Figure 1. Candidate
and confirmed planets in the final Kepler dataset. This image is based on
Figure 7 in Morton et al. (2016), with the addition of the red box outlining
the parameter space occupied by Earth-like planets (radii 0.7-1.4 Earth units
or Rea) orbiting in the habitable zones (periods 100-800 days) of Sun-like
stars (masses 0.7-1.2 Solar units or Msol).
------------------------------
Two weeks
ago, with considerable fanfare, the Kepler team announced the confirmation of
1,284 new transiting planets (Overbye 2016, Kluger 2016). This remarkable feat
was accomplished through an automated analysis of the complete Kepler dataset
(Morton et al. 2016). Following an established tradition, the
announcement highlighted a selection of small planets that some investigators
might consider somewhat Earth-like. This time around, however, the significance
of the selection was downplayed, since the nine objects singled out for
presentation were described only as candidates that “may fall within
the optimistic habitable zones of their host stars” (italics
in original). As we’ll see below, that claim is a surprisingly watered-down response
to the Kepler Mission’s original goal, which was to characterize Earth-size
planets on habitable orbits around Sun-like stars.
All
1,284 new planets were immediately incorporated into the exoplanetary census
maintained by the Extrasolar Planets Encyclopaedia (EPE).
Thus, as of today, the count for all exoplanets detected by all search methods stood
at 3,411. In combination with 19 other transiting planets reported earlier this
year, the new data release has doubled the transit population in a span of only
four months. The grand total is now 2600 transiting planets, representing 76%
of all exoplanets detected by any method.
lonely little planets
Unlike
previous releases of Kepler data, this one is dominated by single-planet
systems. Systems of high multiplicity, defined as those with at least three
planets, are quite scarce. Among 1,284 new detections, 110 occur in 55 newly
announced systems with 2 planets each; 15 occur in 5 new systems with 3 planets
each; and 12 occur in 3 new systems with 4 planets each. In addition, one
planet (Kepler-436c) was found in a system where a single planet was already
known, raising the multiplicity of this system to 2; 14 were found in systems
where 2 planets were known, raising the multiplicity of these 14 systems to 3;
and 9 planets were found in 7 systems where 2 or 3 planets were known, raising
the multiplicity of these 7 systems to 4. No new systems with five or more
planets were identified.
Given such
a modest increase in the population of multiple systems, 87% of the new planets
orbit their stars alone, without any detectable companions. Among all Kepler
planets confirmed since the spacecraft launched (including objects designated EPIC,
KIC, KOI, and K2), singleton planets now comprise 53% of the total. Among all
Kepler planetary systems, 75% contain only one planet, 16% contain two planets,
and 9% contain three or more planets. The latter group represents the
high-multiplicity subsample; only 16 systems in that group contain more than
four detected planets.
One caveat
is in order for these statistics: several Kepler multiplanet systems contain planets
detected by radial velocity measurements or transit timing variations but not
observed in transit. Therefore, counts and percentages will vary depending on
the sample queried. In addition, most Kepler systems of any multiplicity are
likely to contain additional planets that are undetectable by existing search
methods. Thus the terms “single-planet,” “two-planet,” and so on represent our
state of knowledge about a system rather than the true number of planets it
contains.
Caveats
aside, these new numbers signal a dramatic shift in our perspective on the
demographics of transiting systems. The Kepler sample available as recently as March was biased against
singletons, an inescapable product of extant validation methods that relied on
evidence of planetary multiplicity to rule out false positives. By correcting
that bias, the automated approach to validation taken by Morton and colleagues
(2016) has revealed that singletons are just as common in Kepler systems as
they are in the Sun’s back yard.
Table 1. Characteristics
of three exoplanetary populations
Table 1 updates the summary of exoplanetary
demographics originally posted in March. The frequency of selected planetary
and system characteristics is compared across three samples: 1) 129 planets in
73 exoplanetary systems located at a distance of 20 parsecs or less, 2) 707
planets in 606 systems detected by transit or radial velocity searches outside
20 parsecs, excluding Kepler discoveries, and 3) the full Kepler sample of 2342
confirmed planets in 1677 systems, as characterized in a recent query of EPE.
One
demographic feature hasn’t changed: the full Kepler sample is still dominated
by small planets. The median radius is 2.15 Earth radii (Rea), and 95% are
smaller than 8 Rea, which is the practical cut-off between low-mass objects
like Earth and Uranus and gas giants like Jupiter and Saturn. Fully 25% of
Kepler planets are smaller than 1.5 Rea, another practical cut-off that
approximates the boundary between terrestrial planets and gas dwarfs (that is, planets
with hydrogen/helium envelopes accounting for less than half their mass).
Figure 2. Transiting planets by
radius (N = 2570)
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medium coolThat’s right: one in four Kepler planets is potentially rocky, like Earth or Venus. Such odds sound intoxicating until we tune into the orbital characteristics of these small, dense objects (see Figures 1 and 2).
The vast majority of planets of 1.5 Rea or less orbit Sun-like stars. Yet their median semimajor axis is only 0.06 AU, which implies equilibrium temperatures that range from “infernal” to “Tartarean.” The sunny side of rocky worlds on such tight orbits will likely be molten. More than 70% of Kepler planets of 1.5 Rea or less have periods shorter than 10 days, qualifying them as potential Hellworlds. Only six have periods longer than 100 days. Among the cool six, just one – our old friend Kepler-62f – occupies the habitable zone of a Sun-like star.
Given this background, I’m puzzled by the decision of Morton & colleagues (hereafter M16) to showcase nine mostly puffy planets in their Table 3. These objects are described rather oddly as “newly validated planets in the optimistic habitable zone.”
Now, it’s a truism of astrobiology that an orbit in the habitable zone is no guarantee of habitability. That’s because a planet must have an appropriate mass and composition, in addition to an appropriate level of stellar flux, in order to maintain surface bodies of water. Only one of the nine planets presented by M16 is smaller than 1.5 Rea: Kepler-1229b, which has an estimated radius of 1.12 Rea. This value is consistent with a rocky planet of 1.5 Mea, but unfortunately, the planet’s host star is an M dwarf of only 0.43 Msol.
Several characteristics of young M dwarfs (stellar masses 0.12-0.65 times Solar or Msol) are antagonistic to the formation of habitable planets. Their luminosities are one to two orders of magnitude higher than mature stars of the same mass; they emit high levels of extreme ultraviolet radiation; and they are subject to frequent flaring events (Ramirez & Kaltenegger 2014, Luger & Barnes 2015). These factors ensure that volatiles will be stripped from any small, low-mass planets that happen to form in the mature habitable zone of an M dwarf. The likely outcome of such a “boil-off” will be desiccated rocks like Venus, unfriendly to the emergence of life. It’s no coincidence that Kepler’s mission was to seek Earth-size planets around Sun-like stars, not around M dwarfs.
Nevertheless, certain contingencies – possibly quite rare – might still permit the formation of habitable planets around red stars. Imagine a puffy rock/ice planet of a few Earth masses in the mature habitable zone of a young M dwarf. This world happens to support just enough hydrogen and water to boil down to the level of an Earth ocean during the first billion years of system evolution. Thus unveiled, the cooling rocky core could outgas a new atmosphere and rain down shallow seas where life could begin.
Absent such a history, Kepler-1229b does not meet the criteria for a potentially habitable planet. The same verdict applies to five other objects selected by M16, which also orbit host stars with masses ranging from 0.30 Msol to 0.55 Msol. Unlike Kepler-1229b, these five M dwarf planets are quite bulky, with radii ranging from 1.56 Rea to 1.97 Rea. Such values imply either blasted monoliths of 6 to 10 Earth masses (Mea) or less massive worlds originally so rich in volatiles that even the tantrums of a baby red star were insufficient to deplete them.
Only three of the nine planets in the current selection orbit Sun-like stars. Unfortunately, these three have estimated radii in the range of 1.70 Rea to 1.98 Rea. Theory indicates that such objects, if composed entirely of heavy elements, will be too massive for plate tectonics, and thus not habitable (Unterborn et al. 2016). Both theory and observation suggest that objects of this size are likely to include a substantial volatile component – either water or hydrogen or both – around rocky cores (Rogers 2015). Such a composition would lower their mass, but it would also render them uninhabitable.
Yet M16 embellished their text with happy talk about “optimistic habitable zones.” This approach evidently convinced the editors of TIME Magazine that astronomers are closing in on Earth 2, even though not much has changed in years. Jeffrey Kluger, the author responsible for TIME’s coverage of the new data release, described the nine objects selected by Morton’s team as “earth-like planets,” and his headline proclaimed that their detection “Boosts Odds of Life in Space.” Sadly, they’re not, and it doesn’t.
the drought continues
Three years ago, Kepler-62f enjoyed a moment of celebrity as the Holy Grail of exoplanetary astronomy. That distant world (368 parsecs/1200 light years away) remains the best candidate for an Earth-like planet orbiting an alien sun. Yet Kepler-62f is clearly a borderline case, since the planet’s estimated radius of 1.41 Rea falls near the outer limit for a rocky composition, while the estimate itself depends on limited data.
I’m still waiting for a thorough assessment that would explain why Kepler has been unable to detect a significant population of small rocky planets in the habitable zones of Sun-like stars. Maybe those planets are really out there, but transit photometry isn’t well suited to detecting them. Or maybe transit photometry is perfectly fine, but the Kepler Mission ended too soon to disentangle the faint transit signals predicted for such objects from other stellar activity. Maybe if the Kepler spacecraft had kept functioning for three more years, it could have returned enough data to identify dozens of Earth-like planets.
But maybe such planets are so rare that the one we happen to be standing on has very few siblings in the entire Milky Way Galaxy. Maybe all the others are so far away that twenty-first century technologies can never find them. I don’t like that possibility, but if it’s the best explanation for Kepler’s disappointing results, I’d rather know than remain ignorant.
Luger R, Barnes R. (2015) Extreme water loss and abiotic O2 buildup on planets throughout the habitable zones of M dwarfs. Astrobiology 15, 119-143.
Morton TD, Bryson ST, Coughlin JL, Rowe JF, Ravichandran G, Petigura EA, Haas MR, Batalha NM. (2016) False positive probabilities for all Kepler objects of interest: 1284 newly validated planets and 428 likely false positives. Astrophysical Journal 822, 86.
Overbye D. Kepler Finds 1284 New Planets. New York Times, May 10, 2016.
Rogers L. (2015) Most 1.6 Earth-radius planets are not rocky. Astrophysical Journal 801, 41.
Ramirez RM, Kaltenegger L. (2014) The habitable zones of pre-main sequence stars. Astrophysical Journal Letters 797, L25.
Unterborn CT, Dismukes EE, Panero WR. (2016) Scaling the Earth: A sensitivity analysis of terrestrial exoplanetary interior models. Astrophysical Journal 819, 32.
