Perhaps the most profound shift in astrobiology has been the discovery of thousands of exoplanets, or planets orbiting other stars. Before the mid-1990s, we knew of none. Thanks largely to missions like the Kepler Space Telescope (launched in 2009), we now know that planets are common, and that rocky, Earth-sized worlds in the "habitable zone" (where liquid water could exist) are likely numbered in the billions in our galaxy alone.

This changed the search for life from a purely theoretical exercise into a statistical and observational science. We've gone from asking if there are other Earth-like worlds to asking which ones we should study first.

## The Discovery of Subsurface Ocean Worlds

The discovery that some of the icy moons in our own outer solar system harbor vast liquid water oceans beneath their frozen shells has revolutionized our search for life nearby. Data from the Galileo and especially the Cassini missions revealed that Jupiter's moon Europa and Saturn's moon Enceladus are prime candidates.

The discovery of massive plumes of water vapor and organic molecules erupting from cracks in Enceladus's ice in 2005 was a watershed moment. We are seeing the contents of a potentially habitable alien ocean being sprayed directly into space—a tantalizing "free sample" for future missions to analyze. These ocean worlds are now considered among the most promising places to find life in our solar system.

## Confirming a Habitable Past on Mars

While finding life on Mars remains an open question, the work of robotic rovers, particularly Curiosity (landed in 2012) and Perseverance (landed in 2021), has fundamentally rewritten our understanding of the Red Planet.

Curiosity's analysis of ancient lakebed deposits in Gale Crater provided the first definitive proof that ancient Mars once had all the necessary conditions for life: persistent liquid water, the essential chemical building blocks (carbon, hydrogen, nitrogen, oxygen, etc.), and an energy source. Perseverance is now taking the next step by actively searching for signs of past microbial life (biosignatures) and collecting the first rock samples for a future mission to return to Earth.

## Analyzing the Atmospheres of Alien Worlds

One of the most exciting new frontiers is our ability to study the atmospheres of distant exoplanets. Using technology pioneered by the Hubble Space Telescope and now perfected by the James Webb Space Telescope (JWST), scientists can analyze the starlight that passes through an exoplanet's air.

This technique, called transit spectroscopy, allows us to determine the chemical composition of that world's atmosphere. It's our most powerful tool for searching for biosignatures—gases like oxygen and methane that, in certain combinations, could indicate the presence of life. JWST is now providing our first real opportunities to "sniff" the air of potentially habitable worlds hundreds of light-years away.

## The Study of Extremophiles on Earth 🦠

Some of the most important astrobiological discoveries have been made right here on Earth. The study of extremophiles—organisms that thrive in conditions previously thought to be lethal—has dramatically expanded our understanding of the limits of life.

Scientists have found life in the boiling, acidic hot springs of Yellowstone, around crushing deep-sea hydrothermal vents with no sunlight, inside solid rock deep underground, and in the freezing, arid deserts of Antarctica. Each discovery proves that life is far more tenacious and adaptable than we ever imagined, broadening the range of environments we can consider "habitable" on other planets and moons.

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