Published Date Written by Edward Gleason
Merely finding exoplanets — those planets in orbit around other stars — does not settle the extraterrestrial life issue, which remains unresolved. Astronomers have so far confirmed 1055 exoplanet discoveries and are quite confident that they'll find tens of thousands more within the next decade alone.
Many scientists insist that billions of planets lurk within the Milky Way's vast starfields. However, even detecting all of these worlds would not conclusively prove that alien life exists in the Universe. Instead, such a comprehensive list would show where alien worlds might be. The galaxy could harbor a trillion lifeless spheres, which would make Earth seem a singular anomaly of the most improbable sort. Or, life could abound in great profusion throughout the galaxy, making Earth just one sphere amongst millions where intelligence flourishes.
Determining if life exists elsewhere, however, necessitates more than the discovery of other worlds. "Signs of life," in what ever form they might assume, must also be found. Principally, the sign of life the astronomers seek is within a planet's atmosphere, the gaseous layer enveloping the planet. Astrobiologists assume, of course, that any life form would require a sustaining atmosphere. All animal life on Earth would perish in minutes if the atmosphere vanished. It seems logical that life on another world would be similarly reliant on that world's gaseous envelope. The tricky issue is in the composition. Based only on Earth, the only life-bearing planet with which we're familiar, a planet would need an abundance of water vapor and oxygen to support life. Then again, it is not inconceivable that extraterrestrial life might require an entirely different blend of gases. After all, though we're carbon based life forms, it is possible that some aliens might be based on silicon, an element with a bonding affinity comparable, but not equal, to carbon.
The next step in the alien search involves the examination of exo-planet atmospheres. That is quite a step, actually. How does one determine the composition of an alien world's atmosphere? The method involves observations of the planet's spectrum. Just as all our solar system's worlds reflect some incidental sunlight, exoplanets will also reflect radiation from their parent stars. However, before it is reflected, this light passes through part of the atmosphere. During this passage, the gases will absorb various wavelengths of light. As each gas creates its own spectrum, astronomers can identify the gaseous components within the planets' atmospheres.
To understand this concept, imagine a light source cast against a wall. We'll naturally pass the light through a prism. Consequently, we'll see a rainbow of colors on the wall. It is a crisp, clean continuum of color from Red through violet, with a gradation of hues in between. Now, we'll imagine that we pass the light through a a glass container of gas. What do we find? Within that rainbow continuum we observe a series of dark vertical lines resembling a bar code. The lines form because the gas absorbs some of the light at various wavelengths. Each gas produces its own "signature" spectrum. Scientists can identify a material through which light passes by analyzing this spectrum. The same principle applies to stars, as well.
That is the theory and, as is often true, it is much simpler than the practice. Analyzing exoplanet atmospheres is extremely difficult because the planets, from our perspective, are close quite their parent stars. Rarely do our telescopes resolve the planet separately from the star, itself. However, a research team led by University of Chicago astrophysicists examined the atmosphere of a "Super Earth" world called GJ 1214b. The term "Super Earth" refers to planets larger than Earth, but smaller than Neptune. Through a 96-hour series of observations complied over nearly a year, the researchers have detected "clouds" on this super Earth. While the composition of these clouds remains undetermined, their discovery represents an advancement in humanity's ability to discern atmospheric features on an exo-world.
Granted, GJ 1214b was perhaps the easiest Super Earth world to observe, owing to its close proximity of 40 light years and the fact that it passes across the star every two days, enabling astronomers to examine its atmosphere as the light of its Sun passes through it.
The observation series that led to the clouds' discovery was not the first such investigation of this nearby Super Earth since its 2009 discovery. Research teams examining its spectra in 2012 and 2013 found a complete lack of chemical signatures. This subsequent discovery of high altitude clouds suggests, perhaps, that an envelope of gases in compressed into the lower regions, beyond our scrutiny.
Astronomers are convinced that these clouds, however, are not composed of water vapor. The temperatures on GJ 1214b exceed 430 degrees, which precludes the formation of water clouds. Instead, they might consist of an exotic substance such as zinc sulfide; a compound that could form in which sweltering conditions. We don't expect life to thrive on this cloud laden planet. However, its existence supports the notion that planets around the galaxy retain dynamic atmospheres: another step toward finding another world teeming with life ... if such a world exists.
(Edward Gleason is an astronomer and manager since 1999 of the Southworth Planetarium in Portland. He also was employed at the Maynard F. Jordan Planetarium in Orono. Gleason writes the daily e-mail article, "The Daily Astronomer." Visit http://usm.maine.edu/planet for more about the planetarium.)