This is an artist's image of an overhead view of planets in systems that NASA's Kepler observatory just discovered. All the coloured planets have been verified. The grey ones have not.
In January 2012, NASA's Kepler mission, shown below in an artist's rendering, announced that it just found 11 new solar systems containing at total of 26 confirmed planets, ranging in size form 1.5 times the diameter of Earth to larger than Jupiter.
This is just the latest data adding to the 729 known extrasolar planets (in 594 planetary systems) listed in the Extrasolar Planets Encyclopedia. There are a lot of planets out there in the cosmos. There are two questions to answer. We want to know which ones sustain life of some kind. Titan's strange atmosphere gives us tantalizing clues and makes this question is challenging to define: What kinds of different atmospheres could harbor life, and what would those different kinds of life chemistries look like? The second question is more human-centric. Are there any other planets out there that have oxygen-based atmospheres and possibly life just like us?
As we have learned so far, atmospheres have complex and evolving chemistries and in order to understand them we must have some idea of how they form. There are still many unanswered questions about the atmospheres within our own solar system and as we work toward answering them, we can simultaneously refine what kinds of markers we need to look for when we look at atmospheres on extrasolar planets. This is an excellent example of a tenant central to scientific research: Our answers are only as good as the questions we ask.
We already have some basic guidelines to go on:
An atmosphere depends on how far away from its star it formed. Heavier gases like carbon dioxide, oxygen and nitrogen should, in general, be more abundant on closer planets and light gases like hydrogen and helium should be more abundant on more distant planets.
It depends on the gravity of the planet.
It depends on the planet's magnetosphere (and this depends on the planet's interior composition and perhaps other factors such as collision history).
The shape of the planet's orbit is as well as how fast it revolves is important - is it in a geostationary orbit where only one side faces its star?
Does it have an axial tilt - does its atmosphere experience seasons, in other words?
We have two basic challenges to consider:
We don't yet fully understand how atmospheres work (but we have some general ideas).
We are just now able to "see" distant planets. Astrophysicists are still refining the technology needed to analyze their atmospheres.
Hot Jupiters
The first extrasolar planet found orbiting a Sun-like star is 51 Pegasi b, discovered in 1995. This massive Jupiter-sized planet is 51 light years away orbiting very close to its star.
As early as 2001, the Hubble Space Telescope, shown below, was able to detect an atmosphere on a Jupiter-sized planet, named HD 209458, 150 light years away.
This is an artist's concept of what it might look like:
Its spectrometer analyzed light from the planet's star filtered through the planet's atmosphere as it transited it, finding it to be rich in sodium. It is a gas giant orbiting its star even closer than Mercury orbits the Sun, with atmospheric temperature of 1100°C. They believe huge monsters like these can hold onto their atmosphere even at this close range with their intense gravitational field. Tiny Mercury would never be able to hold onto an atmosphere. An international team of scientists is now using Hubble to search for other hot Jupiters focusing on the atmospheric compositions of these planets. This Hubble video describes how they do it:
As they orbit so close to their stars, they all tend to have a rotation synchronous to their orbit so one side always faces the star. It will be very interesting to figure out what kinds of atmospheric dynamics these planets have.
Another hot Jupiter, called XO-2b, was recently found by scientists using the land-based Smithsonian Astrophysical Observatory telescope. This planet, like the other one is about the size of Jupiter and orbits very close to its star. Potassium has been detected in its hot atmosphere.
These giant planets tend to be "puffy." Intense heat from their close-by star and internal heat tend to inflate their atmospheres. They are at most slightly more massive than Jupiter but with much greater diameters. These are the easiest planets for us to see. We can zero in on them because they are large fast orbiting bodies ? they tend to induce a noticeable wobble in the orbits of their stars. Their close orbits also mean we have a good chance of observing them as they transit their star, and we have a opportunity to analyze their atmospheres as they do so.
Many hot Jupiters have been discovered so far. Scientists now estimate that about 7% of the stars in the Milky Way alone should have hot Jupiters orbiting them. This has led scientists to wonder if systems with these planets reduce the likelihood of earth-like planets. Would these giants, orbiting so close to their star knock smaller Earth-sized terrestrial planets from their orbits, or, more likely, hog up all the gas and dust and prevent their formation altogether?
At first, physicists were confused about how such massive planets could form so close to their star. The discovery of hot Jupiters is a good example of how theories evolve as new information comes in. Scientists had to reconsider their theories of planetary formation. They now believe that these giants formed much further out, past the frostline, the orbital distance at which ices don't sublimate away into space. Here, these planets quickly accumulated light volatile elements and migrated inward since. The inward migration may tend to occur in protoplanetary disks that were extra-thick with accreting gases. A Jupiter-sized planet would experience viscous drag making it spiral inward. Computer models show that, with extra-dense gas clouds, there would enough raw material for terrestrial planets to form as well, and in most types of modeled systems, Earth-size planets could exist in stable Earth-like orbits along with a hot Jupiter. There is a catch though. Scientists believe that Jupiter's gravity have defected water-rich asteroids to collide with young Earth and contribute much of our ocean's water. Without Jupiter (in its distant orbit), Earth may have been a much more parched desert-like planet. This makes the formation of life less likely but not impossible. The likelihood of a wet "Earth" within a system containing a hot Jupiter is difficult to calculate and, so far, unknown.
Challenges of Searching for Planets Like Earth
The discovery of hot Jupiters suggests to us that planetary atmospheres are not at all uncommon in the universe. Gas giants simply tend to form out of accreting debris within a certain range from a forming star. This leaves the question of how likely a terrestrial planet atmosphere is, and how many would exist in some kind of equilibrium state like Earth's, rather than the atmospheres of Venus and Mars.
Scientists are approaching these questions from two directions. Life seems highly unlikely in the extreme environment of a gas giant, but what about smaller terrestrial planets more like Earth? This means improving our ability to locate and identify smaller planets and analyze their atmospheres, and that requires better telescopes. Second, astrobiologists are exploring what kinds of environments life could inhabit, in an attempt to focus the search. We have life on Earth to use as a template and we can use our knowledge of biochemistry to extrapolate from that to conditions different from Earth where different kinds of molecules could arrange into structures complex enough to carry out life functions. At the same time new kinds of life are continuously being discovered in environments right here on Earth that were previously thought too extreme for allow the biochemistry of life to work.
Scientists are refining their set of hypothetical conditions that would be necessary for a planet to exist that could support life. And they are building a list of the basic ingredients that life requires. As we explore these two lists, you will may it striking how critical our knowledge of Earth's atmosphere and of atmospheric dynamics in general are to constructing such lists.
A Recipe for Life
95% of all life on Earth is built from only 6 elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur. These elements are the building blocks of biological molecules from which complex (living) structures form. Carbon is especially important because it forms flexible covalent bonds with many other elements and it is the organic basis upon which energy transfer can occur.
Life on Earth requires water as a solvent in which biological molecules can form and grow in complexity.
Life requires an input of energy to sustain metabolic reactions. Solar energy is the basis of almost all Earth life, but electrical energy (lightning) turned a mixture of gases into amino acids, life building blocks. Chemical energy is used on Earth by chemotroph bacteria as well as more complex organisms around hydrothermal vents. These organisms use hydrogen sulphide, sulphur, iron hydrogen or ammonia as their sole energy source. The recent discovery of complex life forms sustained only by chemical energy opened up a whole new possibility for extraterrestrial life. Similar organisms could evolve in subterranean seas on moons such as Europa or ice-covered planets elsewhere in the cosmos.
These elements are abundant in the universe and should exist within every protoplanetary disk, which supplies the raw material of planets. Therefore, many experts suspect that life on other planets would likely be based on similar organic chemistry. Some have speculated on a few possible variations on this theme. Silicon forms bonds similar to carbon and could replace carbon in a silicon-based biochemistry. Earth and other terrestrial planets in our system are very rich in silicon compared to carbon (by a factor of 925:1) and yet carbon seems to be much more successful as a basis for Earth life. Silicon doesn't bond with as wide a variety of other elements as carbon does and, at least with an oxygen respiring organism, the waste product analogous to carbon dioxide would be silicon dioxide, sand in other words rather than gas. Still, there is some speculation that silicon could form the basis of biochemical structures that would be far more stable in very high temperatures where carbon-based structures would dissociate. This 3-minute excerpt from the BBC documentary "Cosmic Safari" explores what silicon-based life might look like:
Life forms using ammonia rather than water as a solvent have also been suggested. Like water, it dissolves many other chemicals, it forms compounds without being either too stable or too reactive and it's abundant. Life on Titan, if it exists, could be based on such chemistry.
Finally, if life is defined as fundamentally self-replicating reaction, it is possible that such a life form could even exist within the intense plasma of a star. This speculation, which seems really out there like an episode from Star Trek, stretches our ideas about how we define life.
Life-friendly Planets - The Goldilocks Zone
The goldilocks zone, or habitable zone as it is often called, is a theoretical orbital zone around a star where conditions are neither too hot nor too cold for liquid water to exist on a planetary surface. Its location and range depends on various factors such as how luminous the parent star is and how massive the planet is, as it must have enough gravity to hold onto water vapour. This zone is extrapolated from water-based life on Earth, the only life we know exists. Planets in this zone offer us the best chance of finding life that is based on biochemistry similar to ours. However, many researchers find it too constrictive, considering that life based on liquid methane as a solvent or silicon as a chemical backbone could exist in theory. If these potential biochemistries are considered, the goldilocks zone must be significantly widened accordingly. The potential for life in subsurface oceans, brought to light by the discovery of deep see thermal vent organisms, also widens the goldilocks zone considerably.
Scientists may need to expand their assumptions about where life could exist even further. For example, microbes called Deinococcus radiodurana are frequently found in highly radioactive nuclear waste. Our DNA is very vulnerable to radiation, but these organisms survive by having multiple copies of DNA and rapid DNA repair mechanisms. Such life forms would have no problem on a highly radiated planet surface as long as it had the right ambient temperature and an energy source. Many more extremophile organisms are being discovered every day in environments that we think should be unlivable. Astrobiology Web provides an excellent list of such organisms.
Meanwhile, NASA, using the Kepler Space telescope, shown below, announced in December 2011, to a great deal of excitement, the first confirmed discovery of an alien goldilocks planet.
This is the focal plane of the Kepler telescope, containing a total of 95 megapixels.
The planet, called Kepler-22b, located 600 light years away, orbits a Sun much like our own. It is about 21/2 times larger than Earth with similar surface temperatures. It might look like this:
We don't yet know if it has surface water, however, just that it could. Its mass still unknown. Once researchers can determine its mass, they can determine the planet's density and make some predictions about its composition, how much rock versus how much water exists on it for example. Planets with low masses and long orbits like this one, and any other Earth-like planets we may discover, make it very difficult to determine mass because their radial velocity signal is very small. Whether Kepler-22b could support life depends critically on what kind of atmosphere is has, if any. Its surface temperature is estimated to be about 22°C on average but this estimate depends on what kind of atmosphere the planet has. If Earth had no atmosphere, for example, its surface temperature would be about -20°C. The greenhouse effect from atmospheric carbon dioxide, water vapour and methane warms it up by trapping some of the Sun's thermal radiation. No one knows yet if the planet is tidally locked or not either. If it is, this could make it inhospitable to life as we know it.
I mentioned Gliese 581g as the first discovered candidate as a goldilocks planet in my article on the goldilocks zone. There was a great deal of excitement in the science community when this planet was announced in 2010. The planet was detected using radial velocity measurements. Since then, several research teams have been unable to confirm its existence. As mentioned, radial velocity measurements of small mass planets tend to be very weak. They are difficult to verify statistically above background noise. To make these measurements, a series of observations of the electromagnetic spectrum emitted by a star are made. If periodic variations in the spectra occur, they may indicate the radial velocity of the star being altered by a planet's mass as it orbits it. If the data is plotted, the curve will indicate the mass of the planet. Apple even has a new Exoplanet app plotting this curve, for the iPhone, iPad and iPod touch, if you're so inclined. Planets a few times greater than Earth mass and smaller and orbiting Earth-distance or further from their star make these measurements extremely challenging. They are often so weak they get lost in background noise. This may be what happened with Gliese 581g; it may not exist after all. Kepler-22b, on the other hand, has been confirmed by the data from the Spitzer Space Telescope, an infrared space observatory shown prior to its 2003 launch below.
As well, Kepler-22b has been observed long enough to confirm at least three transits (it has an orbital period or year of 290 days), further supporting its existence.
Extasolar planets are, for the most part, simply too far away to observe directly. Fewer than 5% of the planets discovered so far have been directly observed, and those that have are especially large hot planets that emit enough infrared radiation to be directly detected. Everything we know about smaller more Earth-like planets, we must measure indirectly and this makes the atmospheric study of these planets very challenging. The James Webb Space telescope, shown below, scheduled for launch in 2014 is a large infrared-optimized telescope that will succeed both the Hubble and Spitzer space telescopes.
Although it was originally designed to study extremely distant (and old) galaxies, it also will be able to search for and study extrasolar planets in a new way. It may be equipped with a star shade that will work like putting your thumb in front of the Sun. It should be possible then to see a nearby orbiting planet within 1 AU (distance from the Sun to Earth) and further out. It may be able not only to directly image a terrestrial Earth-size planet but also distinguish seasonal changes in its atmosphere due to colour changes and determine its rotation. We may actually be able to see the atmosphere of a distant Earth-like planet!
We are just beginning to understand the complex interrelationship between life and atmosphere. Simultaneously, we are beginning to grasp how atmospheres develop, evolve and behave on various moons and planets within our solar system. We are becoming increasingly sophisticated in our understanding of extrasolar atmospheric possibilities, and how life based on different chemistries could exist in them. Yet it sometimes seems as if we have a lot of un-connectable puzzle pieces to work with. Ongoing research in many different fields is making headway toward a more singular understanding of the connections between atmosphere and life. An understanding of how Earth's atmosphere, and atmospheres in general, works is crucial in helping us ask the right questions, in keeping ourselves pointed in the right direction in other words, as we make our way through this murky and sometimes confusing uncharted territory. As our technology improves, we will undoubtedly make significant headway in the next few years.
A Philosophical (and personal) Note
All of this research centers around a basic and profound question. "Are we alone?" Our basic conceptions about where we fit into this unfathomably vast universe are now being challenged. Maybe it is a good time to pause for some introspection as we approach a critical point at which we may soon discover atmospheric traces of alien life. Are we ready to look over the fence into the backyard of our mysterious neighbour? We've recently made giant advances in science and technology and there is a seductive aura around them that is easy to focus on, so much that we risk losing a larger perspective. I suspect that where we are at as humans will be indicated by our approach: When we find another "Earth" (I personally think it is inevitable) will we pounce on it with our heavy guns of reason and acquisition or will we approach it with more delicate and respectful hands of an ecologist, taking the time and energy to gently unfold and ponder over this new mysterious world's secrets?
If you would to explore this further there are many recently published books to choose from. I recommend one in particular: "Lonely Planets - the natural philosophy of alien life" published in 2004. Astronomer David Grinspoon resumes where Carl Sagan left off exploring the possibility of alien life in this intelligent and thoughtful book.
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