On Planets and Dwarfs
Somewhere in the twilight between stars and planets lie objects called brown dwarfs. Below about 8 percent of the Sun’s mass, physical conditions will never allow the fusion of hydrogen into helium, as happens in the Sun. There may be a flickering of energy from the fusion of hydrogen into deuterium, but lower mass objects don’t have a sustainable source of energy. When they’re young, brown dwarfs are easy to observe in the infrared because they generate a lot of heat during their gravitational collapse. As they age, they get cooler and fainter. For example, a puny star 10 percent of the mass of the Sun would have a temperature of 3000 K and a luminosity 1/10,000 that of the Sun. By contrast, a brown dwarf 5 percent of the mass of the Sun would be three times cooler and a further 100 times dimmer. It would take a million of these feeble objects to equal the light of the Sun. Some astronomers consider Jupiter a failed star or a brown dwarf. Models indicate that brown dwarfs likely host moons like those of Jupiter and Saturn, worlds replete with weather systems, geysers, volcanoes, mountain ranges, and oceans—even if, as on Europa, the oceans are frozen over.
Set on a moon orbiting a gas giant planet in the nearby Alpha Centauri star system, James Cameron’s film Avatar (2009) mesmerized audiences with its rendering of Pandora orbiting a Jupiter- like world whose swirled and rippled clouds resemble those of Jupiter and its Great Red Spot. Cameron’s Pandora (one of Saturn’s moons also happens to be named Pandora) abounds in colorful flora and fauna evocative of Earth’s ocean environments, like the giant Christmas Tree Worms that suddenly retract when touched, the seeds of the sacred tree that float with the pulses of a jellyfish, and the photophores that dot the plants, animals, insects, and human-like inhabitants of Cameron’s adventure tale.58 That the Na’vi have blue skin seems no accident or random artistic choice. In ocean waters, blue light is least absorbed and can penetrate into the depths so that most deep-sea animals see solely in blue light. With the announcement in October 2012 of an exoplanet with a similar mass to Earth detected in the Alpha Centauri triple star system, Cameron’s fictional depiction seems even more plausible.59
In part, what captivated film audiences was the bioluminescence with which Cameron painted his Pandoran forests. Having explored and documented deep ocean fauna in the film Aliens of the Deep (2005), Cameron is familiar with myriad bioluminescent sea life and readily projects similar plants and animals onto his imaginary world. Draped with waterfalls alight with bioluminescence, Pandora’s forests teem with glowing fireflies and whirling fan lizards. The forest floor is carpeted with illuminating moss, while its streambeds are lit with the equivalent of sea anemones. In effect,
Cameron anticipates how life might be adapted on an exomoon of a gas giant planet or a brown dwarf star. On such worlds, we might expect to find luminous biota highlighted with photophores as Cameron predicts or species with eyes better adapted to night or low-light conditions. Even as felines have dark-adapted vision superior to humans, the Na’vi have feline features and navigate the night-t ime forest with far better facility than the character Jake. Cameron’s bioluminescent world may have been inspired by Jules Verne’s 20,000 Leagues Under the Sea, in which the characters, strolling on the ocean floor, encounter bioluminescent jellyfish and note their “phosphorescent glimmers.” Actually, nearly all jellies in the deep sea are luminescent. Verne’s voyagers likewise chance upon corals, the tips of which glow. In stark contrast to the nineteenth-century perception that the ocean floor was devoid of life, Verne imagined the seafloor abounding in bioluminescence. An engraved illustration for 20,000 Leagues titled “On the ocean floor” depicts a kelp forest with large corals, crustaceans, and a flotilla of giant jellyfish whose bodies and tentacles radiate light (figure 9.6). Cameron’s vision of bioluminescent organisms flourishing on a nearby exomoon is similarly prescient.
Back on Earth, the first brown dwarf was discovered in 1995. Spitzer has contributed to this research by detecting some of the coolest and faintest examples known, including eighteen in one small region of sky sifted from among a million sources detected.60 Despite their extreme faintness, Spitzer can detect the coolest brown dwarfs out to a distance of one hundred light-years. The outer layers of these substellar objects are cool enough that they’re rich in molecules. The composition of the gas, and so the appearance of narrow lines that act as chemical “fingerprints” in the spectrum, changes as the brown dwarf evolves. Most of the eight hundred cataloged brown dwarfs have atmospheres with temperatures in the range 1200°C to 2000°C. The next category, with temperatures from 1200°C down to 250°C, has strong methane absorption in their atmospheres. There’s overlap (and often confusion) between planets and brown dwarfs because some giant exoplanets are larger and hotter than some brown dwarfs. The very coolest category of the brown dwarf, and the end point of their evolution, has recently been discovered. NASA’s Wide Field Infrared Survey
Figure 9.6. An original illustration from Jules Verne’s 20,000 Leagues Under the Sea depicting bioluminescence. A more modern media representation is in James Cameron’s 2009 motion picture Avatar, where bioluminescent creatures inhabit the exomoon Pandora, where the action takes place. On Earth, thousands of species of sea creatures of all sizes employ bioluminescence (Jules Verne’s 20,000 Leagues Under the Sea).
Explorer (WISE), which was launched in 2009, has finished a scan of the entire sky at infrared wavelengths. In 2011, astronomers reported six of the coolest stars ever found, one of which has a surface at no more than room temperature.61 The WISE data have the potential to reveal brown dwarfs closer than Proxima Centauri, the nearest star to our Sun.
Another promising mission in the search for dwarf stars and transiting exoplanets is headed by Harvard astronomer David Charbonneau and was designed largely by Philip Nutzman, an astronomer at the University of California, Santa Cruz. The MEarth Project is focusing eight small robotic telescopes on 2,000 M dwarf stars.62 The project targets particularly M dwarfs as they are smaller than the Sun and transiting planets would block out a greater portion of the star’s light, making them easier to detect. Studying nearby transiting exoplanets could afford astronomers a better sense of whether Earth-like planets are common and additionally allow astronomers to discriminate the chemistry of their atmospheres. Within six months of launching their project, the team detected their first transiting planet, a super-Earth named GJ 1214b in orbit around a star 13 parsecs from Earth.63 In 2010, the journal Nature reported that the atmosphere of GJ 1214b was found to be comprised either of water vapor, or of thick clouds or haze as on Saturn’s moon Titan.64 Infrared investigations are planned to determine which of these options exist on the planet. With resources like Zooniverse. org, in the coming decade the public will likely contribute to the search for biomarkers such as ozone or water vapor in the atmospheres of these other worlds.
Awash as it is in newborn stars and exoplanets, the universe may be teaming with life. Perhaps in some far future humans will have physical, robotic, or other means of virtual presence on a nearby exoplanet or one of its moons that, similar to Cameron’s Pandora, teems with bioluminescent life. Astronomer Carole Haswell cautions, “If any of this is to happen, however, we need to use our collective ingenuity to understand and repair the effects that our industrial activity and our burgeoning population are having on our own planet.”65 What we learn from exoplanets, Haswell suggests, might be invaluable in understanding Earth’s climate evolution and in ensuring our own survival and that of our companion species. In the meantime, NASA and the astronomical community welcome the public’s contribution to one of humankind’s greatest adventures—locating possible habitable planets and, in time, the signatures of life in some dark and overlooked corner of the vast and silent wastes of interstellar space.