Venus, the second-brightest object in the night sky after our own moon, could harbor the potential to change our notion of life in the cosmos.

An international team of researchers is dusting off a theory first outlined in a 1967 paper co-authored by cosmologist Carl Sagan that touted the clouds of Venus as a favorable habitat for extraterrestrial microbial life. Unlike the surface of Venus — where the average temperature is a scorching 864 degrees Fahrenheit — the lower cloud levels of Venus range between 86 and 158 degrees F and contain sulfur compounds, carbon dioxide and water. They also feature something odd: unexplained dark patches composed of sulfuric acid that persist for days and change their shape.

In a new study published in the journal Astrobiology, the researchers theorize that these dark patches may be alien microbial life akin to similar species here on Earth.

"On Earth, we know that life can thrive in very acidic conditions, can feed on carbon dioxide, and produce sulfuric acid," Rakesh Mogul, a biological chemistry professor who co-wrote the paper, told Phys.Org.

Venus, the blue marble

The blackened, scorched surface of Venus as captured by the Soviet spacecraft Venera 13 in 1981. The blackened, scorched surface of Venus as captured by the Soviet spacecraft Venera 13 in 1981. The craft lasted about 127 minutes before succumbing to the planet's extreme surface temperatures. (Photo: Soviet Academy of Sciences)

While today's Earth is nicknamed the "blue marble," it hasn't always laid claim to that title. Billions of years ago, when the sun was 30 percent dimmer and the Earth was likely covered nearly entirely by ice, Venus may have been a warm and wet water world. A 2006 mission by the European Space Agency's Venus Express spacecraft backed up this theory with the discovery that the trace gases given off by the planet contained twice as much hydrogen as oxygen. It also detected high levels of the isotope deuterium, a heavier form of hydrogen that's common in Earth's oceans.

"Everything points to there being large amounts of water in the past," Colin Wilson, a member of the Venus Express science team, told Time.

According to the researchers, habitable conditions on Venus may have persisted for as long as 750 million years, with surface water lingering for as long as 2 billion years. Such an extended run before the sun warmed and greenhouse gases turned the planet into an inferno may have given rise to life. As study lead and planetary scientist Sanjay Limaye noted, this habitable time period is even longer that the one enjoyed by Mars.

"Venus has had plenty of time to evolve life on its own," he said.

Aliens aloft

Venus as captured in ultraviolet light by the European Space Agency’s Venus Express spacecraft. The dark streaks in the clouds feature the absorption of ultraviolet light by an unknown material. Venus as captured in ultraviolet light by the European Space Agency’s Venus Express spacecraft. The dark streaks in the clouds feature the absorption of ultraviolet light by an unknown material. (Photo: ESA)

While microbial alien life aloft in Venus's atmosphere sounds outlandish, it's actually something that happens here on Earth. Scientists using specially-equipped balloons have previously discovered terrestrial microorganisms carried by winds as high as 25 miles above Earth's surface. The researchers studying Venus's clouds theorize that "atmospheric nutrient transport mechanisms" in the form of surface winds may exist to help carry nutrient-rich minerals to airborne colonies of microorganisms. The right conditions, similar to what would encourage algae blooms here on Earth, may also contribute to the strange episodic dark patches seen in the planet's cloud tops.

The researchers say the next step to proving whether Venus may host life in its atmosphere is to recreate similar conditions here on Earth. To that end, they propose building a specialized chamber to simulate the atmospheric and physical conditions of the clouds, seeding them with "sulfur-metabolizing, acid-tolerant, and/or radiation-tolerant microorganisms" and analyzing their survival.

The next step is to send a probe to literally glide through Venus's clouds and analyze those intriguing dark streaks. Aerospace company Northrop Grumman has already developed an unmanned aerial concept vehicle with a wingspan of over 180 feet and solar-powered propellers that could effectively cruise around the planet's atmosphere for as long as a year.

"To really know, we need to go there and sample the clouds," added Mogul. "Venus could be an exciting new chapter in astrobiology exploration."

You can see a concept for the Venusian UAV in the video below.

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.