Science is often easier to learn and understand when its abstract ideas are presented through the lens of our everyday world. Indeed, a key challenge for science teachers is to make the subject as interesting as possible to students, usually by showing its applications to real-world problems. As a former college lecturer turned science journalist, Trisha Muro is very well suited to this task.

To introduce her new book It’s (Just) Rocket Science: Exploring Physics Through Spaceflight Missions, Muro recalls how, while teaching high-school physics, she had a student drop out of her course because they “thought it would be more like ‘story’ physics”. Muro wasn’t exactly sure what the student meant, but the phrase stuck with her over the years, eventually leading her to write this book. Within its pages, Muro presents quintessential physics concepts in the context of spaceflight. She touches upon everything from the Apollo space programme to the Mars rovers, to provide the narrative aspect that makes abstract science relatable.

I’d argue that rather than story, the examples she uses in the book are more like case studies, as there is not much of a continuing narrative. Indeed, I couldn’t help but wonder if by “story”, Muro’s student meant science-fiction physics rather than real-life examples, but that’s semantics. What Muro has written is a very effective guide to all manner of basic physics that draws upon all her experience as a physics teacher and all her skills as a science writer.

In It’s (Just) Rocket Science, each chapter features an interview with one or two scientists (refreshingly, mostly women) providing insights into how the missions that they work on employ physics. There are some clear and concise explanations and neat analogies throughout. Muro provides a treatise on general relativity by highlighting the Hubble Space Telescope’s observations of gravitational lensing. She also includes a fascinating explanation of momentum via the physics of NASA’s DART mission, which impacted and deflected the little asteroid Dimorphos. Muro’s description of the electromagnetic spectrum is aided by discussions about interplanetary radio communications, the spectroscopic measurements of Venus that the forthcoming DAVINCI mission will make, and the X-ray vision of the Chandra Observatory.

The chapters I enjoyed best were in part two of the book where, as a self-confessed Space Cadet, Muro talks about how she attended a NASA Space Camp in the 1980s. (She is donating 100% of the proceeds from this book towards funding scholarships for children to attend the camp too.) In this section, Muro takes us through the science of rocket launches and landing on other worlds. She explains how the balance of forces as described by Newton’s laws of motion will enable NASA’s Dragonfly helicopter mission to fly on Saturn’s moon Titan. I especially liked the ice-skating analogy that Muro used to describe gravitational slingshots. Imagine two ice skaters, one being yourself and the other a champion speed skater, writes Muro. You get a head start, then the speed skater races up alongside you, takes your hand just for a moment and then lets you go, giving you an energy boost.

Muro gets through an impressive amount of science in a short space – the first 50 pages alone are full of heavy topics such as Kepler’s laws of orbital motion, planetary transits, exoplanet science, Lagrange points, gravitation, the three-body problem, general and special relativity, the concept of space–time, the speed of light, the Doppler effect, and even an equation or two. Phew! That’s a lot of science, and while at times it might feel dense to the uninitiated, at no point is it too confusing or impenetrable for the science or space enthusiast.

Muro’s willingness to include equations (there are even two interludes featuring derivations) reminds us of her teaching background. I found it refreshing to see maths presented front and centre when most popular-science books shy away from it, in an attempt not to lose readers. In this book, the maths isn’t complex and it is crucial in showing us why various physical laws work the way that they do.

One small issue for me was Muro’s decision to incorporate both imperial and metric units, which made for some confusing reading. In some instances, she mixes the systems in the same sentence – for example, giving a distance in kilometres followed by a weight in pounds, or a speed in miles per hour followed by energy in joules. Muro comments in the chapter about momentum and the DART mission that she has been using both systems to help readers become more comfortable with them. However, if the conversions between imperial and metric are not going to be given, then I’m not sure she really succeeds at making people more comfortable using metric.

If I were to nitpick further, I’d point out that when Muro strays from straight physics and engineering to astrophysics, some mistakes do crop up. These include stating that objects in the early universe are “red” because they have been cooling for billions of years. What is really happening is we are seeing them as they were long ago and their colour is the product of cosmological redshift, not temperature. Another is implying that gravitational waves are what cause the orbits of asteroids and comets to be perturbed. This ignores the fact that gravitational interaction from the planets, particularly Jupiter, is the dominant influence. However, these errors do not detract from the otherwise clear and correct explanations of the physics, which are easy to read and pitched perfectly for students below university level.

Overall, It’s (Just) Rocket Science is an excellent book that turns abstract physics into something more relevant. With the popularity of the recent Artemis II mission, the book’s focus on space missions will hopefully help it find an audience who will really take all that Muro has to teach on board.

• 2026 Johns Hopkins University Press £25.50 hb 384pp

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A possible “exosatellite” has been discovered 73 light years away. Bizarrely, it is roughly the size of Jupiter and orbiting a failed star called a brown dwarf, which in turn is orbiting the red dwarf star CD-35 2722.

This huge alien “moon” was found by the Very Large Telescope’s CRIRES+ spectroscopic instrument, which measured the Doppler shift in the brown dwarf’s light as the exosatellite gravitationally tugged on it. This is the same method used to discover the first known exoplanet around a Sun-like star, namely 51 Pegasi b, in 1995.

Only a handful of exosatellite candidates have been discovered thus far, with none being particularly convincing. This newly detected satellite, reported in Nature, is possibly the best of the bunch. The brown dwarf, known as CD-35 2722b, has a mass about 37 times greater than Jupiter and its exosatellite has a minimum mass of 90% that of Jupiter.

Moon, satellite or planet?

Planetary mass objects around brown dwarfs have been discovered before. Take the example of 2M1207b, which is an object up to six times more massive than Jupiter orbiting a brown dwarf, discovered in 2005 by the Very Large Telescope. Such objects have not been referred to previously as moons or satellites, so why now?

“This system is meaningfully different from 2M1207 because of the mass ratios and separations involved in CD-35 2722,” says Kevin Hoy, a PhD candidate at the European Southern Observatory and the Instituto de Estudios Astrofísicos in Chile, who led the discovery.

Whereas 2M1207b is approximately a quarter of the mass of its host brown dwarf and on a very wide orbit, possibly as long as 20,000 years, CD-35 2722b’s satellite has about 2.5% of the mass of its host brown dwarf and is on a much closer 170-day orbit.

There’s also the hierarchy of the system to consider. In the 2M1207 system, the brown dwarf doesn’t orbit anything, but in CD-35 2722 the brown dwarf is on a 5000-year-long orbit around its companion star. The candidate exosatellite therefore orbits something that orbits a star, just as a given moon in our Solar System orbits a planet that orbits a star. Since our Solar System has nothing like this brown dwarf–satellite combination, the language that we use to describe such objects is seemingly inadequate.

Mary Anne Limbach of the University of Michigan, who was not involved in this study, agrees that our nomenclature is increasingly ill-suited to the bizarre objects that we are discovering beyond our Solar System.

“Most people in the astronomical community would probably hesitate to call this an exomoon,” she tells Physics World. “But I think ‘exosatellite’ is appropriate. The field needs language for companions that do not neatly fit into the traditional categories of planets and moons, and ‘satellite’ seems to be the broader term that the field is gravitating towards.”

Forming exosatellites

Definitions are important, adds Limbach, because they allow us to distinguish between how otherwise similar objects formed.

The moons in our Solar System formed from debris discs encircling their parent planets. Material in the discs clumped together, forming cores that were then able to sweep up the remainder of the material. Astronomers refer to this as core accretion formation.

“If I had to put money on it, I would doubt that the CD-35 2722b satellite formed via core accretion for several reasons,” says Hoy. These reasons include the size of the disc required to build such a massive satellite, the speed with which it would have done so (it is only 150 million years old) and its eccentric orbit. Moons born in situ around their parent typically adopt a more circular orbit.

Instead, Hoy suspects that the brown dwarf and its satellite collapsed directly out of a gas cloud in much the same way that stars do. “That would be my best guess,” he says.

The formation mechanism makes all the difference to Limbach, who opines that, “an object can reasonably be called a moon if it formed through a process analogous to the formation of the moons around the Solar System’s planets.”

Limbach cautions that since we only know the satellite’s minimum mass, we cannot yet rule out this being a binary system of two brown dwarfs. Either way, Limbach sees the system as being close to that of a binary giant planet. Our understanding of how giant planetary bodies and low-mass brown dwarfs form tells us that binary giant planets should be rare, “but the first emerging examples hint that they may not be,” says Limbach. These include about 40 pairs of rogue binary giant planets, called Jupiter Mass Binary Objects (JuMBOs), found by the James Webb Space Telescope in the Orion Nebula’s Trapezium star cluster.

Tip of the iceberg

Another unanswered question is how far down does CD-35 2722’s hierarchy go? Could the exosatellite have moons of its own?

“I think it’s definitely possible,” says Hoy, so long as they are on tight orbits so that gravitational tides cannot destabilize them. “Unfortunately such an object is likely impossible to detect with our current methods.”

However, our current methods could soon learn that CD-35 2722 is just the tip of the iceberg.

“Looking at directly imaged planets and brown dwarfs is the most likely way to yield more exosatellites,” says Hoy, while also citing astrometry (looking for deviations in the motion of a star through space) and microlensing (where the gravity of an otherwise unseen exosatellite creates a temporary gravitational lens) as key techniques that could bring dividends.

Meanwhile, it is hoped that the European Space Agency’s PLATO mission, which will discover exoplanets via transits, will detect significantly large terrestrial moons once the mission launches at the beginning of 2027. All in all, we could be on the cusp of a new era of exomoon and exosatellite discovery.

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