HIP 41378: A Compact Planet Sampler

Figure 1. Candidate planets of HIP 41378 represented at their relative sizes (radii from Vanderburg et al. 2016). Planets b and c are the only objects with formal validation. The orbital periods of the three outer candidates remain uncertain, and the discovery team provided no data on semimajor axes. Nor could they establish whether the period of candidate e is longer or shorter than that of candidate d.

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The ongoing K2 Mission can’t compare with the original Kepler Mission in terms of the sheer number of planet candidates identified. Nevertheless, some K2 discoveries rival the astrophysical interest of Kepler’s crown jewels. Last year’s game-changing results from WASP-47 have been discussed a few times in this blog. A few weeks ago the world received advance notice of another amazing K2 find: a rich, high-multiplicity, mixed-mass system of five planets transiting HIP 41378, described in a recent preprint by Andrew Vanderburg & colleagues (Figure 1).

As the discovery team notes at the outset, “K2 is not as sensitive to planetary systems with complex architectures as the original Kepler mission” (Vanderburg et al. 2016). That’s because K2 can observe a given star for a maximum of approximately 80 days, instead of the three-plus years of continuous staring enabled by Kepler. But in the case of HIP 41378, a happy coincidence enabled the detection of five different companions within the available period of data collection (Figure 2). Two of them have repeating transits and the other three have a single transit each.

Figure 2. Light curves for five transiting planet candidates around HIP 41378

Phase-folded light curves for each of the candidate planets in the HIP 41378 system. Adapted from Figure 2 in Vanderburg & al. 2016.

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A few definitions are in order. “High-multiplicity” is now in general use as a descriptor for systems with three or more planets. “Mixed-mass” is (I believe) my own coinage, referring to systems with at least one low-mass planet (an object less massive than about 50 Earth masses [50 Mea] with a bulk composition dominated by heavy elements) and one gas giant planet (an object of 50 Mea or more with a bulk composition dominated by hydrogen/helium). “Rich mixed-mass” is my own concept, referring to mixed-mass systems containing a minimum of two low-mass planets. (According to my terminology, then, all “rich mixed-mass” systems are automatically “high-multiplicity.”)

 

Note that all three of these descriptors apply to our Solar System. To date, however, fewer than 20 exoplanetary systems – among the many thousands now known – can be described as “rich mixed-mass.” HIP 41378 adds to their number.

 

Among systems published to date, the one that most closely resembles HIP 41378 is our friend Kepler-90, which supports an astonishing ensemble of seven planets ranging in radius from 1.2 Rea (Earth-like) to 11.3 Rea (Jupiter-like) on orbits shorter than a single Earth year.

 

Before cueing the Hallelujah Chorus, however, we should be aware of the limits of the K2 data. As the investigators concede, only two of the candidate planets around HIP 41378 have been formally validated by analysis of at least two transits. These two are HIP 41378 b and c, with respective orbital periods of 15.6 days and 31.7 days. For the other three candidates, Vanderburg & colleagues used the duration of each recorded transit to distinguish among them. They applied the letters d through f to designate objects with increasingly longer transit times and (potentially) longer orbital periods.

 

The investigators then calculated the probability that one or more of the candidates with a single transit were false positive detections. Using the K2 dataset, they found that a system with five transiting planets was 100 million times more likely than a system with two planets and three false positives. But they also found that the five-planet model was only 200 times more likely than a model with four planets and one false positive. Thus, the five-planet interpretation is probable but hardly certain.

 

The SIMBAD database assigns a spectral type of G1 IV to the host star, HIP 41378. The investigators prefer to describe it as a “slightly evolved late F-type star.” Its mass is 1.15 times Solar (1.15 Msol),  similar to Kepler-90 at 1.13 Msol. Its metallicity (endowment of elements heavier than hydrogen) is -0.11, meaning that the star is less enriched in metals than our Sun. Its distance is estimated at 116 parsecs, much nearer than most systems in the Kepler sample. Nevertheless, HIP 41378 still lies outside the region where radial velocity searches have typically been able to identify low-mass planets in the Super Earth range.

 

Vanderburg & colleagues argue that candidates HIP 41378 d through f have likely orbital periods exceeding 100 days. Their best guess for each period appears in Table 1. For simplicity, both the table and Figure 1 depict the five planets in alphabetical order, since the confidence intervals for the period of planet d overlap with those for planet e. Luckily, the estimates for planetary radii are more firmly grounded in transit data.

 

Table 1. HIP 41378 system parameters

Tags: Radius = planet radius in Earth units (Rea); Preferred Period = likely orbital period in days; Period Range = likely range in days of orbital period.

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According to comparisons with transiting planets that have well-constrained mass estimates, planets b and c have likely masses in the range of 2 to 10 Mea; planet d is in the range of 5 to 26 Mea; planet e is a tween in the range of 15 to 40 Mea; and planet f is a gas giant in the range of 63 Mea (the mass of HAT-P-18b) to 318 Mea (the mass of Jupiter). Given their ample radii, even the smallest of these planets must have hydrogen/helium envelopes.

 

Despite the absence of purely rocky planets, HIP 41378 counts as a rich mixed-mass system. Its assortment of radii likely corresponds to an assortment of planetary species and bulk compositions. Like Kepler-90, it resembles a compact planet sampler (Figure 3), offering a planet population almost as diverse as our own Solar System within a space comparable to the orbital radius of the Earth.

 

Vanderburg & colleagues note that the system’s host star is bright enough for high-precision radial velocity measurements. Such data could confirm the reality of the transiting candidates and constrain the mass and orbital period of HIP 41378 f, and potentially of the smaller planets as well. The information could also help us understand the formation process of systems like HIP 41378 and our own Solar System.

Figure 3. A chocolate candy sampler

 

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REFERENCE

Andrew Vanderburg, Juliette C. Becker, Martti H. Kristiansen, Allyson Bieryla, Dmitry A. Duev, Rebecca Jensen-Clem, Timothy D. Morton, David W. Latham, Fred C. Adams, Christoph Baranec, Perry Berlind, Michael L. Calkins, Gilbert A. Esquerdo, Shrinivas Kulkarni, Nicholas M. Law, Reed Riddle, Maissa Salama, Allan R. Schmitt. Five Planets Transiting a Ninth Magnitude Star. Astrophysical Journal Letters, in press. Abstract: 2016arXiv160608441V

 

K2 and the Tweens

Figure 1. Selected low-mass planets with measured masses that have radii between those of Uranus and Saturn.

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The Kepler space telescope was launched in 2009 on an ambitious mission to find Earth-like planets transiting stars in deep space. Initial results started arriving in 2010, and by 2011 it was clear that Kepler had revolutionized our understanding of exoplanets. In 2013, Kepler scientists began publicizing their best candidates for potentially Earth-like planets. Then, just one month after the first such announcement, the spacecraft’s pointing system malfunctioned, ending the original mission.

 

Unlike the Hubble Space Telescope, which is an artificial satellite orbiting the Earth at a low altitude, the Kepler spacecraft follows an independent orbit around the Sun. Thus no rescue operation was possible. Nonetheless, the astronomical community responded to Kepler’s mishap by applying the same ingenuity that they devoted to the initial problem of identifying Earth-like planets around distant stars.

 

Kepler originally relied on four reaction wheels to aim its grid of light sensors with the exquisite precision needed for continuous observation of the many thousands of stars selected for study. Although one reaction wheel broke down early in the mission, the spacecraft was designed to function with three, so the search for transiting planets continued. It was the breakdown of the second reaction wheel that terminated the telescope’s capacity for precision pointing.

 

Astronomers determined that Kepler could still observe stars whose sky-projected locations coincided with the ecliptic – that is, the plane of the Earth’s orbit around the Sun. Gravity combined with the pressure of Solar radiation could keep the sensors pointed for observing runs lasting a maximum of about 80 days. At the end of each run, the spacecraft’s thrusters could be fired to reposition the field of view (Howell & al. 2014).

 

This plan was nicknamed “K2” or “Second Light” (Figure 2). It went into effect early in 2014, and the Kepler telescope has continued to find new planets ever since. Given finite fuel supplies and the need to fire thrusters periodically, K2 has a limited lifetime.

 

Figure 2. Overview of the K2 Mission

 

As of early July, K2 has unveiled 44 new planets, including four new Hot Jupiters and a few dozen low-mass planets. Some are members of multiplanet systems. K2 has also contributed new data on a handful of previously discovered Hot Jupiters. Many results have been spectacular, such as the discovery of WASP-47d and e, two low-mass planets in a previously known Hot Jupiter system; K2-25b, the first small planet ever identified in a star cluster (the Hyades, our next-door neighbor in the Orion Spur); K2-33b, a puffy newborn planet orbiting an M dwarf in the nearby Upper Scorpius star-forming region; and HIP 41378, a compact mixed-mass system orbiting a bright F star.

 

One bonus associated with K2 is that the stars it observes are generally closer than those targeted by Kepler. Their proximity makes it easier to collect follow-up data with other instruments. In the most recent release of Kepler findings, 76% of Kepler host stars have distance estimates, with a median of 772 parsecs. A similar percentage of K2 host stars (71%) also have distance estimates, with a much smaller median of 183 parsecs. (All population-level data on K2 planets were retrieved from the Table of Confirmed K2 planets at http://kepler.nasa.gov/Mission/discoveries/.)

 

Unfortunately, K2 doesn’t seem very sensitive to rocky planets like Earth, defined as those with radii of 1.5 Earth units (1.5 Rea) or less. Whereas 25% of planets in the full Kepler sample have radii in this range, only one K2 planet does (K2-19d). Similarly, the median radius among Kepler planets is 2.15 Rea, notably smaller than the median of 2.59 Rea for the K2 sample. (Check out the postscript at the end for a more recent perspective.)

 

The silver lining in this cloud is the trove of K2 tweens, which I define as planets with radii between those of Uranus (4 Rea) and Saturn (9.4 Rea). Objects of this size are unknown in our Solar System and relatively rare in the Kepler sample (7% of the total). To date, however, K2 has returned 10 such exoplanets, representing almost 25% of the total. Seven out of 10 have mass measurements, obtained by analysis of transit timing variations (TTVs) or radial velocity (RV) observations or both. These discoveries make a valuable contribution to the field of planetology, since the blurry boundary between the two major planetary species – low-mass planets and gas giants – falls somewhere between Uranus and Saturn.

 

Although everyone agrees that Jupiter and Saturn are gas giant planets, the exact definition varies somewhat from source to source. Here’s the one I find most appropriate: A gas giant planet is an object whose bulk composition is more than 50% hydrogen and helium. The range of masses among extrasolar giants is approximately 60 to 4000 Mea, corresponding to about 0.20 to 13 Jupiter masses (Mjup). The median mass is about 1.8 Mjup. However, the precise upper and lower mass boundaries of the mass distribution are uncertain. Some authors have placed the lower boundary as low as 30 Mea (0.10 Mjup), while others have placed the upper boundary as high as 25 Mjup.

 

Figure 1 shows three well-characterized K2 tweens alongside four better-characterized planets in our Solar System and its immediate neighborhood. All the objects pictured have hydrogen/helium atmospheres, but the proportion of heavy elements (rock, metal, and ice) varies substantially.

 

Table 1. Twenty Transiting Tweens with Measured Masses

*Several conflicting masses have been reported for K2-19b and c. Mass estimates for the K2 planets are generally less secure than those available for the other exoplanets of this selection.

Tags: Rea = planet radius in Earth units; Mea = planet mass in Earth units; H/He = planet mass fraction attributed to hydrogen/helium; Period = planet orbital period in days; a = planet semimajor axis in Earth units; Msol = star mass in Solar units; Dist. = system distance in parsecs (1 parsec = 3.26 light years)

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Table 1 offers a larger and more representative sample of tweens selected from various populations: the Kepler catalog, which encompasses a prominent subset of circumbinary tweens; ground-based transit searches, sometimes supplemented by space-based observations; the CoRoT catalog; and the current K2 sample. As available, the table includes an estimate of the mass fraction attributable to the hydrogen/helium (H/He) envelope around each planet (Dodson-Robinson & Bodenheimer 2009, Doyle & al. 2011, Cochran & al. 2011, Welsh & al. 2012, Lopez & Fortney 2014, Bakos & al. 2015, Petigura & al. 2016).

The criterion for inclusion in this table is the availability of a mass estimate. Nevertheless, some estimates are more robust than others. For example, the five nearest tweens have more precise RV data than any of the K2 tweens.

The K2-19 system in particular presents a unique conundrum. Two of its three planets (b and c) orbit near a 3:2 mean motion resonance, which produces significant TTVs, while the host star is bright enough to permit ground-based RV studies. This happy coincidence might be expected to yield highly precise results for the masses of both planets. However, that’s not how things have turned out. Through an analysis of TTVs, Barros & al. (2015) estimated masses of 44 Mea for K2-19b and 15.9 Mea for K2-19c. In another TTV study, Narita & al. (2015) could not determine a mass for K2-19b, but they found a value of about 20 Mea for K2-19c, formally consistent with the results of Barros & al. A subsequent RV study by Dai & al. (2016) found 28.5 Mea for K2-19b and 25.6 Mea for K2-19c. Finally, a study by Nespral & al. (2016) obtained new RV data indicating a mass of 71.7 Mea for K2-19b, but K2-19c was undetectable in their dataset. The authors followed up with a TTV analysis that found a mass of 57.7 Mea for K2-19b and 9.4 Mea for K2-19c.

The conflicting results of these four studies give me pause: it looks like less data is better than more! We can keep pursuing our dreams of illumination by distant stars only if we’re ready to shrug off a lot of troublesome details.

Having done so, let’s look again at the tweens in Table 1. First, the bulk mass fraction of H/He appears to increase along with increasing radius, consistent with the hypotheses of Lopez & Fortney (2014). Second, overall mass also tends to increase with radius, although the trend is less consistent. Third, the two tweens with the largest radii – Kepler-16b and Kepler-34b – come close to fulfilling my definition of a gas giant, since they have H/He fractions in the vicinity of 50%, and they fall in the expected mass range for giant planets (both in excess of 60 Mea).

However, this neat picture is complicated by predictions for the internal structure of the four most massive tweens in the table. Although CoRoT-8b, HD 149026 b, Kepler-16b, and Kepler-34b have larger endowments of H/He than most of the other tweens, their envelope fractions are still much smaller than the norm for well-constrained gas giant planets, which range from 75% to 95%. Given estimated heavy element masses of 30 to 60 Mea, each of these big tweens could have contributed solid cores to as many as six planets like Jupiter and Saturn.

super-puff tweens

 

Note that my selection in Table 1 is biased in favor of RV data. Many studies have found that mass estimates derived from RV measurements are systematically larger than those based on TTV analyses. Furthermore, for systems where both TTVs and RV observations are available, the two methods frequently produce widely divergent results. The exceptions are those rare cases with data of extremely high precision.

 

When we turn to the sample of tweens for which TTVs provide the only evidence of mass, we find unexpectedly lightweight objects. Such tweens have been nicknamed “super-puffs.” The classic example is Kepler-11, whose radius (4.18 Rea) is about 5% larger than Neptune’s, while its TTV mass (8.4 Mea) is less than 50% of Neptune’s. Lopez & Fortney (2014) calculate its mass fraction of H/He as 15%. The four well-constrained tweens in Table 1 with similar mass fractions (GJ 3470 b, GJ 436 b, HAT-P-11b, HATS-7b) are notably more massive, with values in the range of 13 to 38 Mea.

 

Another multiplanet system, Kepler-223, rivals the compact structure and extensive TTVs revealed in Kepler-11. The four planets of Kepler-223 have orbital periods ranging from 7 to 20 days, arranged in an interlocking pattern of resonances: planets b and d orbit very close to a 2:1 mean motion resonance, as do planets c and e (Mills & al. 2016). TTV analyses yield a narrow selection of masses for these four, ranging from 4.8 Mea for planet e to 8 Mea for planet d. The two bookends of this range also happen to fit my definition of tweens: Kepler-223d has a radius of 5.24 Rea and Kepler-223e of 4.6 Rea. Although Mills & al. do not calculate the H/He mass fraction for these objects, they must surpass our estimates for Uranus and Neptune.

 

Despite their status as outliers, the four super-puffs orbiting Kepler-223 are easier to explain than the four massive tweens discussed in the previous section. Considerable theoretical work has established that planets as lightweight as 2 Mea can accrete H/He envelopes from their native nebulae and retain them against stripping by stellar flux, as long as their obits are cool enough. Still larger core masses can withstand still higher levels of irradiation, maintaining extended atmospheres over the main sequence lifetimes of their host stars. The real problem is to account for musclebound tweens like CoRoT-8b and HD 149026 b.

 

why tweens but not giants?

 

As Erik Petigura & colleagues recently noted (2016), the K2 tweens and their cousins in other extrasolar catalogs raise critical questions about planet formation. How can these objects assemble such massive heavy element cores and deep H/He envelopes without undergoing runaway accretion – the pathway that leads to the birth of a bona fide gas giant?

 

Before Kepler, we already knew that the mass range for planets dominated by H/He extended as low as 60 Mea. HAT-P-18b, with a mass of only 63 Mea, is a twin of Jupiter in radius, requiring a highly inflated H/He envelope. Similarly, HAT-P-12b has a radius of 10.8 Rea (about 4% smaller than Jupiter’s) and a slightly larger mass of 67 Mea, requiring an envelope almost as deep.

 

Petigura & colleagues suggest that puffy tweens (and presumably puffy gas giants like these two HATs) formed far beyond their systems’ ice line, where extensive envelopes can be accreted quickly and easily, and then migrated inward to their present orbits. Ironically, this is similar to the scenario that Dodson-Robinson & Bodenheimer (2009) proposed some years earlier to account for the opposite problem – the anomalously massive core of another tween, HD 149026 b.
In both cases, long-range migration has been invoked: through a gas-rich nebula to form tweens with unusually deep H/He envelopes, and through a metal-rich nebula to form tweens with unusually large heavy-element cores.

 

I suspect that the true explanation for these dissimilar outliers will be more complicated, likely involving the chemical composition of the nebula but also requiring  evolving structures that alternately concentrate mass at particular radial locations and then encourage migration, either inward or outward. Recent advances in the imaging and analysis of planet-forming nebulae have inspired a growing number of theoretical studies that are bound to shed light on these questions.


postscript on July 23, 2016

This past week, Ian Crossfield & colleagues issued a preprint entitled “197 Candidates and 104 Validated Planets in K2’s First Five Fields” (hereafter C16). It upends some of the descriptive statistics on the K2 sample in this blog post, even though I used data retrieved only three weeks ago. C16 offer a table listing key parameters of the planet candidates they identified (their Table 8). It includes 93 confirmed planets, versus NASA’s list of 44 confirmed planets on July 4. Where the two samples overlap, the stellar and planetary parameters listed in C16 are often at odds with the ones offered by NASA, and it’s not clear to me which numbers are preferable.

C16 find 16 planets smaller than 1.5 Rea (meaning that they’re likely to be terrestrial rather than gassy), whereas I identified only one in the recent NASA data. Among those 16 small planets from C16, all but 3 have orbital periods shorter than 10 days, and all but 2 orbit M dwarfs. The 2 proposed terrestrials orbiting Sun-like stars – K2-36b and K2-80c – have periods of 1.42 and 5.61 days, respectively, implying surface conditions that are literally hellish.

I also observed a median orbital period of 8.5 days and a median radius of about 2.6 Rea. C16 find a similar median period (8.6 days), but for radii they present a smaller median of 2.3 Rea. Oddly, my own inspection of their Table 8, using the same approach that I applied to the older NASA data, suggests a median of 2.4 Rea. Regardless of which number is more accurate, the new value is closer to the median in the Kepler sample, which is just 2.15 Rea.

Finally, C16 report 12 planets with radii between Uranus and Saturn, similar to the 10 I found in the NASA selection. Given their larger overall sample, the fraction of tweens amounts to 13% of the total, versus 23% for the NASA sample.

C16 conclude with a cautionary statement: “Lists of K2 candidates and/or validated planets are not currently suitable for the studies of planetary demographics that Kepler so successfully enabled.” I am duly chastened.  

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REFERENCES

Bakos GA, Penev K, Bayliss D, Hartman JD, Zhou G, Brahm R, et al. (2015) HATS-7b: A hot super Neptune transiting a quiet K dwarf star. Astrophysical Journal 813, 111.
Barros SCCC, Almenara JM, Demangeon O, Tsantaki M, Santerne A, Armstrong DJ, et al.. (2015) Photodynamical mass determination of the multiplanetary system K2-19. Monthly Notices of the Royal Astronomical Society 454, 4267-4276. Abstract: 2015MNRAS.454.4267B
Cochran WD, Fabrycky DC, Torres G, Fressin F, Desert J-M, Ragozzine D, Sasselov D, Fortney JJ, et al. (2011) Kepler-18b, c, and d: A system of three planets confirmed by transit timing variations, light curve validation, Warm-Spitzer photometry and radial velocity measurements. Astrophysical Journal Supplement 197, 7.
Dodson-Robinson SE, Bodenheimer P. (2009) Discovering the growth histories of exoplanets: The Saturn analog HD 149026b. Astrophysical Journal 695, L159–L162.
Doyle L, Carter JA, Fabrycky DC, Slawson RW, Howell SB, Winn JN, Orosz JA, and 42 others. (2011) Kepler-16: A Transiting Circumbinary Planet. Science 333, 1602-1606.
Dai F, Winn JN, Albrecht S, Arriagada P, Bieryla A, Butler RP, et al. (2016) Doppler monitoring of five K2 transiting planetary systems. Astrophysical Journal 823, 115.
Howell SB, Sobeck C, Haas M, Still M, Barclay T, Mullally F, et al. (2014) The K2 Mission: Characterization and early results. Publications of the Astronomical Society of Pacific 126, 398-408. Abstract: 2014PASP..126..398H
Lopez ED, Fortney JJ. (2014) Understanding the mass-radius relation for sub-Neptunes: Radius as a proxy for composition. Astrophysical Journal 792, 1. Abstract: 2014ApJ...792....1L
Mills SM, Fabrycky DC, Migaszewski C, Ford EB, Petigura E, Isaacson H. (2016) A resonant chain of four transiting, sub-Neptune planets. Nature 533, 509-512.
Narita N, Hirano T, Fukui A, Hori Y, Sanchis-Ojeda R, Winn JN, et al. (2015) Characterization of the K2-19 multiple-transiting planetary system via high-dispersion spectroscopy, AO imaging, and transit timing variations. Astrophysical Journal 815, 47. Abstract: 2015ApJ...815...47N
Nespral D, Gandolfi D, Deeg HJ, Borsato L, Fridlund MCV, Barragán O, et al. (2016) Mass determination of K2-19b and K2-19c from radial velocities and transit timing variations. In press.
Petigura EA, Howard AW, Lopez ED, Deck KM, Fulton BJ, Crossfield IJM, et al. (2016) Two transiting low density sub-Saturns from K2. Astrophysical Journal 818, 36. Abstract: 2016ApJ...818...36P
Van Eylen V, Nowak G, Albrecht S, Palle E, Ribas I, Bruntt H, Perger M, Gandolfi D, Hirano T, Sanchis-Ojeda R, Kiilerich A, Arranz JP, Badenas M, Dai F, Deeg HJ, Guenther EW, Montanes-Rodriguez P, Narita N, Rogers LA, Bejar VJS, Shrotriya TS, Winn JN, Sebastian D. (2016) The K2-ESPRINT Project II: Spectroscopic follow-up of three exoplanet systems from Campaign 1 of K2. Astrophysical Journal 820, 56. Abstract: 2016ApJ...820...56V
Welsh WF, Orosz JA, Carter JA, Fabrycky DC, Ford EB, Lissauer JJ, et al. (2012) Transiting circumbinary planets Kepler-34 b and Kepler-35 b. Nature 481, 475-479.

 

1,284 New Kepler Planets, None Like Earth

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).

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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)

Distribution of sizes among 2570 transiting planets with radii up to 18 times Earth (18 Rea). The letters M, E, U, S, J indicate the positions of Mars, Earth, Uranus, Saturn, and Jupiter on the same scale. All data were retrieved from the Extrasolar Planets Encyclopaedia on May 13, 2016. Although the recent confirmation of 1,284 new planets in the Kepler dataset doubled the number of planets observed in transit, the relative frequency of planetary radii remains essentially unchanged since the previous Kepler dump in 2014; compare this graph with the one posted two years ago.

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medium cool

That’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.

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 REFERENCES

Kluger J. Kepler Telescope’s New Planets Discovery Boosts Odds for Life in Space. TIME, May 10, 2016.
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.