What Is Astrobiology?

Astrobiology is the study of life in the Universe. The search for life beyond the Earth requires an understanding of life, and the nature of the environments that support it, as well as planetary, planetary system and stellar interactions and processes. To provide this understanding, Astrobiology combines the knowledge and techniques from many fields, including Astronomy, Biology, Chemistry, Geology, Atmospheric Science, Oceanography and Aeronautical Engineering. Astrobiologists can work alone on particular scientific questions, but often Astrobiologists from different scientific disciplines work together to examine complex questions that no one field can answer alone.

These questions cover topics such as:

1. How does life originate?
2. How does life evolve?
3. What kind of environment is necessary for life to survive?
4. What are the environmental limits or “extremes” under which life can survive?
5. What might life look like on another world?
6. Is there or has there been life elsewhere in our solar system?
7. How can we observe and identify a habitable – or even inhabited – world?
8. What is humanity’s future on Earth and beyond?

The World of Astrobiology

While the United States of America (USA) and NASA have pioneered many lines of Astrobiological study, we are hardly alone – as is only proper for an effort to address such enormous and universal questions. Few Astrobiology missions launch without significant international partnership, and the trend is for ever greater interdependence. The much-anticipated mission to bring to Earth the rock and grain being now being cached by the Perseverance rover in Jezero Crater will feature major roles for both NASA and European Space Agency (ESA).

The Chinese Space Agency now has a rover on Mars as well, and is planning for the Tianwen-3 mission to collect Mars samples for transport back to Earth. Another high-profile example is the ExoMars mission headed by the European Space Agency (ESA), which includes a series of missions, the first phase of which was the Trace Gas Orbiter that launched in 2016. One of the major efforts of the Trace Gas Orbiter is to look for methane on Mars, a key question for NASA as well. Astrobiology cooperation is often intellectual as well as operational.

The next step for ExoMars is the delivery of the Rosalind Franklin rover to the Martian surface, which initially included input from the Russian Space Agency (Roscosmos) and NASA. The ExoMars rover cooperation was suspended in March 2022, but efforts are underway to find alternatives.

Astrobiology may sometimes seem most defined by high-profile missions, but often those missions represent years of prior theoretical and laboratory work. And once data from missions have been obtained, more lab analysis and testing are necessary. The data is then challenged and critiqued by colleagues before the results can be released as a significant finding. For instance, it took almost two years of intensive lab work and data analysis before members of the Curiosity science team could announce that they had teased out from Mars drill samples the presence of long-sought organic compounds, the building blocks of life as we know it.

Astrobiology, a multidisciplinary field dealing with the nature, existence, and search for extraterrestrial life (life beyond Earth). Astrobiology encompasses areas of biology, astronomy, and geology.

Although no compelling evidence of extraterrestrial life has yet been found, the possibility that Biota might be a common feature of the universe has been strengthened by the discovery of Extrasolar planets (planets around other stars), by the strong suspicion that several moons of Jupiter and Saturn might have vast reserves of liquid water, and by the existence of microorganisms called Extremophiles that are tolerant of environmental extremes. The first development indicates that habitats for life may be numerous. The second suggests that even in the solar system there may be other worlds on which life evolved. The third suggests that life can arise under a wide range of conditions. The principal areas of astrobiology research can be classified as (1) understanding the conditions under which life can arise, (2) looking for habitable worlds, and (3) searching for evidence of life.

For life like that on Earth (based on complex carbon compounds) to exist, a world must have liquid water. Because planets either too close to or too far from their host stars will be at temperatures that cause water either to boil or to freeze, Astrobiologists define a “habitable zone,” a range of orbital distances within which planets can support liquid water on their surfaces. In the solar system, only Earth is inside the Sun’s habitable zone. However, photographs and other data from spacecraft orbiting Mars indicate that water once flowed on the surface of the red planet and is still present in large quantities underground. Consequently, there is a sustained international effort to use robotic probes to examine Mars for evidence of past, and even present, life that could have retreated to subsurface, liquid aquifers.

Also, discoveries primarily due to the Galileo space probe (launched in 1989) suggest that some of the moons of Jupiter—principally Europa but also Ganymede and Callisto—as well as Saturn’s moon Enceladus, might have long-lived liquid oceans under their icy outer skins. These oceans can be kept warm despite their great distance from the Sun because of gravitational interactions between the moons and their host planet, and they might support the kind of life found in deep sea vents on Earth.

Even Titan, a large moon of Saturn with a thick atmosphere, might conceivably have some unusual biology on its cold surface, where lakes of liquid methane and ethane may exist. The European space probe Huygens landed on Titan on January 14, 2005, and saw signs of liquid flow on its surface. Such discoveries as these have strongly promoted the emergence of astrobiology as a field of study by broadening the range of possible extraterrestrial habitats far beyond the conventional notion of a “habitable zone.”

An additional impetus has been the discovery since 1995 of hundreds of Extrasolar Planets around other normal stars. Most of these are giant worlds, similar to Jupiter and therefore unlikely to be suitable for life themselves, although they could have moons on which life might arise. However, this work has shown that at least 5 to 10 percent (and possibly as much as 50 percent or more) of all Sun-like stars have planets, implying many billions of solar systems in the Milky Way Galaxy. The discovery of these planets has encouraged astrobiology and in particular has motivated proposals for several space-based telescopes designed:

1. to search for smaller, Earth-size worlds and

2. if such worlds are found, to analyze spectrally the light reflected by the planets’ atmospheres in the hope of detecting oxygen, methane, or other substances that would indicate the presence of Biota.

While no one can say with certainty what sort of life might be turned up by these experiments, the usual assumption is that it will be microbial, as single-celled life is adaptable to a wide range of environments and requires less energy. However, telescopic searches for extraterrestrial intelligence (SETI) are also part of astrobiology’s extensive research palette.