Figure 1. Just as
the exoplanet census passed 900, a new
study reported that HD
189733 b, the nearest transiting Hot Jupiter, is a deep blue world where
molten glass rains through seething hydrogen.
-----------------------------
On Monday, July 1, the census
ticker at the venerable Extrasolar Planets
Encyclopaedia (EPE) turned over a new
planetary century: 900 confirmed exoplanets, including the results of all
current detection methods. Since then the count has moved on, hovering at 919
planets today. All these worlds are the harvest of 25 years of searching.
What planetary flavors does this
numerical abundance offer? How much variety do we find in the Milky Way? How
many Earth-like planets are out there?
To answer these questions, we need
to take into account the fact that different search methods find different
kinds of planets. The strengths and limitations of each method introduce enormous
bias into the exoplanetary census.
search
methods & planet populations
Most (58%) of the known exoplanets
have been discovered by the radial
velocity method. It returns data on the masses and orbital periods of specific
kinds of planets: gas giants like Jupiter orbiting within about 5 AU of their
host stars, and low-mass planets (Super Earth to Neptune-size) orbiting within
about 1 or 2 AU. Under optimal conditions, it can also detect Earth-mass
planets orbiting within a few tenths of an AU. This method is sensitive to multiplanet
systems of many kinds, as well as to single planets. However, it isn’t very
good at finding systems
like our own or planets like
Earth.
The next largest share of
exoplanets (34%) have been observed in transit
across the face of their host stars, so that we can calculate their radii
and orbital periods, but not their masses. The transit method is sensitive only
to planets that happen to orbit their stars precisely along the line of sight
from Earth. Its effectiveness in ground-based surveys is limited, since 97% of
transiting objects detected from the ground are Hot Jupiters (i.e., gas giants with
periods shorter than 10 days – a monotonous selection indeed). Fortunately,
detections by space-based programs now outnumber those from the ground, and the
vast majority of those detections represent planets of Neptune size or less. Space-based
observations have also been remarkably successful in finding low-mass multiplanet
systems similar to our radial velocity neighbors, HD 40307 and HD 69830. Until other search
methods improve, they remain our only hope of
detecting Earth-size planets with habitable temperatures. Meanwhile, all the planets on which we have enough data to establish that they are rocky (like Earth) and smaller than two Earth radii (2 Rea) are Hellworlds, orbiting their stars in periods of a few days or less.
Figure 2. Small Planets with Measured Masses & Radii
The major drawback of transit data
is the absence of constraints on an object’s mass, except in rare cases where a
planet’s orbital period is modified by the influence of an adjacent transiting planet
(a phenomenon known as transit timing variations or TTV). The major strength of
transit data is that, when they can be supplemented by radial velocity data,
the resulting synergy provides our clearest understanding of exoplanetary
environments. Figure 2 shows artist’s
views of half a dozen small, well-constrained planets (“Super Earths” and “Mini
Neptunes”) with masses (M) and radii (R) expressed in Earth units. The three
Kepler planets are located at distances of 173 to 613 parsecs (565-2000 light
years away); the other three are next-door neighbors of our Sun, located within
22 parsecs (72 light years). Similar line-ups of low-mass planets are available
here
and here.
All five methods have been in use
for more than a decade, each contributing its own piece of the picture. In the
past few years, a new approach to detection – reanalysis of existing radial velocity and transit data, using Bayesian inference – has also proved
fruitful. In fact, the exoplanetary census just surged to 900 and beyond partly
as a result of two brand-new reanalyses of radial velocity data, discussed here
and here.
two kinds
of planets
Figure 3. Radii of 298 Transiting
Planets < 18 Earth Radii: July 2013
Figure 3 illustrates
the double-peaked distribution of exoplanet radii. The letters E, U,
S, and J indicate where the radii of Earth,
Uranus, Saturn, and Jupiter fall
on the same scale. Recent analyses argue that the population peak between 2 and
3 Rea is real, and not an artifact of detection bias (Petigura et al. 2013). In other words, the population of low-mass planets does
not continue to increase at radii smaller than 2 Rea. At best, the actual number
of Earth-size planets – admittedly harder to detect than larger planets – may
rival that of their siblings between 2 and 3 Rea, but it probably does not exceed
that population. Some well-known examples of the latter are illustrated in Figure 2.
Current radial velocity data
indicate that the median mass of all extrasolar gas giants is 1.5 Jupiter
masses (Mjup). This figure has remained fairly consistent over the past five
years, even as the number of exoplanets has tripled. Ninety percent of gas
giants have minimum masses of 7 Mjup or less. Unlike the case in lightweight
planets, we see little correspondence between mass and radius for the more
massive gas giants observed in transit. Instead, temperature plays the defining
role. With a single exception, the objects in Figure 3 with radii above 12 Rea are Hot Jupiters, which have very
high equilibrium temperatures and usually inflated atmospheres (see Figure 1 for an example).
The fraction of Hot Jupiters is another
relatively consistent feature of the overall sample. They constitute 35% of all
known gas giants and 25% of all well-constrained exoplanets of any kind. However, several studies have demonstrated that the true occurrence rate for these planets is much lower: Hot Jupiters orbit far fewer than 1% of the stars in the Milky Way.
More than half (53%) of all
exoplanets with well-defined orbits have semimajor axes smaller than Mercury’s
(0.49 AU), while only 23% have semimajor axes larger than that of Mars (1.52.
AU). All the cooler planets are gas giants, and rather massive ones at that. Just
four objects on cool orbits fall in the low-mass tail of the gas giant range,
with minimum masses between 45 and 60 Mea. The median mass for all cool giants
(i.e., trans-Martians) is 2 Mjup, illustrating the tendency of more massive planets
to travel on wider orbits.
Among low-mass planets, hot orbits
predominate. Although the range of orbital periods for this group extends from
8.5 hours to 600 days, almost 40% have periods shorter than 10 days, and only 5%
have periods longer than 225 days (the period of Venus around our Sun). However, this distribution is certainly the result of detection bias; a hidden population of small planets on longer-period orbits still awaits discovery. Among
low-mass planets for which masses can be estimated, the median is 10 Mea,
corresponding to an object more reminiscent of Uranus than of Venus. Again, this figure might be the result of detection biases instead of a reflection of reality.
three
kinds of systems
One welcome trend is the
increasing share of multiplanet systems within the full census of
well-constrained exoplanets. Currently, just 60% of planets occur in systems
with only one planet, and within this subpopulation, 92% are gas giants (among
which almost half are Hot Jupiters). Another 23% of the full census are found
in systems with two planets. Like the singleton systems, these are dominated by
gas giants (60% of all planets in two-planet configurations). However, only
5% of planets in these systems are Hot Jupiters. The two-planet sample provides an interesting illustration of the
adage that birds of a feather flock together, since 84% of systems consist of either
of two gas giants or two low-mass planets, while only 16% have one of each.
Even among the odd couples, the vast majority of giant companions are less
massive than 0.5 Mjup.
Finally, 17% of the total appear
in systems with three or more planets; 73% of planets in these systems are
low-mass, and as in the two-planet configurations, only 5% of systems contain
Hot Jupiters.
This breakdown of systems by planeticity (an indispensable coinage
that hasn’t yet found its way into standard dictionaries) tells us something significant
about the character of the two exoplanet species. The little folk are
gregarious and abundant, while the heavyweights are hostile and aloof. The
former play well with others, while the latter prefer solitude, or at most the
company of a few representatives of their own kind. When the two species manage
to co-exist in the same system, something special sometimes happens, such as life.
The study of highly planetic
systems – i.e., those with three or more planets – opens a unique window on
extrasolar system architectures. In a previous post I proposed three
basic architectural types, defined by their planet populations: low-mass,
mixed-mass, and high-mass. I continue to find these categories useful for
conceptualizing the exoplanet census.
Low-mass systems
contain closely-spaced planets of similar size traveling on circular orbits
near their host stars. They are often extremely compact. For example, Kepler-33
has five planets (three of them larger than Neptune) within a semimajor axis of
0.26 AU, while Kepler-11 has six planets within a semimajor axis of 0.5 AU.
Mixed-mass systems,
such as the Solar System, 55 Cancri, and HD 10180, contain a broader range of
planet masses, with at least one, sometimes two, and rarely three gas giants. Orbits
in these systems tend to be more expansive than those of low-mass systems, while
their giants are typically much less massive than the median for the full
sample. None have Hot Jupiters. Overall, the mixed-mass systems look more like low-mass systems than like high-mass systems
High-mass systems containing
three or more gas giants, but no low-mass planets, are quite rare. (Nevertheless,
one of the best-studied systems detected by direct imaging, HR 8799, seems to host at least
four giants on wide orbits). The components of these systems tend to be more
massive than the median for all gas giants, and their orbits are wider and more
eccentric than those in the other two architectural types. Half have Hot
Jupiters.
In general, gas giants are
intolerant of other gas giants, especially in close proximity. Among all extrasolar
systems detected to date, only six (less than 1%) contain three or more giants.
Two are mixed-mass systems; the rest constitute the high-mass sample shown in
the table below. The more massive the giant, the less likely it is have neighbors, in particular diminutive neighbors.
Table 1. 38 systems with three or more
planets, binned by architectural category
Early in 2012, after a few months
of unusually rapid growth in the census at EPE, I made a
prediction about where the numbers were headed: “At this rate, we can
expect the census to reach the magic number of 1000 before the end of 2013.”
Given the much slower growth over the past year, my prediction may be on shaky
ground. It will be interesting to see how things pan out.
REFERENCES
Bennett
DP. (2008) Detection of extrasolar planets by gravitational microlensing.
In Exoplanets, ed. John Mason. Berlin: Springer, 2008. Abstract.
Mann AW, Gaidos E, Gaudi BS. The invisible majority? Evolution and detection of outer planetary systems without gas giants. (2010) Astrophysical Journal 719, 1454–1469. Abstract: http://adsabs.harvard.edu/abs/2010ApJ...719.1454M
Petigura EA, Geoffrey MW, Howard AW. (2013) A plateau in the planet population below twice the size of Earth. In press.
Mann AW, Gaidos E, Gaudi BS. The invisible majority? Evolution and detection of outer planetary systems without gas giants. (2010) Astrophysical Journal 719, 1454–1469. Abstract: http://adsabs.harvard.edu/abs/2010ApJ...719.1454M
Petigura EA, Geoffrey MW, Howard AW. (2013) A plateau in the planet population below twice the size of Earth. In press.
Nice article. It will be interesting to see how this holds up with better / different instruments.
ReplyDeleteIn particular: as we do surveys for nearby transiting systems where we can do follow up RV work; and as we get data without various biases. For example:
Against "cool Jupiters" for both RV and transits. If we can get a large sample across the metallicity range e.g. is there a "tail" to the distribution at low metallicities?
Against detecting multiplanet transiting systems - we only see transits for planets with orbits that are tightly coplanar.
Towards transiting low mass (compact) systems around smaller stars (relative transit depth, transit frequency and looser constraints on coplanarity)
Against RV & transiting bias smaller, higher period planets
Extremely interesting times!!!
"Extremely interesting times!!!"
DeleteI couldn't agree more. (And thanks for reading!) I'm very curious to see if some of the really compact systems like Kepler-11 have gas giants in their outer regions. My hunch is that they don't.
Mass in a planetary system seems to be concentrated either in a few gas giants, which tend to stay on cooler orbits, or a lot of Neptune-like planets, which tend to stay on warmer orbits. But a few new discoveries could change that picture.