Between Earth and Uranus: Part I

Seekers after alien life tell us to follow the water

For some years now, extrasolar headlines have heavily featured Super Earths – objects not quite like our ordinary Earth, yet much more reminiscent of home than the typical high-mass worlds that dominate the exoplanet census. Over the last several months in particular, as radial velocity and transit searches have attained the requisite degree of sensitivity, we’ve been hearing enthusiastic reports of habitable Super Earths: big sisters of our blue marble that orbit at just the right distance from their host stars to support bodies of liquid water, if only their physical and chemical compositions are just right, too.

Recent candidates for the Goldilocks prize – not too hot, not too cold, not too hard, not too soft – have been announced around a nearby K dwarf (HD 85512 b), a nearby M dwarf (GJ 667 Cc), and a faraway Sun-like star (Kepler-22b). Each announcement has been accompanied by an artist’s view of the exciting new real estate – appealing spheres with frothy white clouds and a hint of blue oceans that seem inspired more by the allegedly Earthly aspect of their subjects than by their demonstrably Super features.

 Artist’s view of Kepler-22b, an object in the telluric to gas dwarf range (“Super Earth”) orbiting a G-type star about 620 light years away. Image credit: NASA/Kepler Mission

Super Earths are typically understood as planets with masses between 2 and 10 times Earth's and radii up to about 3 times Earth's. The term seems to conjure images of big planets with wide oceans and lots of leg room. But the cold, hard reality is that candidate Super Earths are less likely to be scaled-up versions of home than they are to be scaled-down versions of Uranus. To unpack this claim, we need to take a closer look at our distant neighbor in the outer Solar System, as well as at those still more distant extrasolar planets. Only then can we make a sober assessment of the biotic potential of telluric planets orbiting alien suns.

the other ringed planet

Uranus is an odd and interesting place: greenish, very cold, and pitched almost sideways on its axis. Among the Solar System’s eight planets, it is third in diameter, fourth in mass, and seventh in distance from the Sun. It has a deep, stormy hydrogen atmosphere and apparently no solid surface; an astronaut descending through its cloud decks would eventually become enveloped in plasma and slush. Uranus is distinguished by an impressive ring system (second only to the rings of Saturn) and extreme obliquity. While Earth rotates on an axis tilted about 23 degrees with respect to our system’s orbital plane, Uranus is tilted a whopping 97 degrees, so that its north polar region points at the Sun on the northern summer solstice.

Uranus has an extremely tilted axis

Like our system’s other giant planets, Uranus supports an entourage of icy moons. However, given its extreme axial tilt, the Uranian moons circle their host like compartments on a Ferris wheel rather than horses on merry-go-round, as is the case for the other giants’ satellites.

Our understanding of extrasolar planets in other star systems depends heavily on our knowledge of the much more accessible worlds in our Solar System. These fall neatly into three categories on the basis of mass, composition, and distance from the Sun.

toy planetology

The terrestrial or telluric planets have one Earth mass (1 Mea) or less. They are composed almost entirely of elements heavier than helium, with metallic cores, silicate mantles, and relatively thin atmospheres of heavy gases (Venus, Earth, Mars) or no atmosphere at all (Mercury). All travel in the warm, inner regions of the Solar System, where freely orbiting ice particles sublimate in the Sun’s heat.

The gas giant planets have masses in the range of 95 Mea (Saturn; 0.3 Mjup) to 318 Mea (Jupiter; 1 Mjup), with their bulk composition dominated by hydrogen. Although both planets have modest cores of rock, metal, and high-pressure ices, heavy elements constitute only 5%-15% of their overall mass. The remaining 85%-95% is contributed by deep envelopes of hydrogen and helium, in a gaseous phase at the top and metallized at the bottom. Both gas giants orbit outside our system’s ice line, the boundary past which free-floating water molecules stay frozen.

The Sun and the 8 planets at their approximate relative sizes

All these telluric and gas giant planets have naked-eye visibility from the Earth’s surface, so they have been known since prehistory. Well beyond the orbit of Saturn lie the “telescope planets,” Uranus and Neptune, which were discovered only when optical technology achieved the necessary sophistication. They are similar in mass, at 14.5 Mea and 17.2 Mea, respectively, and composed primarily of rock, metal, and ice, with hydrogen envelopes contributing about 15%-20% to their bulk composition. For much of the 20th century, however, astronomers regarded these two objects as smaller versions of Jupiter and Saturn, so that all four outer planets were simply known as “giant planets” or “gas giants.” Only after a series of robotic missions to the outer Solar System returned a better understanding of each planet’s atmosphere and interior structure did it become common to draw a distinction between the gas giants proper, which are mostly hydrogen, and the ice giants (Uranus & Neptune), which are mostly heavy elements.

This neat tripartite schema – rocky planets, icy planets, gas planets – works well for our Solar System. It supports a widely endorsed theory of planet formation, according to which each type of planet in our system formed more or less in the same region where it now orbits. The small rocky planets coalesced along the dry orbits of the inner system; the more massive gas giants emerged in the “sweet spot” of planet formation, where abundant ices supplemented refractory elements, and the resulting cores captured huge quantities of hydrogen; and the middling ice giants grew in the outer regions, where ice is still abundant but slow orbits mean smaller planets with thinner envelopes.

Of course, this toy model of planetology is too simple to explain the substantial gap in mass and radius between Earth, the largest telluric planet, and Uranus, the less massive of the two ice giants, or the equivalent gap between Neptune, the more massive ice giant, and Saturn, the less massive gas giant. Nor can it tell us why the three planetary species have remained near their original homes in the Solar System, while migrating inward to converge on short-period orbits in so many extrasolar systems. For a fuller understanding, we need to confront the accumulated data on that mediagenic new type, the Super Earths, which the past year has brought forth in such abundance.

Extrasolar evidence can now populate the structural void between Earth and Uranus with more than 90 worlds whose estimated or minimum masses fall between 1 Mea and 14.5 Mea, as well as another 80 in the void between Neptune and Saturn, with masses between 17.2 Mea and 95 Mea. In the next installment we’ll take a deeper look into the first of these gaps and investigate the mysteries of the Super Earths.