The Crowded Universe

Artist’s impression of a three super-Earths that were detected around the star HD 40307 in 2008.
Credit: ESO

Planets, planets everywhere. Many have been detected in our cosmic neighborhood, but none of them resemble our own. One planet guru thinks that is about to change. He argues in his new book that we are on the verge of uncovering a universe crowded with Earths.

Alan Boss, a research scientist at the Carnegie Institution of Washington, is well-known for his work on theories of planet formation. He has been in the thick of NASA’s planet search for over 20 years, and has become a frequent commentator (and widely-read blogger) on scientific developments in this field. His book, "The Crowded Universe," provides a play-by-play analysis of the planet detections, mission design decisions and theoretical breakthroughs of the past 15 years.

"A new space race is under way," Boss says at the outset of his book. The contestants in this race are NASA’s Kepler mission and Europe’s CoRoT (Convection, Rotation and Planetary Transits) mission, both of which have the potential to detect the first Earth-like planet around a distant star. Boss is betting that together these spacecrafts will find not one but many Earths. He writes:

"If this bold assertion is proved correct by Kepler and CoRoT, the implications will be staggering indeed: it will suggest that life on other worlds is not only inevitable but widespread. We will know that we cannot be alone in the universe."

Mission control

Artist’s impression of the Kepler spacecraft, which launched on March 9, 2009.
Credit: NASA

Despite Boss’ confidence in their success, Kepler and CoRoT did not start out as sure things. After the watershed detection of the first extrasolar planet in 1995, "the exclusive club of planet finders became increasingly crowded," and space agencies began seriously considering missions that would extend the planet search into space. However, the transiting method that Kepler and CoRoT rely on was dismissed early on as being too impractical. Because the transit method of detection tracks the reduction in starlight when a planet passes in front of its star, the method requires that the orbital plane of the planet be lined up nearly perfectly with our viewing angle from Earth. Because this is so rare, one would have to look at tens of thousands of stars to have any hope of seeing a habitable planet.

The astronomy community instead preferred those methods that could see planets in a wider range of orbital inclinations. The basic strategy focused on the Space Interferometry Mission (SIM) and the Terrestrial Planet Finder (TPF). SIM would detect a planet by the sideways wobbles in the host star’s position that are induced by the planet’s orbit. The TPF would attempt to view planets directly by either canceling out the light from the parent star with a coronagraph (TPF-C), thereby making it easier to see the light reflected by the much fainter planet, or by radically increasing spatial resolution with a multi-telescope interferometer (TPF-I), creating clearer and more detailed images.

Artist’s impression of the CoRoT spacecraft, which launched on December 27, 2006.
Credit: CNES

In the beginning — according to Boss’ telling of the story — there was a great deal of enthusiasm for SIM and TPF. But budget troubles and agency politics soon soured this optimism. As Boss reports, it was the modestly-priced Kepler mission that made it through the NASA gauntlet first:

"October 31, 2006: Nearly all of NASA’s astronomy budget would be needed to support just three programs: Hubble, [the James Webb Space Telescope], and the Stratospheric Observatory for Infrared Astronomy (SOFIA). There was no room for anything else… Planet hunting seemed to have disappeared from NASA’s agenda, with the exception of the Kepler Mission. It was looking as though NASA headquarters might decide to rename Kepler TPF-K and forget about TPF-C and TPF-I altogether."

It is still not clear if NASA will ever build SIM or TPF. Their fate partly depends on what Kepler and CoRoT find. If Earths turn out to be rare, it may not be worth it to send more hardware up into space. Currently, NASA and the European Space Agency (ESA) are considering combining TPF-I and a similar ESA mission. If it flies, it would be called Emma Darwin, but Boss isn’t holding his breath.

Artist’s impression of the SIM Lite Astrometric Observatory (formerly called the Space Interferometry Mission).
Credit: JPL

"Emma Darwin would now have to hope for a launch in 2025-2035, a time frame that seemed impossibly distant for someone of my age who had been involved in NASA’s planet search efforts since 1988….[I]t was becoming evident that Emma Darwin would need young astronomers and engineers to be involved in her development if they were to have a good chance of living long enough to enjoy the discoveries that Emma would surely make."

Top dog for top-down

Boss’s interest in planet searching is not solely wrapped up with the possibility of life on other worlds. He is also hoping to vindicate the disk instability theory of planet formation, of which he is the most vocal supporter. He explains how this "top-down" mechanism could form gas giants like Jupiter.

Artist’s impression of the Terrestrial Planet Finder Coronagraph (TPF-C).
Credit: JPL

"Clumps of gas and dust [form] directly out of the planet-forming disk as a result of the self-gravity of the disk gas. The clumps would result from the intersections of random waves sloshing around the disk, waves that look much like the arms in spiral galaxies such as the Milky Way. When two spiral arms pass through each other, they momentarily merge to form a wave with their combined heights, just as waves do on the surface of an ocean. Such a rogue wave might rapidly lead to the formation of a clump massive enough to be self-gravitating and so hold itself together against the forces trying to pull it apart."

This contrasts with the more widely accepted bottom-up method for making planets, called core accretion. Here, Jupiters start off forming like Earths through the coalescence of dust grains and other rocky material. The difference is that a Jupiter core grows big enough to pull in large quantities of gas from the surrounding disk. Boss thinks this mechanism is not likely to form many Jupiters.

"The first step of core accretion might be so slow that the disk gas would be gone by the time most cores grew large enough to accrete the gas, resulting not in gas giants but in "failed cores." Disk instability does not have this problem. If disk instability worked, extrasolar Jupiters would be the rule rather than the exception…"

Jupiters are considered vital for the habitability of Earth-like planets, since they swat incoming comets out of the way like "the batsman protecting the wicket in a cricket match." Computer models have shown that the number of habitable worlds might be 50% higher if Jupiters form rapidly rather than slowly. "Disk instability seems to be not just compatible with, but also highly supportive of, the formation of habitable worlds," Boss writes.

Life lessons

Artist’s impression of the Terrestrial Planet Finder Interferometer (TPF-I).
Credit: JPL

Boss devotes almost the entire book to the notion of habitable planets without addressing whether any might actually be inhabited. But in the final pages he argues that, with so many available planets to choose from, life has almost certainly arisen on more than one. Although we all may hope that this alien life is intelligent, Boss would settle for something simpler.

"Even if life surely exists elsewhere in the Milky Way Galaxy and throughout the universe, the closest habitable worlds are likely to be in a phase of the development of life that is either pre-intelligence or post-intelligence. But this likelihood by no means detracts from our strong desire to search for such life. The former will tell us about our past, and maybe about how we originated and evolved, whereas the latter may tell us about our future. Finding a world of methanogenic bacteria would be just as mesmerizing as finding an Earth-like planet that has been repopulated by a thermophilic species better adapted to the runaway greenhouse heating caused by the short-sighted inhabitants who went before."