Kepler-452b: Latest Hope for Another Earth

Figure 1. Imaginary view of the surface of an Earth-like exoplanet, created to illustrate the formal announcement of Kepler-452b, a Super Earth or gas dwarf orbiting a metal-rich yellow star. Unfortunately, the estimated radius of this object is 63% larger than Earth, This value is inconsistent with an Earth-like composition. Credit: SETI Institute/Danielle Futselaar 

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Earth 2.0! Again! No really! This time the hype machine seems to be on overdrive. After decades of waking to National Public Radio, the first time I ever heard an exoplanet headline on the morning news was Thursday, July 23, and it was all about Kepler-452b. More details emerged over the course of the day (e.g., video, press release), and eventually I found a copy of the official discovery paper by Jon Jenkins and colleagues.

With that data under my skull, I can tell you that the planet in question is no match for Earth. The host star and the orbital elements are everything you could wish for – spectral type G2, local year longer than ours – but the gosh darn radius (1.63 times Earth) is just too big. I noticed skepticism even in mainstream news accounts, which generally referred to “Earth’s cousin” instead of “Earth’s twin,” along with murmurs that an object that is (or was) possibly potentially habitable under certain improbable conditions isn’t really all that sexy. Indeed, Wired suggested that the whole story was “kind of meaningless.”

Table 1 summarizes the confirmed Kepler planets that have been proposed to date as Earth-like worlds orbiting in their host stars’ habitable zones (HZ). (For other postings on this topic in 2015, see Much Ado About Earth 2 and What Kepler Hasn’t Told Us.)

Table 1. Small, Cool Extrasolar Planets

Column 1 presents the host star’s name; column 2 the stellar effective temperature (Teff) in Kelvin; column 3 the stellar mass in Solar units (Msol); column 4 the stellar metallicity; column 5 the stellar age in billions of years (Gyr); column 6 the distance to the system in parsecs; column 7 the planet name; column 8 the KOI number; column 9 the planet radius in Earth units (Rea); column 10 the orbital semimajor axis in astronomical units (Earth’s orbit = 1); column 11 the planet equilibrium temperature (Teq) in Kelvin; and column 12 the orbital period in days. 

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Kepler-452b stands out from the pack in several ways. Most important are the statistics on its host, whose mass and effective temperature are almost identical to our Sun’s. Accordingly, a habitable orbit in this system exceeds 300 days. Thus Earth’s new cousin has one of the longest orbits in the Kepler dataset, as well as the widest semimajor axis of any HZ candidate.

At 430 parsecs (1400 light years), this system is also more distant than any of the others, suggesting that detectable Earth-size planets around G-type stars are notably less abundant than those around M or K stars. Kepler-452 itself is also older than all but one star in Table 1, with a radius 11% larger than the Sun’s (data not shown), indicating that it is expanding and growing hotter with age. These factors result in an estimated equilibrium temperature (Teq) of 265 K for Kepler-452b. This is a little hotter than Earth, which has Teq of 255 K and a mean surface temperature of 288 K. 

Kepler-452 is also the most metallic star in Table 1, raising the chances that it might harbor one or more gas giants in addition to its single confirmed planet. Analogs of Jupiter or Saturn on wider orbits would make a very interesting system architecture; maybe future radial velocity observations can assess their potential presence. 

But the least appealing superlative associated with Kepler-452b concerns its radius, which at 1.63 Rea is by far the largest in Table 1. Including error margins – minus 0.20, plus 0.23 – blurs the picture a bit, but overall makes it uglier. At 1.43 Rea, Kepler-452b could be an iron/silicate planet of about 3 Earth masses (3 Mea) or a 50% rock/50% ice world of 1.5 Mea (Dressing et al. 2015). The iron/silicate option would make it a good “potentially habitable Super Earth,” but the icy option would raise questions about the chemical diversity needed for biogenesis. At the preferred value of 1.63 Rea, however, an Earth-like composition with an iron core and a silicate mantle would raise the mass to 5.5 Mea (Dressing et al. 2015), which is arguably too high for plate tectonics or a carbon cycle. A 50% rock/50% ice composition would correspond to a mass of 2.5 Mea. More likely for a radius around 1.6 Rea, however, is a planet with the same mass as Earth and a puffy atmosphere with 1% hydrogen/helium (Rogers 2015), corresponding to a lifeless Hellworld. 

Finally, at the high value of 1.86 Rea, current theoretical models would readily define Kepler-452b as a low-mass, low-density gas dwarf with a substantial hydrogen/helium envelope (Rogers 2015, Dressing et al. 2015, Wolfgang et al. 2015). Indeed, one recent study (Lammer et al. 2014) suggests that virtually all planets of 2 Mea or more orbiting in the circumstellar HZ will retain hydrogen atmospheres. The authors singled out Kepler-62f – perhaps the most impressive candidate in Table 1 – as a likely gas dwarf with an extensive hydrogen envelope, presenting a detailed argument to invalidate claims by Borucki et al. (2013) that the planet could have an Earth-like composition. Ouch!

And so – while I can’t retroactively cancel my joyous response to the announcement of Kepler-62f – the skepticism I’ve honed over the duration of the Kepler mission stifles any similar rejoicing for Kepler-452b. That doesn’t mean I’ve lost my optimism regarding the possibility of complex life beyond the Earth. Indeed, Kepler data demonstrate that small planets are abundant in the Milky Way, and what we know about the evolution of the Earth argues that some of those planets are bound to sustain life. But it’s equally plain that on orbits of 100 days or more, Earth-size planets are no more common (and probably less common) than small gas dwarfs of 2-3 Rea. Even telescopes as sensitive as Kepler have great difficulty detecting Earth-size planets on such long orbits. 

present ideals & future likelihoods 

Ideal radii for potentially habitable terrestrial planets are about 1.3 Rea and smaller, implying iron/silicate planets of 2.5 Mea or less. At 1.2 Rea, the corresponding mass is about 2 Mea, while at 1.1 Rea, it’s 1.5 Mea. That relatively narrow range of masses and radii appears to define the population of bona fide Super Earths. Larger radii require either large quantities of rocky mass that might jeopardize geologic activity; fractions of ices that might be too high to enable mixing of heavy elements with water; or hydrogen/helium envelopes that would rule out water in its liquid state altogether. Indeed, a version of Earth scaled up to 10 Mea (a mass so far not attested for purely rocky objects) would have a radius of only 1.9 Mea. 

As we’ve seen, Kepler isn’t very sensitive to planets of 1.3 Rea or less. A recent iteration of the Extrasolar Planets Encyclopaedia listed 160 transiting planets in this range, representing about 15% of the confirmed Kepler sample. The vast majority have orbits shorter than 25 days. Only six occur in systems with a single transiting planet (a description that applies to Kepler-452). This bias toward multiple companions has everything to do with the protocols used to validate Kepler candidates. Members of multi-planet systems are much easier to confirm than singletons, which might require data on transit timing or radial velocity variations to ensure their reality. Yet future confirmations of Kepler planets, including those in the desired range of radii and masses, are likely to involve increasing numbers of singleton systems. The low-hanging fruits of planetary multiplicity have already been mostly plucked.  

In the discovery paper, Jenkins and colleagues include an interesting paragraph on potential system architectures for Kepler-452. They calculate that if the system included a Venus analog with the same inclination to planet b’s orbital plane as Venus to Earth, the likelihood of its detection (along with Kepler-452b) would be only 10%. An analog of Mars would be still more difficult to detect, while all three planets together would have just a 2% chance of discovery.

Could the likeliest Earth-like planets in the Kepler dataset turn out to be in systems with only one transiting object?

REFERENCES
Borucki WJ, Agol E, Fressin F, Kaltenegger L, Rowe J, Isaacson H, et al. (2013) Kepler-62: A five-planet system with planets of 1.4 and 1.6 Earth radii in the habitable zone. Science 340, 587-590. DOI:10.1126/science.1234702 Abstract: http://adsabs.harvard.edu/abs/2013Sci...340..587B. 

Dressing CD, Charbonneau D, Dumusque X, Gettel S, Pepe F, Cameron AC, et al. (2015) The mass of Kepler-93b and the composition of terrestrial planets. Astrophysical Journal 800, 135. Abstract: http://adsabs.harvard.edu/abs/2015ApJ...800..135D 
Jenkins J, Twicken JD, Batalha NM, Caldwell DA, Cochran WD, Endl M, et al. (2015) Discovery and validation of Kepler-452b: A 1.6 Rea Super Earth exoplanet in the habitable zone of a G2 star. Astronomical Journal 150, 56. 
Lammer H, Stokl A, Erkaev NV, Dorfi EA, Odert P, Gudel M, Kulikov YN, Kislyakova KG, Leitzinger M. (2014) Origin and loss of nebula-captured hydrogen envelopes from ‘sub’- to ‘super-Earths’ in the habitable zone of Sun-like stars. Monthly Notices of the Royal Astronomical Society 439, 3225-3238. Abstract: http://adsabs.harvard.edu/abs/2014MNRAS.439.3225L 
Rogers L. (2015) Most 1.6 Earth-radius planets are not rocky. Astrophysical Journal 801, 41. Abstract: 2014arXiv1407.4457R 
Wolfgang A, Rogers LA, Ford EB. (2015) Probabilistic mass-radius relationship for sub-Neptune-sized planets. Astrophysical Journal, in press.

Four Fabulous Dwarfs

Figure 1. Small Solar System objects photographed by the Dawn and New Horizons missions, shown at their relative sizes with radii in kilometers (km). Ceres is the largest object in the Asteroid Belt; Pluto is the largest object in the Kuiper Belt. Both are currently designated dwarf planets, although even Pluto is notably smaller than the Earth’s Moon (radius 1737 km). Now that Vesta has been imaged as a battered, non-spherical remnant, it no longer seems as planet-like as it used to.

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Dawn and New Horizons – dutiful space robots launched years ago into realms beyond Earth – have fulfilled their missions over the past few months by returning spectacular images of Ceres and Pluto, the principal dwarf planets of our Solar System. These missions have also illuminated two of their companions: Vesta, which shares a similar orbital space with Ceres, and Charon, which can be described as Pluto’s moon, but is really one constituent of a unique “binary system” otherwise unattested in planetary science. Until very recently, these four worlds had been imaged only as blurry dots. Now astronomers are using the latest photos to make high-resolution maps of their surface features.

Ceres, Vesta, and Pluto were all thought to be planets when they were discovered, respectively in 1801, 1807, and 1930. In each case, however, research by later generations established that these objects are not only much smaller than originally estimated, but also accompanied by thousands of other similar, smaller objects in the same orbital space. 

Figure 2. Terrestrial planets, dwarf planets, and moons

This figure represents 3 of the 4 terrestrial planets, 2 dwarf planets, and 12 of the 19 spheroidal moons in our Solar System. All four terrestrial planets and two moons (Io and Luna) are rocky. All the other small spheroids in our system, which have orbits wider than Mars, are a combination of rock and ice.

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We now know that Ceres and Vesta are respectively the largest and third-largest objects in the Asteroid Belt, a swarm of debris orbiting mostly between 2 and 3.5 astronomical units (AU). Both were demoted from planetary status in the mid-nineteenth century. We also have brand-new evidence that Pluto is the largest object in the Kuiper Belt, another swarm of debris orbiting mostly between 30 and 50 AU. Pluto was demoted to dwarfdom only in 2006, after more than a decade of new evidence revolutionized our understanding of the Kuiper Belt.

I still run into people who resent Pluto’s demotion. I kind of understand their feelings, since I was a child once myself, fond of underdogs and eager to back contrary viewpoints. Nevertheless, between Dawn and New Horizons, I was much more excited by the former, given my interest in asteroids and their role in the Solar System’s evolution. That inclination changed dramatically just the other day, when those amazing photos of Pluto and Charon were beamed back to Earth (e.g., Figure 1). Vesta and Ceres are truly fascinating, but Pluto and Charon are something else again! 

Figure 3. Two views of Vesta

These photos of Vesta’s south pole, taken from different distances at slightly different angles, hint at the object’s original spheroidal shape.

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Vesta 

The asteroid known as 4 Vesta was evidently once a rocky spheroid, more than four billion years ago when our system was young. Although 2 Pallas is slightly larger, Vesta is second in mass only to Ceres among the asteroids. Radionuclide analyses suggest that Vesta was born just 3 million years after the Sun, making it one of the oldest offspring of our parent star. As such, Vesta is probably a remnant protoplanet – a rare survivor of the ancestral swarm from which all the planets and moons in our system coalesced. Over the aeons, collisions with other objects have blasted away big chunks of Vesta’s mantle and left a multitude of surface scars in the form of craters and chasms. The result is a peculiar object resembling a slightly squashed pumpkin that some knife-wielding maniac has carved at random.

This battered world orbits the Sun at a semimajor axis of 2.36 AU in a period of 1325 days. Its orbit is as steeply inclined to the plane of the Solar System as that of Mercury (7 degrees) and as eccentric as that of Mars (0.09). 

Figure 4. Comparison view of Ceres and our Moon

The topography of Ceres looks even flatter and gentler than the outward-facing hemisphere of the Moon, which is remarkable for its blandness in comparison with the Earth-facing side. Note that these images are shown at different scales; the diameter of the Moon is 3.7 times that of Ceres.

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Ceres 
Well-resolved photos and other data on Ceres began arriving only in January, so astronomers have had less time to digest this information than they’ve had for Vesta, which Dawn visited in 2011-2012. The photos reveal a slightly oblate sphere with a thoroughly cratered surface. Some craters feature bright white spots that have incited considerable scientific interest, but so far no robust explanations (icy patches? salts?). Overall, the landscape looks rather Lunar, except that Ceres has neither dark areas nor mountain ranges. Although much the same description applies to the far side of our Moon, the Lunar far side still looks more rugged than Ceres (Figure 4).

According to a widely accepted view, Ceres has a rocky core covered by an icy mantle accounting for about 50% of its volume and 25% of its mass. This is similar to the structure proposed for the spheroidal moons of Jupiter and Saturn. This model, along with recent observations of water vapor escaping from discrete areas on the surface of Ceres, led some people (e.g., me) to entertain the possibility that Ceres would resemble Europa or Enceladus: flat frozen plains with markings suggesting the activity of subsurface seas. As it turns out, Ceres bears a much closer visual resemblance to Rhea and Tethys, two of Saturn’s icy moons (Figure 5). Its crust might be an amalgam of ices and clays.

Figure 5. Ceres and Tethys at their relative sizes

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Ceres orbits the Sun in an orbit wider than Vesta’s, with a semimajor axis of 2.77 AU and a period of 1682 days. Its inclination (almost 10 degrees) and eccentricity (0.12) are also larger than Vesta’s. Nevertheless, the eccentricity of Mercury – the most eccentric of the eight planets – is even higher, at 0.21. Both the orbital elements and the morphology of Ceres and Vesta should eventually shed light on the origin of the Asteroid Belt. For now, though, we have no compelling hypotheses that are also consistent with the widely endorsed Grand Tack model of system history. 

Pluto and Charon 
If we compare asteroids to Kuiper Belt objects (hereafter Kuiperians), we can’t avoid noticing that size matters. Pluto and Charon are significantly larger than the largest asteroids, providing scope for much more varied, planet-like topographies. But environment matters, too: Vesta and Ceres both orbit solo, while Pluto and Charon constitute a binary system, with each member of the pair tidally locked to the other.

Figure 6. The Pluto System

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The two Kuiperians zip around their common center of mass – which falls outside the volume of Pluto – in a period of only 6.4 days, maintaining a mutual separation of 19,570 km (just 5% of the 384,400 km separating Earth from our Moon). Four other objects orbit the central binary – tiny, irregular moons christened Nix, Hydra, Kerberos, and Styx, all with circular orbits that are coplanar with the motion of Pluto and Charon (Figure 6). Remarkably, all four were discovered in the past 10 years, and thanks to New Horizons, we already have photos of two of them.

Figure 7. Nix and Hydra, moons of Pluto

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Contrary to expectations, the surface morphology of the two big Kuiperians at the center of this knot of satellites reflects recent geologic activity. Charon has few visible craters, Pluto even fewer. The New Horizons team estimates that parts of Pluto’s surface assumed their current form only within the past 100,000 million years, unlike the truly primeval topographies of Mercury, Vesta, and the Moon. 

Pluto is thought to be composed of a rocky core with an icy mantle. Its surface consists largely of frozen nitrogen with traces of methane, and it also supports a tenuous atmosphere containing nitrogen, methane, and carbon monoxide. These atmospheric gases evidently arise from evaporation of surface deposits during the long Plutonian summer, which happens when Pluto is near perihelion (i.e., its closest approach to the Sun). The gases create a haze that floats more than 100 km above the surface. New Horizons observed a large volume of nitrogen escaping into space, creating a comet-like tail. As summer ends, the gases likely freeze again and rain onto the surface, creating the variegated landscapes we see.

Figure 8. Ice mountains in Tombaugh Regio on Pluto

Above, the Norgay Mountains amid a craterless plain. Below, icy mountains in the southwest, adjacent to the black, cratered terrain of Chthulhu.

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To my great surprise, the Plutonian landscape is colorful, ranging from charcoal black to orange to snow white. Recent telescopic observations suggest that the landscape’s colors are changing over time, becoming brighter and redder; these changes might be seasonal. A few striking geographical features on Pluto have already been named. The big white heart-shaped area visible in Figure 1 is now called Tombaugh Regio, after Clyde Tombaugh, Pluto’s discoverer. The New Horizons team reports that this area is covered with frozen carbon monoxide – not exactly the most romantic chemical compound! Tall, icy mountain ranges have also been imaged in this area (Figure 8), which appears to be devoid of impact craters. Adjacent to Tombaugh is a dark region named Chthulhu, after the unspeakably dreadful entity who haunts the tales of H.P. Lovecraft (Figure 9).

According to the latest news (July 24), Pluto's surface deposits of frozen methane, nitrogen, and carbon monoxide qualify as glaciers, flowing across the landscape much like glaciers on Earth.

Figure 9. Two hemispheres of Pluto

These images were recorded as New Horizons approached Pluto, explaining the difference in resolution.

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Charon looks just as interesting as its binary partner. At the north pole is a very dark crater-like formation nicknamed Mordor, after the land of the Shadow (AKA Sauron) in the tales of J.R.R. Tolkien. Elsewhere the new photos reveal smaller craters, chasms, white and dark patches, and broad plains. Recent publications indicate that Charon’s composition includes a larger proportion of ices than Pluto’s, and that water ice in particular is common on Charon. These characteristics, combined with its size and surface features, lend Charon a distinct resemblance to the four largest moons of Uranus (Figure 10).

Pluto and Charon orbit the Sun in a period estimated at 248 years, so astronomers have observed only about one-third of a Plutonian year so far. With an eccentricity of 0.249, Pluto’s projected orbit is more elongated than that of any full-size planet. Neptune, the nearest planet to Pluto and the outermost of the canonical eight, follows an almost perfectly circular orbit with a period of 164 years. As a result, the Pluto-Charon system regularly crosses Neptune’s orbit. Yet this configuration appears to be stable on billion-year time scales, thanks to its machine-like precision. Neptune and Pluto are engaged in a 3:2 resonance, such that Neptune completes three orbits for every two orbits of Pluto. This “time-sharing” arrangement guarantees that the two objects are always far apart during their regular orbit crossings, preventing collisions or perturbations. 

Figure 10. Charon alongside the four largest moons of Uranus, shown to scale

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The intricacy of this configuration suggests a sort of Baroque timepiece worthy of Kepler or Leibniz. It certainly injects an exciting counterpoint into the grand symphony of the spheres. For those of you who like music along with your astronomy, I recommend two Plutonian excursions: “Pluto Drive,” a spooky tune by The Creatures, recorded in 1989, and “Orfeo, vincesti,” a lyrical bass aria sung by Pluto in the opera Orfeo, composed by Antonio Sartorio in 1672 and performed by Harry Van Der Kamp in a recording from 1999 (unfortunately not on Youtube). As Sartorio intuited and New Horizons has proven, even Pluto reveals an unexpected beauty - and a surprisingly big heart! - when observed in the right light. 

Figure 11. Pluto's atmosphere back-lit by the Sun