The potential discovery of extraterrestrial life would undeniably be one of science's most monumental findings. With hundreds of millions of potentially habitable planets swirling in our Milky Way, astronomers are on a thrilling quest. They are deploying advanced space telescopes to hunt for molecular hints of life in the atmospheres of the most Earth-like exoplanets.

However, as of now, we've yet to find compelling proof of life beyond our planet. A recent paper claimed to detect a biological signature in the atmosphere of the exoplanet K2-18b, but the scientific community remains cautious. Even the authors of the study hesitate to jump to conclusions about extraterrestrial life.

**Extraordinary Claims Need Extraordinary Evidence**

Renowned astronomer Carl Sagan famously said, "Extraordinary claims require extraordinary evidence." This guiding principle insists that any remarkable assertion—like the existence of life beyond Earth—should be backed by robust data.

As an astronomer and author specializing in astrobiology, I've witnessed many groundbreaking discoveries, but establishing life beyond Earth demands meeting several critical criteria.

**Three Essential Criteria for a Valid Discovery**

To be considered a legitimate discovery and not merely speculation, a scientific finding must fulfill three stringent criteria. So, how does the latest claim about K2-18b stack up?

Firstly, the measurement must capture a significant quantity. The research team utilized the James Webb Space Telescope to observe K2-18b and identified dimethyl sulfide in its atmosphere, a molecule typically linked to biological activity on Earth, particularly from bacteria and oceanic plankton. However, this molecule can also be produced through non-biological processes, leaving its implications unclear.

Secondly, the detection must be strong. Each detector picks up some noise due to electron movement, so the scientific signal needs to clearly stand out from this background. The K2-18b detection achieved a 3-sigma significance—indicating a 0.3% chance of being a random occurrence. While this may sound remarkable, many scientists believe it's a weak signal when compared to the gold standard of 5-sigma, which signifies a mere 0.00006% chance of accidental detection.

Lastly, scientific results must be repeatable. Reliable findings are reinforced when corroborated by independent studies or varied instruments. In the case of K2-18b, further exploration is necessary, with other biological markers like oxygen needed to strengthen claims.

**Debunked Claims Closer to Home**

Historically, tantalizing claims have been made about discovering life closer to Earth, such as those surrounding Mars. Over a century ago, astronomer Percival Lowell insisted he saw artificial canals on Mars—quite an astounding assertion, yet it faltered under scrutiny for lack of robust evidence.

In 1996, a NASA team proposed evidence of life in a Martian meteorite, ALH 84001, showcasing images hinting at microfossils. Still, subsequent research offered alternative explanations, ultimately dimming excitement around that claim.

More recently, astronomers recorded trace amounts of methane on Mars. This gas, like dimethyl sulfide, can be produced by life but is also generated by geological processes. Mixed findings from various missions make it clear: definitive evidence for Martian life remains elusive.

**Hunting for Intelligent Life**

While the detection of microorganisms would be groundbreaking, discovering advanced extraterrestrial civilizations would be the crowning jewel of astrobiology.

The Search for Extraterrestrial Intelligence (SETI) has been ongoing for 75 years, yet no solid signals have emerged. The most famous anomaly, the Wow! Signal received in 1977, was a transient burst that has never been repeated.

Additionally, interstellar object ‘Oumuamua sparked debates about being an alien artifact due to its peculiar shape and trajectory, yet many scientists believe it's just a natural comet. As it stands, no finding has yet satisfied Sagan's stringent criteria.

**The Bigger Picture: Researching the Universe**

These stringent criteria extend beyond life searches to cosmology. A notable case was a 2014 claim of evidence for cosmic inflation—a brief period of rapid expansion post-Big Bang. Later retractions showed the signals were confused with dust in our galaxy.

Conversely, the discovery that the universe is expanding—first posited by Edwin Hubble in 1929 and later determined to be accelerating—stands as a model for scientific rigor. Two independent teams examining supernovae arrived at the same conclusion, dramatically increasing confidence in the results, earning a Nobel Prize.

In both the cosmos and our quest for life, astronomers maintain a commitment to high standards of evidence before accepting any claims. Until then, as the search for extraterrestrial life continues, we will remain in suspense—wondering whether we are alone in the universe.