It is the refrain of teachers everywhere preparing their students for standardised tests: 鈥淢ake an educated guess.鈥� An answer is always better than no answer, they say, and if you can apply what you know to the question to rule out some of the choices, even better. Of course, scientific research isn鈥檛 a multiple-choice exam, and its bedrock is the idea that an educated guess without solid evidence just isn鈥檛 good enough. But the concept is still a useful one, particularly when our questions outpace the technology we currently have to answer them.

The key part of the phrase 鈥渆ducated guess鈥� is the first word. Put another way, a guess based on nothing may be worse than no guess at all. 鈥淏ut Leah,鈥� I hear you asking, 鈥淒o you have a framework, or a favourite example, for educated guesses?鈥� And just because you are clamouring for it, and I鈥檓 the kind of person who has favourite equations, I will tell you about it. The Drake equation, which turns 65 years old next month, is an incredibly useful tool for framing our thinking about life in the cosmos 鈥� even though, at its core, it is simply a method for making an educated guess.

Astrobiologist Frank Drake came up with his eponymous equation in 1961, before convening the first scientific conference on the search for extraterrestrial intelligence (SETI). According to written by his daughter, science journalist Nadia Drake, he sketched out the equation on a bit of a whim to get the few attendees of the meeting talking and was, even into his later life, astonished by how important it is now considered.

The equation has seven key terms, and when multiplied together, they provide an estimate of N: the number of intelligent civilisations in the Milky Way galaxy with whom we might someday be able to communicate. It looks like this:

N = R_* 脳 f_p 脳 n_e 脳 f_l 脳 f_i 脳 f_c 脳 L

The first few terms are relatively simple. R_* is the rate of star formation in our galaxy, which has been measured at about 1 to 3 stars per year. The fraction of those stars with planetary systems is f_p, and we now think that number is about 1 as well 鈥� most stars seem to have planets. The number of planets per star with an environment suitable for life is n_e, and that is where things start to get more complicated. 鈥淪uitable for life鈥� is a broad idea, and one that鈥檚 difficult to define. Even the most basic requirement 鈥� that the planet be in its star鈥檚 habitable zone, where temperatures allow liquid water on the surface 鈥� is required for Earth-like life, as far as we know, but it鈥檚 certainly possible that other lifeforms have different needs. In our own solar system, there are either 2 or 3 planets in the habitable zone, depending how you slice it 鈥� but Venus and Mars don鈥檛 seem particularly suitable for life, and there are several moons in the outer solar system that may well be.

That is the first time the equation forces us to make an educated guess. The second comes hot on its heels with f_l, the fraction of the habitable planets that go on to develop life. On this, we have an even smaller sample size. We haven鈥檛 found life anywhere but Earth, so this number could be very small, or it could be 1 鈥� remember, this includes life that arises at any point in the history of the cosmos. But until we do find life elsewhere, it will be more of a philosophical argument than anything else. The same is true of the next two terms of the Drake equation: f_i, which denotes the fraction of life that is intelligent 鈥� the definition of which is also wily 鈥� and f_c, the fraction of intelligent lifeforms that eventually broadcast their existence into space and are thus capable of communicating with us. The final term is L, the length of time in which a civilisation of intelligent lifeforms sends detectable signals out into space. As the astronomer Carl Sagan pointed out, this depends heavily on whether such civilisations can avoid self-destruction, which isn鈥檛 something we can know for sure 鈥� but we make our best guess based on the only evidence we have: human history.

If you combine all the lowest estimates, N could be one-trillionth or even lower, implying that we are totally alone in our galaxy. If you combine the highest estimates, there could be tens of millions of civilisations in the Milky Way that we could communicate with, which to me would say it鈥檚 only a matter of time before we find them. At Drake鈥檚 original SETI meeting, the researchers set N between 1000 and 100,000,000, and the lower end of that sounds about right to me 鈥� an estimate that, like those at the meeting, is based largely on my not-particularly-scientific opinions about how long our civilisation might last.

But when it comes down to it, the number itself isn鈥檛 the important part of the equation; the important part is the process of estimating each of its terms. In doing so, we force ourselves to consider some of the biggest questions in science: How similar are other places in the universe to our own cosmic neighbourhood? How likely are those most-similar places to develop life? What is life, anyway? What does life truly need? What is intelligence? How can we search for it? And how long will we be capable of searching for it? None of those questions are answered definitively, but pondering them can help us direct our scientific research 鈥� and more than that, it gives us an idea of our place in the grand scheme of the universe. That鈥檚 why teachers tell their students to make an educated guess, and it鈥檚 why the Drake equation is still considered so important. The value in an educated guess is in taking account of what you know and what is still out there, waiting to be known.

I never start writing a science fiction book without first asking scientists a lot of extremely strange questions. My next novel 鈥� A Wall Is Also a Road, which comes out on 6 October 鈥� was a particularly tough case. I decided that my main character would be a cellular slime mould who is also a graduate student. Why not? I certainly spent most of graduate school feeling like an amoeba.

Gardenpath, the slime mould in question, is studying environmental science at a university that floats high above our galactic plane in a gaseous region known as the circumgalactic medium. You may not have heard of it, but this roughly spherical space is where the Magellanic clouds orbit 鈥� two dwarf galaxies near the Milky Way 鈥� along with rogue stars, planets and other chunks of random crap squirted out by Sagittarius A*, the black hole at the centre of our own galaxy. It鈥檚 full of all kinds of stuff that could support an advanced civilisation, from metals to water.

Before I could even begin to imagine what Gardenpath looked like, I needed to understand where they live. So I called up at the Space Telescope Science Institute in Maryland, whose lab made stunning simulations of the circumgalactic medium (you can see them on the web at ). She gave me a tour of this distant region, which is full of spots where the swirling gas is warmer or colder, almost like weather.

I decided that the slime moulds would experience the circumgalactic medium the way humans experience the atmosphere on Earth. Their single-celled bodies could be absolutely enormous in this low-pressure environment full of diffuse gases. Living in this region of space would affect the slime moulds鈥� understanding of what is scientifically possible, too. That鈥檚 why Gardenpath鈥檚 fellow scholars think the galactic plane 鈥� where we live 鈥� is utterly lifeless and uninhabitable. Who could possibly survive in that radiation-saturated gravity well?

Gardenpath literally fumes when their professors scoff at their proposed area of study: a little planet inside one of the Milky Way鈥檚 spiral arms. But they don鈥檛 express rage by emitting sound waves that agitate molecules in a soupy, thick atmosphere like we do. Instead, they spurt electrons, hormones and other chemicals, opening receptors in their membrane to receive answers. On Earth, we call this cellular signalling. To Gardenpath, it鈥檚 just talking.

, a molecular biologist at Northern Illinois University, spent a long afternoon with me on a video call brainstorming about what kind of culture would develop among polymorphous people who communicate by swapping smells through apertures in their skin. A diffuser would be like a megaphone, I suggested, and a specific gas like methane could serve as a question mark. Swingley added that the slimes would need an organelle for storing memories, which I dubbed a gnosome. Because their membranes are permeable, Gardenpath鈥檚 professors can actually reach inside their body to probe their gnosomes during a test, which is just as creepy as it sounds.

Cells have many single-purpose receptors, which led us to think about something I called an 鈥渙ath receptor鈥�. Think of it as a biological friendship ring. When Gardenpath鈥檚 classmate Oriel wants to become BFFs forever, the two of them reach inside each other to create special receptors that can only receive their friend鈥檚 signals. It would be as if your bestie gave you a set of custom ears that can only hear their voice.

What else do slime mould graduate students need? A way to publish their findings, of course. The slimes store data inside fungi, so their libraries look like nurseries packed with containers of mushrooms. Want to send some data over the network? Plug your fungus notebook into a vent and release a bunch of spores that will bring your information to its destination.

I also created slime mould body language. These blobby little guys get bumpy when they are nervous and stretch up tall to laugh. They sprout more eyestalks when something intrigues them and dangle an extra-long arm to add a sarcastic edge to whatever they are emitting.

This makes things especially weird for Gardenpath on Earth, because humans look like people who are laughing sarcastically while paying very little attention to anything with their paltry two eyes. Still, they manage to find a friend among these weird creatures, a woman named Murtis who lives in a city called Pompeii, at the foot of a very intriguing volcano.

Don鈥檛 worry 鈥� I talked to a lot of archaeologists to get ancient Roman culture just right. Thankfully, Pompeii expert at the University of Massachusetts Amherst walked me through Pompeiian plumbing and told me about the famous 鈥減rosciutto sundial鈥� discovered in Herculaneum, a site nearby. If you want to know how that fits into the story, you鈥檒l have to read it!

The US Department of Defense (DoD) released a trove of files on UFOs. The files include images along with government documents and correspondence, some of which were classified until now.

鈥淭hese files, hidden behind classifications, have long fueled justified speculation 鈥� and it鈥檚 time the American people see it for themselves,鈥� said US Secretary of Defense Pete Hegseth in a statement on where the files are displayed.

The images are mostly photographs taken by members of the US military showing small dots or indistinct shapes in the sky. Of more interest are the hundreds of pages of files relating to UFOs, also called unidentified anomalous phenomena (UAPs), from the FBI, Air Force and various other government departments.

Many of the pages are correspondence between the government and concerned members of the public. Some are pamphlets from special-interest groups such as the Amalgamated Flying Saucer Clubs of America聽or the Fraternity of Cosmic Sons and Daughters, and others are requests from children to the head of the FBI 鈥� many to J. Edgar Hoover, who served in the role from 1935 to 1972聽鈥� for help with school projects.

Letters from UFO enthusiasts through the years, from the 1940s to now, show remarkable similarities in the sentiments they express: a feeling that UFO sightings have been mounting, that the government must be hiding something and that they are sure to be persecuted for saying as much.

The responses to these letters, and the internal government communications that have been released, seem to show something different 鈥� many thousands of reports of UFO sightings have been taken seriously and investigated, and there has been no indication that any of them have been extraterrestrial. This mirrors the 2023 results from NASA鈥檚 task force on UAPs, which found that normal aircraft and weather phenomena account for most reports, with only a few remaining unexplained due to blurry images and low-quality data.

The documents that have made the biggest splash are images and transcripts from NASA鈥檚 Gemini 7, Apollo 12 and Apollo 17 missions, each referencing some bright lights that the astronauts saw in space and could not explain. Most of these sightings have already been investigated and explained as micrometeoroid impacts on the moon or the spacecraft, bits of floating debris, and camera or film defects, although a few have remained unexplained thus far.

The DoD has opened a new investigation into the images that remain unexplained, and has been directed by the administration of President Donald Trump to 鈥渃onduct separate reporting鈥� on any unresolved UAP cases. According to the website, this is only the first of many rolling document drops expected 鈥渆very few weeks鈥� as more files are found and declassified.