Figure 1. Familiar
bright stars and selected exoplanetary host stars within 20 parsecs (65 light
years) of our Sun. This stylized diagram illustrates the perspective of an
observer located in the north celestial hemisphere, in the direction of the
constellation Draco. Approximate stellar spectral types are coded. Names of
host stars appear in turquoise; names of stars without known planets appear in violet.
------------------------------
With a stellar population in excess of 100 billion and a
diameter of at least 30,000 parsecs (100,000 light years), our Milky Way is a good-sized spiral galaxy. Astrophotography
has unveiled some of its most spectacular features, from the claw-like filaments just north of Sagittarius A* (our resident
black hole, about 7900 parsecs away) to the billowing clouds and blazing young
stars of the Carina Nebula (2300
parsecs away) and the delicate filigree of supernovae remnants such as Simeis 147 and Vela (920 and 250 parsecs away, respectively). Yet
even as these panoramic views of our local universe swim into focus, other
details remain fuzzy, especially when we turn to the question of potential planets
orbiting all those billions of stars.
Three decades of searching, using several different techniques, have returned data on 2098 confirmed extrasolar planets in more than 1300 different star systems. The most complete and broadly comparable information continues to flow from radial velocity (RV) and transit searches. Apart from two outliers (SWEEPS-04 and -11, both in the Galactic Bulge), the most remote star with planets detected by either method lies at a distance of 3200 parsecs. That’s more than 10% of the diameter of the Galactic Disk.
But if we take a closer look at the combined sample of RV and transiting planets available in early January (n = 1867), stellar distances shrink fast. Searches of the Extrasolar Planets Encyclopaedia (EPE), the Kepler Discoveries Table, and various discovery papers found me distance estimates for just 713 stars in that sample. Although they range from 3 to 3200 parsecs, 50% are closer than 90 parsecs, and 70% are no farther than 200 parsecs. The size of those two fractions exposes a critical deficiency in our picture of extrasolar planets: the subset of robustly characterized systems skews sharply in favor of the nearest stars. Even though more than 1000 transiting planets have been confirmed by the Kepler Mission, fewer than 10% of their stellar hosts have distance estimates. The ones with estimates typically reside between 200 and 2700 parsecs.
Evidently our knowledge of transiting as well as RV planets has gaps of near-cosmic proportions.
If we want to grasp the true diversity of extrasolar planets and system architectures, we could do worse than examine a clearly delimited sample of well-constrained systems. I suggest that the best place to find them is the volume of space within 20 parsecs, where diversity is balanced by precision of observation.
Three decades of searching, using several different techniques, have returned data on 2098 confirmed extrasolar planets in more than 1300 different star systems. The most complete and broadly comparable information continues to flow from radial velocity (RV) and transit searches. Apart from two outliers (SWEEPS-04 and -11, both in the Galactic Bulge), the most remote star with planets detected by either method lies at a distance of 3200 parsecs. That’s more than 10% of the diameter of the Galactic Disk.
But if we take a closer look at the combined sample of RV and transiting planets available in early January (n = 1867), stellar distances shrink fast. Searches of the Extrasolar Planets Encyclopaedia (EPE), the Kepler Discoveries Table, and various discovery papers found me distance estimates for just 713 stars in that sample. Although they range from 3 to 3200 parsecs, 50% are closer than 90 parsecs, and 70% are no farther than 200 parsecs. The size of those two fractions exposes a critical deficiency in our picture of extrasolar planets: the subset of robustly characterized systems skews sharply in favor of the nearest stars. Even though more than 1000 transiting planets have been confirmed by the Kepler Mission, fewer than 10% of their stellar hosts have distance estimates. The ones with estimates typically reside between 200 and 2700 parsecs.
Evidently our knowledge of transiting as well as RV planets has gaps of near-cosmic proportions.
If we want to grasp the true diversity of extrasolar planets and system architectures, we could do worse than examine a clearly delimited sample of well-constrained systems. I suggest that the best place to find them is the volume of space within 20 parsecs, where diversity is balanced by precision of observation.
the
immediate solar neighborhood
Figure 1 represents
a sphere centered on our Sun with a diameter of 40 parsecs. Although this
region is devoid of O- and B-type stars – the rarest and most massive species
in our Galaxy – all of the more common varieties of main sequence and evolved
stars are represented. The brightest nearby objects are young blue-white stars
of type A, such as Vega and Sirius, and older red giants, such as Pollux and
Aldebaran. Many bright stars in our immediate neighborhood are binary or
multiple. For example, both Sirius and Procyon have tiny white dwarf companions
that were born as B stars and then rapidly bloomed and withered into their
present state. Capella, the third brightest star in northern skies, is actually
a quadruple system, consisting of a pair of tightly bound red giants accompanied
by a pair of red dwarfs at a separation of 10,000 astronomical units (AU). More
than a dozen exoplanetary host stars in our immediate neighborhood are also members
of binary or higher-order multiples.
Google couldn’t find me an authoritative estimate of the full stellar population within 20 parsecs, but I did locate some approximate counts of G-type stars in this space (about 130). A rough extrapolation from available sources (Turnbull 2013, RECONS 2012) would suggest a population of 1500-2000 stars inside our 20-parsec radius, with about 75% classed as M dwarfs. (If anyone reading this knows of an authoritative census, please let me know!) Within the full population are 73 host stars accompanied by a total of 129 planets, suggesting that only 3%-5% of stars in this volume can be described as “planetic.” Yet recent studies argue that virtually every star of spectral types M through G (and probably types F through A) harbors at least one planet (Winn & Fabrycky 2015). Evidently a vast population of planetary systems remains to be discovered right on our Galactic doorstep.
Figure 2 sorts the known exoplanetary systems within 20 parsecs according to their distance from our Sun, with each successive ring in the blue background representing an increment of 5 parsecs in the radius of the expanding sphere.
Figure 2. All host stars at 20 parsecs or less, arranged by distance from the Sun
Google couldn’t find me an authoritative estimate of the full stellar population within 20 parsecs, but I did locate some approximate counts of G-type stars in this space (about 130). A rough extrapolation from available sources (Turnbull 2013, RECONS 2012) would suggest a population of 1500-2000 stars inside our 20-parsec radius, with about 75% classed as M dwarfs. (If anyone reading this knows of an authoritative census, please let me know!) Within the full population are 73 host stars accompanied by a total of 129 planets, suggesting that only 3%-5% of stars in this volume can be described as “planetic.” Yet recent studies argue that virtually every star of spectral types M through G (and probably types F through A) harbors at least one planet (Winn & Fabrycky 2015). Evidently a vast population of planetary systems remains to be discovered right on our Galactic doorstep.
Figure 2 sorts the known exoplanetary systems within 20 parsecs according to their distance from our Sun, with each successive ring in the blue background representing an increment of 5 parsecs in the radius of the expanding sphere.
Figure 2. All host stars at 20 parsecs or less, arranged by distance from the Sun
73 exoplanetary host stars sorted by distance
from our Sun. Spectral types are indicated by the color key at lower right. The
inner circle has a radius of 5 parsecs, and each successive ring represents an
increment of 5 parsecs. Stellar icons are arranged in 2 dimensions by right
ascension, which is marked at the edge of the outer circle. (Declination is
necessarily ignored.) This diagram shows the relative distance of planetary
systems from our Sun, but not from each other.
-------------
Inspection of the diagram reveals that the frequency of
confirmed host stars per cubic parsec falls rapidly with increasing distance,
even within this limited space. So does the ratio of dim stars (spectral type
M) to brighter stars. G-type stars, on the other hand, appear to be over-represented.
They constitute less than 10% of field stars within 20 parsecs, but almost
one-third of the sample of host stars in this space.
About 57% of the planets in Figure 2 are low-mass objects in the same classification as Uranus and Earth (Table 1). These planets have masses smaller than about 0.15 Jupiter masses (0.15 Mjup), equivalent to 48 Earth masses (48 Mea). The remainder are gas giants like Jupiter and Saturn, although very few of them have orbital periods as long as our giants. A similar majority of nearby host stars (55%) harbors a single detected planet. Nonetheless, multiplanet systems are abundant enough that only one-third of all planets in this space occur in singleton systems.
Thus the “average” planet in our neighborhood is a low-mass object with at least one other companion orbiting the same star. Notably, this description applies to six out of eight planets in our Solar System.
Figure 3. Transiting low-mass planets within 20 parsecs
About 57% of the planets in Figure 2 are low-mass objects in the same classification as Uranus and Earth (Table 1). These planets have masses smaller than about 0.15 Jupiter masses (0.15 Mjup), equivalent to 48 Earth masses (48 Mea). The remainder are gas giants like Jupiter and Saturn, although very few of them have orbital periods as long as our giants. A similar majority of nearby host stars (55%) harbors a single detected planet. Nonetheless, multiplanet systems are abundant enough that only one-third of all planets in this space occur in singleton systems.
Thus the “average” planet in our neighborhood is a low-mass object with at least one other companion orbiting the same star. Notably, this description applies to six out of eight planets in our Solar System.
Figure 3. Transiting low-mass planets within 20 parsecs
All these planets have been observed in
transit from Earth’s orbit. A single transit has also been reported for HD
219134 b. The extrasolar planets in this image have orbital periods shorter
than 3 days; the period of 55 Cancri e is shorter than 18 hours.
----------------------
Our neighborhood also contains several distinctive architectural designs. The most common (11 systems; 15%) features at least two gas giants. More than half of this sample harbors three or more planets in total, and in 9 out of 11 systems, at least one planet orbits outside 2 AU. Again, this description fits our Solar System, but it also applies to quite different systems, including Upsilon Andromedae, 47 Ursae Majoris, and HD 128311.
The next most common architecture (12%) is the compact low-mass configuration, in which at least three low-mass planets orbit inside 1 AU and no gas giants are detected. This architecture is mutually exclusive with the previous type, and it is extremely common in the Kepler sample. Well-studied examples within 20 parsecs include GJ 581 and HD 69830.
The third most common architecture overlaps with the two-giant design. This is the mixed-mass type (10%), in which a minimum of three planets are present: at least one gas giant and at least one low-mass object. 55 Cancri, GJ 876, and Mu Arae are notable nearby examples containing at least two gas giants each.
In the full exoplanetary sample, the most abundant and easily detectable architecture is the Hot Jupiter configuration, defined as a star accompanied by a gas giant planet with a period of 10 days or less. However, this is only the fourth most common architecture in our immediate neighborhood. Just five (7%) of the systems detected to date in this space contain a Hot Jupiter. Three of them (HD 189733, Tau Boötis, and 51 Pegasi) are singletons. Each of the others(Upsilon Andromedae, HD 217107) harbors at least one additional gas giant on a wider orbit, so these two systems can also be assigned to the two-giant architecture.
Among singleton systems within 20 parsecs, giants have a slight edge, accounting for 58% of this subsample. Hot Jupiters are in a minority, however: the median semimajor axis for single giants in this volume of space is 1.3 AU, compared to 0.1 AU for low-mass singletons. Single giants also tend to orbit more massive stars: inside 20 parsecs, the median host star mass in such systems is 0.9 Msol (maximum 1.65 Msol), whereas the median for hosts of low-mass singletons is 0.49 Msol (maximum 0.97 Msol). These numbers suggest a certain antipathy between hot stars and low-mass planets, whether real or the result of observational bias. Low-mass planets might truly be scarce around hot stars, or they might simply be harder to detect in such environments.
One of the least common architectures revealed to date is the Solar System analog. I define this configuration as a Sun-like star (0.7-1.2 Msol) hosting a gas giant on a circular orbit wider than 3 AU, without any gas giants on interior orbits either occupying or perturbing the system habitable zone. This configuration is a subset of the mixed-mass type. Inside a radius of 20 parsecs, only one system besides our own features such an architecture: HD 154345. More than a dozen others have been reported outside this volume, but the most distant is only 57 parsecs away (see How Weird is Our Solar System? and Almost Jupiter). None are known within the Kepler sample.
Their absence is not surprising, since Kepler collected data for less than four years and observed only planetary transits. The cool gas giant in a Solar System analog will have an orbital period longer than five years (and probably at least twice as long) and it might not be co-planar with the inner planets. In such a configuration, an observer with a fixed viewing angle could never observe both the inner and outer planets in transit.
Table 1. Characteristics of three exoplanetary populations
Among objects listed in EPE in
early 2016, two substantially larger, non-overlapping samples are available to
compare with the discrete population in our immediate neighborhood: 1) 472 confirmed Kepler systems hosting
1038 planets, and 2) 606 confirmed
transiting and RV systems outside 20 parsecs hosting 707 planets, excluding
Kepler discoveries. Table 1 compares
both samples to the population of 73
host stars and 129 planets within 20 parsecs.
The descriptive statistics summarized in the table suggest at least six salient points:
1. Although all three groups are strongly biased against M dwarfs (defined as stars < 0.7 Msol) and in favor of hot stars (those ≥ 1.2 Msol), our neighborhood population appears to have the most representative selection of stellar hosts. For example, if we want a “typical” M-type sample, the 26 systems orbiting red dwarfs within 20 parsecs are a good place to start.
2. Only the Kepler sample has a minority of single-planet systems, alerting us that even the host stars in our local sample must have many additional planets. These objects have eluded detection so far because they are either very low in mass (and thus undetectable by ground-based RV monitoring) or traveling on long-period orbits (which require a substantial investment of resources to ensure regular observations over a period of decades).
3. The abundance of multiplanet systems in the Kepler sample (78%) is evidence of the opposite bias. It is much easier to confirm the detection of a transiting planet with one or more candidate companions than it is to rule out all sources of false positives for a single planet with similar transit data. Hence Kepler systems with a single transiting planet (“tranet”) are less likely to be confirmed than those with multiple tranets. Nevertheless, a system with a confirmed tranet might still harbor invisible companions if their orbits are widely spaced (Xie et al. 2014) or non-coplanar (Johansen et al. 2012).
4. The relative fractions of low-mass planets in these three samples indicate a more straightforward bias: the RV technique is simply less sensitive to small planets than transit monitoring is, and sensitivity declines rapidly outside 10 parsecs. That limitation probably indicates that hundreds of terrestrial planets are awaiting discovery within our immediate neighborhood.
5. The diversity of system architectures is greatest within 20 parsecs, as demonstrated by the low frequencies of two-giant, compact low-mass, and mixed-mass systems in the other two populations. This feature makes our neighborhood sample especially valuable for comparison studies.
6. Hot Jupiters are heavily represented in all three populations, with the most obvious excess in the RV sample outside 20 parsecs. Nevertheless, the Kepler fraction is quite similar to the fraction in the local population. If only about 1% of Sun-like stars harbor a Hot Jupiter, as recent studies conclude (Bayliss & Sackett 2011, Wright & al. 2012, Wang & al. 2015), both of these samples are unbalanced.
local bubble
If we expand our perspective by an order of magnitude to a radius of 200 parsecs, we see a more varied starscape (Figure 3). This larger volume of space encompasses most of the region known as the Local Bubble, an irregular, gas-free cavity surrounded by molecular hydrogen clouds that trace the Orion Spur of our local spiral arm. Whereas stars in the immediate vicinity of our Sun appear to be a random sample of the Galactic population, our enlarged perspective reveals localized structures.
The descriptive statistics summarized in the table suggest at least six salient points:
1. Although all three groups are strongly biased against M dwarfs (defined as stars < 0.7 Msol) and in favor of hot stars (those ≥ 1.2 Msol), our neighborhood population appears to have the most representative selection of stellar hosts. For example, if we want a “typical” M-type sample, the 26 systems orbiting red dwarfs within 20 parsecs are a good place to start.
2. Only the Kepler sample has a minority of single-planet systems, alerting us that even the host stars in our local sample must have many additional planets. These objects have eluded detection so far because they are either very low in mass (and thus undetectable by ground-based RV monitoring) or traveling on long-period orbits (which require a substantial investment of resources to ensure regular observations over a period of decades).
3. The abundance of multiplanet systems in the Kepler sample (78%) is evidence of the opposite bias. It is much easier to confirm the detection of a transiting planet with one or more candidate companions than it is to rule out all sources of false positives for a single planet with similar transit data. Hence Kepler systems with a single transiting planet (“tranet”) are less likely to be confirmed than those with multiple tranets. Nevertheless, a system with a confirmed tranet might still harbor invisible companions if their orbits are widely spaced (Xie et al. 2014) or non-coplanar (Johansen et al. 2012).
4. The relative fractions of low-mass planets in these three samples indicate a more straightforward bias: the RV technique is simply less sensitive to small planets than transit monitoring is, and sensitivity declines rapidly outside 10 parsecs. That limitation probably indicates that hundreds of terrestrial planets are awaiting discovery within our immediate neighborhood.
5. The diversity of system architectures is greatest within 20 parsecs, as demonstrated by the low frequencies of two-giant, compact low-mass, and mixed-mass systems in the other two populations. This feature makes our neighborhood sample especially valuable for comparison studies.
6. Hot Jupiters are heavily represented in all three populations, with the most obvious excess in the RV sample outside 20 parsecs. Nevertheless, the Kepler fraction is quite similar to the fraction in the local population. If only about 1% of Sun-like stars harbor a Hot Jupiter, as recent studies conclude (Bayliss & Sackett 2011, Wright & al. 2012, Wang & al. 2015), both of these samples are unbalanced.
local bubble
If we expand our perspective by an order of magnitude to a radius of 200 parsecs, we see a more varied starscape (Figure 3). This larger volume of space encompasses most of the region known as the Local Bubble, an irregular, gas-free cavity surrounded by molecular hydrogen clouds that trace the Orion Spur of our local spiral arm. Whereas stars in the immediate vicinity of our Sun appear to be a random sample of the Galactic population, our enlarged perspective reveals localized structures.
Figure
3. Selected star clusters and exoplanetary systems within 200 parsecs
Selected exoplanetary systems and star
clusters within 200 parsecs (652 light years) of our Sun. Each successive ring
in the colored circle represents an additional 50 parsecs. In this flattened,
two-dimensional schema, stellar icons are distributed by right ascension
according to their distance from our Sun, not by their distance from each
other. Right ascension is marked by the numbers ascending counterclockwise
around the outer ring.
-------------
Visible are 10 star clusters, some young and bright,
others older and dimmer. A star cluster is a compact association in which the
space density of stars is much higher than the surrounding environment, and
member stars share a similar motion through space. Each cluster population is
far from random, since cluster members were born together in the same molecular
cloud, and thus have very similar ages and chemical compositions. All stars of
the same mass in the same cluster are at the same evolutionary stage.
Star clusters can be described as either embedded or open, depending on the presence (embedded) or absence (open) of remnant hydrogen clouds. Several nearby clusters are still embedded: Rho Ophiuchi, Taurus-Auriga, Chamaeleon I, and Coronet. Their estimated ages range from 1 to 10 million years. The open clusters are substantially older, with the Pleiades about 125 million years old, Praesepe 600 million, and the Hyades 625 million. Notably, planets have been discovered in both Praesepe and the Hyades.
Apart from these astronomically interesting and aesthetically captivating structures, we can also see several field stars that harbor unusual planetary systems. Kepler-16, located only 65 parsecs (212 light years) away, was the first circumbinary system ever detected, and the nearest by a margin of several hundred parsecs. This system includes a K-dwarf and an M-dwarf sharing a binary orbit of 41 days, with an undersized gas giant (Kepler-16b) orbiting their common center of mass in a period of 229 days. Remarkably, even though gas giant environments are inimical to life as we know it, this giant happens to reside in the system’s habitable zone. While we can be pretty sure that circumbinary planets are rare, they might not be so rare that Kepler-16 is alone in our Local Bubble.
Venturing into deeper space, we find not one but two examples of another rare species: conjoined planetary systems, defined as stellar binaries in which each star hosts its own system of planets. XO-2NS comprises two G-type stars with a binary semimajor axis of 4600 AU. Star N hosts a Hot Jupiter; star S hosts two warm gas giants. WASP-94AB is a pair of F-type stars with a binary semimajor axis of 2700 AU. Both stars host Hot Jupiters, with no other planets in evidence.
Then we find the rarest of the rare, the only one of its kind: WASP-47, which includes a Hot Jupiter flanked by two low-mass planets inside 0.1 AU. No other Hot Jupiter has any nearby companions. WASP-47b also belongs to another small club: Hot Jupiters with outer giant companions, of which we identify two inside 20 parsecs, but few others in deeper space.
An unmistakable feature of Figure 3 is the Kepler treasure trail extending outward along right ascension 19 into infinity. Joining Kepler-16 in this procession of wonders is Kepler-444 and its Hot Martian quintuplets (five planets less massive than Earth orbiting a K-type star in periods shorter than 10 days) as well as two M dwarf systems (Kepler-186, 438) hosting small planets that have been widely promoted as potentially habitable Super Earths. Also represented is one of the few mixed-mass systems in the Kepler sample: Kepler-68, which hosts two small transiting planets inside 0.1 AU as well as a non-transiting gas giant detected by radial velocity measurements orbiting just outside 1 AU. All these Kepler systems enhance the diversity of the region between 50 and 200 parsecs, which otherwise presents a bland continuum of relatively bright stars hosting lonely giants.
Star clusters can be described as either embedded or open, depending on the presence (embedded) or absence (open) of remnant hydrogen clouds. Several nearby clusters are still embedded: Rho Ophiuchi, Taurus-Auriga, Chamaeleon I, and Coronet. Their estimated ages range from 1 to 10 million years. The open clusters are substantially older, with the Pleiades about 125 million years old, Praesepe 600 million, and the Hyades 625 million. Notably, planets have been discovered in both Praesepe and the Hyades.
Apart from these astronomically interesting and aesthetically captivating structures, we can also see several field stars that harbor unusual planetary systems. Kepler-16, located only 65 parsecs (212 light years) away, was the first circumbinary system ever detected, and the nearest by a margin of several hundred parsecs. This system includes a K-dwarf and an M-dwarf sharing a binary orbit of 41 days, with an undersized gas giant (Kepler-16b) orbiting their common center of mass in a period of 229 days. Remarkably, even though gas giant environments are inimical to life as we know it, this giant happens to reside in the system’s habitable zone. While we can be pretty sure that circumbinary planets are rare, they might not be so rare that Kepler-16 is alone in our Local Bubble.
Venturing into deeper space, we find not one but two examples of another rare species: conjoined planetary systems, defined as stellar binaries in which each star hosts its own system of planets. XO-2NS comprises two G-type stars with a binary semimajor axis of 4600 AU. Star N hosts a Hot Jupiter; star S hosts two warm gas giants. WASP-94AB is a pair of F-type stars with a binary semimajor axis of 2700 AU. Both stars host Hot Jupiters, with no other planets in evidence.
Then we find the rarest of the rare, the only one of its kind: WASP-47, which includes a Hot Jupiter flanked by two low-mass planets inside 0.1 AU. No other Hot Jupiter has any nearby companions. WASP-47b also belongs to another small club: Hot Jupiters with outer giant companions, of which we identify two inside 20 parsecs, but few others in deeper space.
An unmistakable feature of Figure 3 is the Kepler treasure trail extending outward along right ascension 19 into infinity. Joining Kepler-16 in this procession of wonders is Kepler-444 and its Hot Martian quintuplets (five planets less massive than Earth orbiting a K-type star in periods shorter than 10 days) as well as two M dwarf systems (Kepler-186, 438) hosting small planets that have been widely promoted as potentially habitable Super Earths. Also represented is one of the few mixed-mass systems in the Kepler sample: Kepler-68, which hosts two small transiting planets inside 0.1 AU as well as a non-transiting gas giant detected by radial velocity measurements orbiting just outside 1 AU. All these Kepler systems enhance the diversity of the region between 50 and 200 parsecs, which otherwise presents a bland continuum of relatively bright stars hosting lonely giants.
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