One of the biggest mysteries of astrobiology is why we've yet to discover any evidence of life elsewhere in the universe. The problem is especially jarring because the more we learn about our own solar system and others, the more we find that wet, rocky worlds with the ingredients and energy sources necessary for life are actually relatively common. So where are all the aliens?

A new theory by Dr. Aditya Chopra and colleagues from Australia National University may offer a solution. He thinks life probably has developed in many places throughout the universe, perhaps even on other planets within our solar system. But the volatility of most planetary environments prevents almost all life from ever evolving. Life hasn't been detected because it almost always goes extinct immediately following its initial emergence.

"The universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens," explained Chopra. "Early life is fragile, so we believe it rarely evolves quickly enough to survive."

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He continued: "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."

For instance, scientists believe that Mars and Venus were habitable planets at one time in the past; life may even have emerged on these planets. But like with most young worlds, these favorable conditions were only temporary. Any life that may have emerged would not have been able to evolve fast enough to stabilize the environment, at least not on a large enough scale.

Gaining a foothold

Chopra and colleagues believe that what made Earth so unique was that, for whatever reason, life here was able to gain a large enough foothold to play a part in stabilizing its own environment. In other words, life itself is the stabilizing force that maintains balanced planetary ecosystems. Meeting such a threshold before planetary conditions change is a very improbable event. That's why life in the universe is rarer than expected.

"The mystery of why we haven't yet found signs of aliens may have less to do with the likelihood of the origin of life or intelligence and have more to do with the rarity of the rapid emergence of biological regulation of feedback cycles on planetary surfaces," said Chopra.

If this theory is correct, it means there might exist countless planets throughout the cosmos full of fossils, but without any actual living organisms. The astrobiologists may need to have their jobs replaced by astropaleontologists.

It all goes to show just how precarious our own existence may be on Earth, in spite of life's success here. The ecosystems that stabilize our world and make it habitable are fragile indeed, maintaining a delicate balance. If it weren't for that balance, Earth too could become unstable. Our dainty blue world's natural state, if left to its own devices, could be to end up like Mars or Venus: vacant, lifeless. Nothing more than an abode for old bones.

Bryan Nelson ( @@brynelson ) writes about everything from environmental problems here on Earth to big questions in space.

Aliens haven't been discovered because they've all gone extinct, says scientist
New theory suggests that life may begin on most planets, but rarely survives long enough to evolve.