The next obvious step is sending a robotic probe to take a closer look, something NASA and its Russian counterpart seem interested in doing. The U.S. space agency was already considering going forward with two different Venus missions, either or both of which could potentially be modified to look for phosphine or attempt a more direct detection of alien organisms.

Russia, meanwhile, has a long history of sending robotic spacecraft there, and this past week, the head of Roscosmos, that country’s space agency, called Venus a “Russian planet.”

But both agencies could also get scooped. The day after the news broke, a privately funded organization backed by Russian billionaire investor Yuri Milner, called Breakthrough Initiatives, announced that it had enlisted a team of top scientists to study ways to pull off a near-term Venus exploration mission.

“We just want to do something small and fast and focused,” said Sara Seager, an astrophysicist at MIT who is leading the Breakthrough study and is also a co-author of the paper in Nature Astronomy that announced the detection of phosphine.

“Can we send a microscope and look for life directly?” she asked. It’s conceivable, she said. “Obviously, we’d love to see little microbes swimming around.”

Like other scientists, Seager knows that any claim of a discovery of extraterrestrial life would require tremendously persuasive evidence. Finding phosphine is not the same thing as finding hard evidence of life.

On Earth, the molecule can be produced through the metabolic processes of life as we know it, and through industrial manufacturing. The scientists who reported the phosphine discovery on Venus said they could think of no natural explanation, other than the presence of life, for the abundance of the molecule. And they detected it in the part of the Venusian atmosphere assumed to be most congenial to life.

“All we can say is we have a confident detection of phosphine gas at 20 parts per billion coming from 50 to 60 kilometers from the surface, where the temperatures are not too hot, not too cold, but just right for life,” Seager said.

What kind of life? No one knows. The Venusians might have a truly alien biochemistry adapted to one of the harshest environments in the solar system.

Venus is so hot — about 900 degrees Fahrenheit — that the rocks literally glow at the surface. But this was not always the case.

Venus, Earth and Mars all formed more than 4 billion years ago, and for a good chunk of their early history, they had many similarities, including moderate temperatures that allowed water to be liquid at the surface. It is conceivable that life existed on all three planets several billion years ago.

But Venus’s location so near the sun turned out to be disastrous. As the young sun aged, it grew hotter. Venus’s oceans eventually boiled away. That flooded the atmosphere with carbon dioxide, which had previously been largely dissolved in the ocean. The result was a runaway greenhouse effect.

Astrobiologist David Grinspoon of the Planetary Science Institute in Tucson notes that the reflectivity of Venus’s thick cloud cover is what makes the planet so bright in our night sky. The clouds are the result of ongoing volcanic activity, he said.

“So with your naked eye, you can confirm the presence of volcanic activity on our twin planet,” Grinspoon said in an email. “It’s hot enough on the surface to melt lead and zinc and some aluminum alloys. The Soviets sent a metallic bust of Lenin in one of their early landers, which must have quickly become a puddle.”

But higher in the atmosphere, the temperature is more benign. In 1967, astronomer Carl Sagan speculated that microbes could survive in the Venusian clouds. Because the clouds are shot through with sulfuric acid, however, these microbes would probably need a protective shell.

The Soviet Union had a special fascination with Venus and sent pioneering space probes there as part of its Venera program starting in the 1960s. The Soviets eventually managed to land six spacecraft on Venus, and they sent back images of a stark, rocky landscape.

“By the end of the ’60s, the romance of Venus was gone,” said Sanjay Limaye, a University of Wisconsin at Madison planetary scientist who, like Grinspoon, is a longtime proponent of the Venusian life hypothesis.

Another Venus advocate is Darby Dyar, a planetary scientist at Mount Holyoke College and the chair of the Venus advisory committee for NASA. She knows her favorite planet is overshadowed by Mars, estimating the number of Venus specialists as no more than a tenth of those for Mars. There hasn’t been a major mission to explore Venus in a quarter-century, she said.

But Venus, she argues, is unfairly overlooked, and the planetary science budget should be more balanced, in part because new research suggests that Venus probably had a congenial environment for several billion years.

In fact, she said, Venus probably had oceans before Earth did. It had plenty of time for life to evolve — potentially into complex organisms. It’s not out of the question that the rocks of Venus have fossils, she said.

“The probability of there ever having been life on Venus has skyrocketed,” Dyar said, referring to research on the longevity of Venusian oceans. “There are atmospheric models that show that Venus had as much water as Earth and had it in fact earlier.”

Dyar is one of the scientists pushing for a mission called Veritas, which would send a probe to Venus to map its surface. It is one of four proposed planetary missions that are competing for NASA funding as part of the agency’s Discovery Program, which backs relatively low-cost robotic explorations of the solar system.

“Right now, we know the topography of Pluto” — the dwarf planet in the exurbs of the solar system — “better than we know the topography of Venus,” she said.

Another proposed mission, DaVinci+, which is under development at NASA’s Goddard Space Flight Center in Greenbelt, Md., would send a probe through the atmosphere of Venus and to the surface with the goal of understanding the history of water on the planet. According to NASA, the mission’s instruments would be “encapsulated within a purpose-built descent sphere to protect them from the intense environment of Venus.” The mission would send back images of the surface.

The other two planetary missions competing for dollars are Trident, which would send a probe to study Neptune’s large, icy moon Triton — which may have a subsurface ocean — and the Io Volcano Observer, or IVO, which would study one of the four large moons of Jupiter.

NASA is expected to make its selection next April. The agency’s reaction to the phosphine announcement was notably cautious. Its administrator, Jim Bridenstine, posted to his blog a description of the many astrobiology programs backed by NASA but did not promise any special treatment for Venus.

A separate statement from the agency was similarly equivocal, but it mentioned one of the attractive features of Venus as a target of exploration: It is “a planetary destination we can reach with smaller missions.”

Proximity, in other words, is a cosmological virtue.