"Most early planetary environments are unstable. To produce a habitable planet, life forms need to regulate greenhouse gases such as water and carbon dioxide to keep surface temperatures stable," he said.
PHOTOS: How Aliens Can Find Us (and Vice Versa)
Unlike Earth, most worlds will likely not find this balance, ultimately succumbing to being cooked by a runaway greenhouse effect (like Venus) or frozen by a thinning atmosphere (like Mars). Life will often not be fortunate enough to win the race against environmental fluctuations to become a stabilizing factor.
Earth, which already has the stunning fortune to exist at just the right spot around a stable star, spawned life and that life had a role to play in stabilizing its atmosphere as it evolved over that last 4 billion years.
"Life on Earth probably played a leading role in stabilizing the planet's climate," said co-investigator Charley Lineweaver, also from ANU.
And this could be why we're not finding a galaxy filled with alien life - just because there's a habitable world out there, it doesn't mean it's suitable for life for long. It's yet another hurdle against life from gaining a foothold.