If you rank the most habitable places in our solar system Venus lands pretty low, with surface temperatures hot enough to melt lead and sulphuric acid rain.

And yet it may have just jumped to the front of the pack.

In fact we may have detected the signature of alien life - Venusian life -for the first time.

Our searches for life-beyond Earth have tended to focus on the Martian subsurface and the ocean moons Enceladus and Europa and even the methane lakes of Titan.

And yet the horrible hellhole that is the atmosphere of Venus has just yielded perhaps the most exciting lead for extraterrestrial life.

Arguably, it’s our only lead - the faint signature of the chemical phosphine in Venus’s atmosphere, which may have been produced by living organisms.

I joked more that the weirdest end to this very weird year would be the discovery of alien life.

Let’s see if 2020 can make up for itself.

Now, before we get to the evidence, let’s talk about Venus.

It’s the brightest thing in the sky besides the sun and moon, hanging just above sunrise and sunset.

It’s bright because it’s close and it’s big, at 90% Earth’s diameter, and its our closest planetary neighbor.

In many ways it’s Earth’s twin - it’s even inside that band around the Sun where liquid water is possible - the so-called habitable zone.

But for the longest time Venus was thought to be among the LEAST habitable places in the solar system.

See, Venus has what we call a runaway greenhouse effect.

Its atmosphere is 100 times the mass of Earth’s and mostly carbon dioxide.

This traps so much heat that the surface of Venus reaches a temperature hot enough to melt lead, and with a pressure that would crush most submarines.

That atmosphere also sports a permanent thick layer of sulfuric acid clouds, So yeah, when we realized just out just how awful Venus was, our telescopes and hopes for finding ET swiveled outwards.

To the outer solar system - Mars, Enceladus and Europa in particular, and ultimately to planets around other stars.

But Venus wasn’t ready to give up our attention so easily.

Our landers and orbiters - Venera, Pioneer, and Magellan - caught hints of unusual atmospheric chemistry.

Various sulphur-bearing molecules were out of the expected equilibrium quantities.

But non-living, or abiotic processes like volcanism could still explain these.

The other weird thing is that the clouds of Venus appear to absorb the Sun’s light in a weird way - more short wavelength visible and UV light is sucked up than expected, leading to the yellow colour of the clouds and dark patches that change over time.

This could be due to dust churned up from the surface - or it could be gigantic colonites of microbial life.

Although the surface is completely hellish, at around 50km altitude both the temperature and pressure are close to those at Earth’s surface, with the only problem being that pesky sulphuric acid rain.

But compared to the death-zone of Venus’s surface, it’s a paradise, and we talked about the possibility of human cloud-city settlements in a previous episode.

In 1967, Carl Sagan and Harold Morowitz were among the first to suggest that life might exist permanently aloft in Venus’s relatively habitable upper atmosphere.

These guys speculated about both microbes and larger creatures, alien gas bags perhaps supported by in-built hydrogen balloons.

But all these ideas were still pretty fringe.

The attention of ET-hunters remained focused outwards.

One of the most promising avenues in the search for life is to detect so-called atmospheric biosignatures - chemicals in a planet’s atmosphere that are very hard to explain without the presence of life.

There are lots of ways to do this - for example seeing the effect on a star’s light as it passes through its own planets atmospheres.

Another possibility is to look for the absorption of the planet’s own light as it emerges from deep within its atmosphere.

This can be done at far infrared and submillimeter radio wavelengths where the star’s own glare doesn’t kill the signal.

One possible biosignature in this range is phosphine, which absorbs photons of around 1.1mm wavelength.

Phosphine is produced in abundance by some microbes, but not so easily produced by non-biological processes.

A team of researchers led by Professor Jane Greaves, mostly out of the United Kingdom, decided to explore phosphine as a possible biosignature.

They thought they’d take a look at Venus using the James Clerk Maxwell telescope - more as a control to help guide their studies of planets beyond our solar system, but not really expecting to see anything so close to home.

But to their extreme surprise, they found phosphine.

The team followed up with ALMA - the Atacama Large Millimeter/submillimeter Array - and confirmed that there had to be phosphine in the upper atmosphere of Venus.

Vastly more than expected.

So there are two big questions here.

Is the detection of phosphine real?

And if so, how likely is it that the phosphine came from life?

Well, the fact that both JMCT and ALMA detected the feature makes it pretty convincing that it’s real, although there’s still a chance that both observatories just happened to see a dip in the exact spot where phosphene produces an absorption line.

Followup observations will confirm or refute this pretty quickly, but the detection is looking pretty solid at this point.

Okay assuming it is real, does this mean we’ve found life?

Well, let’s first discuss why the presence of a simple molecule might get people excited about the possibility of life.

So phosphine is a tetrahedtron - pyramid or d-4 - shaped molecule with one phosphorus and 3 hydrogen atoms.

It's a common byproduct of living metabolisms, although its also highly toxic.

Now you don’t need biological processes to make the stuff.

It can form anywhere that you might have free phosphorus and hydrogen atoms.

That means anywhere that the more stable phosphorus and hydrogen-containing molecules might be broken apart.

For example, it’s formed deep beneath the surfaces of Jupiter and Saturn and carried to the surface.

We’ve spotted the characteristic absorption features of phosphine in the two gas giants - but no one screamed life, because there’s a clear mechanism for producing that phosphine by abiotic processes.

So why the excitement with Venus?

Well, although phosphine can be produced in a number of different ways, it rarely lasts long no matter how it’s made.

The stuff is very quickly oxidized - particularly in the highly acidic atmosphere of Venus.

That means something on Venus is making phosphine faster than it can be destroyed.

On Venus there are a few ways to make this stuff - you’d definitely get some in the extreme heat and density near the surface.

You might get some produced by lightning strikes in the cloud layers, or by cosmic rays hitting the upper atmosphere.

Most of the non-biological production is likely to happen at the surface, and would take years to diffuse to the upper atmosphere.

However the scientists calculate that almost all of the phosphene would be destroyed in that time.

The abiotic production rate of phosphine is expected to be 10,000 to a million times too low to produce the amount of phosphine that was observed.

So we’re left with two possibilities - either there’s some unknown non-biological process that produced this phosphine, or Venus has life.

Honestly, the former still seems the more likely because, well, it’s never aliens until every other possibility is ruled out.

But this may be the most promising lead to extraterrestrial life we’ve ever had, so let’s talk about what that life might look like.

Almost certainly it would be floating microbes like Sagan and co. talked about half a century ago.

But the requirements for life in the clouds of Venus are pretty strict.

The entire lifecycle would have to remain in the regions with survivable temperature and pressure, and the critters need to somehow survive the crazy acidity.

Sara Seager from MIT and collaborators proposed a lifecycle that might just do the trick.

Firstly, they assert that these critters are microbes living in tiny droplets.

Liquid environments are universally thought to be essential to life, and in the extreme dryness of the Venusian atmosphere free-floating microbes would quickly dry out.

But those droplets would be mostly hydrochloric acid - maybe 85% concentration, with the rest water.

By comparison, the most extreme extremophiles on Earth can survive in pools with something like 5% concentration of sulphuric acid - in the Dallol pools in Ethiopia.

So these Venutian microbes would have to be very different to anything found on Earth.

These droplets can only exist in a narrow range in the atmosphere.

Droplets are expected to form at a particular height, but as they find each other and grow they begin to fall back down into the crushing oven of the lower atmosphere, where they quickly evaporate again.

So how does a droplet-dwelling organism survive this?

Seager and co. have an idea.

What if the microbes enter a resilient spore state as their droplets descend and dry out?

Earth bacteria can transform into dormant states in adverse conditions - bacterial spores.

We previously talked about how these spores can survive crazy conditions - including the vacuum of space and the extreme temperatures and pressures of meteor impact or atmospheric reentry.

So what if Venusian microbes passed through such a spore state in the lower atmosphere?

The life cycle would look like this: microbes are in a metabolic state in sulphuric acid droplets in the upper atmosphere.

Those droplets find each other and grow.

Perhaps microbes multiply in this phase.

As they fall and their droplets evaporate they enter a spore state.

These spores are extremely tiny and light, and so they float in the haze below Venus’s cloud banks.

Updrafts then carry spores into the temperature range once again where they act as nucleation centers for new droplets to form, at which point they become metabolically active again.

And, like, churn out a bunch of phosphene I guess.

OK, nice story.

The main stretch here is the whole sulphuric acid thing.

We just don’t know if life can exist in those conditions.

An important point here is that Venus’s hellish conditions are relatively young only perhaps only around 700 million years old.

And before that they likely had oceans of liquid water.

Any lifeforms in its atmosphere today must have evolved from presumably much more sensible critters that existed prior to the runaway greenhouse effect.

Could microbes really evolve to withstand the transition from a watery to a sulphuric-acid-y environment?

I guess we’re about to find out.

And how exactly will we find that out?

Well, the first thing is to confirm the presence of phosphine with more observations.

The previous observations were relatively brief, but there’s now motivation to point our telescopes for much much longer at Venus.

We'll need to look for other phosphine absorption features also, and entirely different biosignatures.

But ultimately we’re going to want to go to Venus, to search for life signatures there or, even better, to bring samples back to Earth.

NASA is about to decide between two proposals for the next mission to Venus.

Neither mission has a direct life-detecting instrument, but that might change given these developments.

However it may be time to refocus our solar system explorations.

Ultimately we want to bring back samples of the Venusian atmosphere - to actually get microbes under a microscope, if they exist.

The discovery of alien life in our nearest planetary neighbor would totally change our calculations about the frequency of life in the universe.

We may be forced to conclude that life is cosmically abundant.

2020 is the year of weird microbes - but this latest one, if it’s real, may prove extremely good news - granting us a grander perspective on humanity’s place in a life-filled space time.