recent study, using the largest available datasets, concluded that their results “cast some doubt on the reality of this candidate” (Anglada-Escude & Butler 2012). So the space junk is unquestioned, but the planet most certainly is not.

Tau Ceti (also HD 10700) is another very nearby star, as well as a perennially popular candidate for the honor of hosting a planetary system. As a single yellow star of type G8 and 0.83 Msol, located only 3.65 parsecs (12 light years) away, it is our nearest Solar analog. Like our other next-door neighbor, Epsilon Eridani, Tau Ceti supports an outer debris field much more massive than our Kuiper Belt, as well as an inner belt of warmer debris similar to our Asteroid Belt: evidence that rocky bodies have had no trouble accreting in this system (DiFolco et al. 2007). Unlike Epsilon Eridani, Tau Ceti has long since matured out of its magnetic tantrums, so it makes a good target for radial velocity observations.

For decades, however, Tau Ceti returned no data consistent with planetary companions. One potential explanation for this silence is that we view the star in a pole-on orientation (Gray & Baliunas 1994), which is the least favorable to the detection of radial velocity variations. This view is not universally accepted, however, since Greaves and colleagues obtained images of the debris disk that suggest an edge-on orientation (Greaves et al. 2004).

Late in 2012, Mikko Tuomi and colleagues reported their reanalysis of existing radial velocity data on Tau Ceti. Using sophisticated and highly complex modeling, they recovered tentative signals corresponding to a system of five planets (b-f) that range in minimum mass from 2 to 6.6 Mea and in period from 14 to 642 days. Most notable is their candidate planet e, with a hypothetical minimum mass of 4.3 Mea and a hypothetical period of 168 days. Given the host star’s effective temperature of 5344 K, this orbit occupies the circumstellar habitable zone. Thus, provided it exists, Tau Ceti e meets many definitions of a potentially habitable Super Earth (though not my own: it’s too massive; see GJ 667C, above).

Yet no one, not even Tuomi and his collaborators, is pressing this claim too strongly. If we do in fact view Tau Ceti in an almost pole-on perspective, then the actual masses of these candidates would be much higher than their minimum masses, disqualifying them as Super Earths. Only an edge-on view would result in truly sexy planets. The Extrasolar Planets Encyclopaedia lists the Tau Ceti Five as “unconfirmed, controversial,” and Tuomi’s study concludes, “these issues remain merely speculative until the planetary origin of the signals can be verified by an independent detection.” Given this abundance of scientific courtesy and caution, I can’t decide whether we have a controversy or simply an asterisk.

And that wraps up my review of the treasures and junk – the dreams and doubts – the gas, dust, and rock that populate (or maybe just litter) our Sun’s back yard.

 

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  2. "The average metallicity for G dwarfs in our neighborhood is –0.20, while the median for all exoplanet host stars is +0.10 (Adams 2010). From this perspective our Solar System stands out as a likely haven for planets."

    Since that's mentioned, I wonder if the relatively nearby (approx. 20 ly away) and sunlike star Delta Pavonis which has the estimated metallicity of some 200% of Sun's, could host planets - especially those terrestrial. Sources say it's becoming a subgiant or nearing the end of it's main-sequence life but it might still be for millions (or more) years capable of sustaining an Earth-like planet, potentially the closest one to Solar System - because I don't see much promise in the nearer ones like Tau Ceti, of course from the perspective of a scifi buff and not a professional astronomer.

    Currently Del pav has zero detected planet but it could be the fault of RV limitations you mentioned in the article. The transit method probably wouldn't be more useful though.

    Just a few thoughts from me, great article!

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