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Home » Page 1326

Keeping the lights on after 2100

As the presidential election campaign hots up here in the US, it is inevitable that energy issues will loom large on the political agenda. Being Americans, our focus will inevitably be local and short term, although I admit to hoping that maybe – just maybe – this time around, politicians on both sides will finally tell the public what they mean by “clean energy”. Right now, I haven’t got a clue.

Putting such limited optimism aside, however, it is clear that energy strategies for the future will pose challenges far beyond the next election (and the next, and the next…), and not only for those of us living in North America. By the year 2100 our planet’s population will exceed 10 billion souls, all striving for a North American or European standard of living, with its attendant thirst for energy. This is a staggering (and likely unsustainable) prospect, but it is just this scenario that the Stanford University physicist and Nobel laureate Robert Laughlin addresses in his book Powering the Future.

As with any complex and fundamentally nonlinear physics problem, the devil is always in the details. Fortunately, Bob Laughlin is a details kind of guy. His subtitle, “How we Will (Eventually) Solve the Energy Crisis and Fuel the Civilization of Tomorrow”, tells us who that devil is, and that he or she resides within the parenthetical remark “eventually”. Through 11 chapters and accompanying notes (which make up half of the book’s total 224 pages), Laughlin guides us through the jungle of the energy economy: from coal to its combustion; gas to gasoline (petrol); fission dynamite to deuterium fusion; the transport of energy by electrons and protons; the possible future generation of electricity and fuels from waste substances such as manure and maize husks; and prospects for exploiting sources of energy that stem from cosmic radiation or pressures from within the ocean depths.

All of these processes are governed by a “Jungle Law”, the title of Laughlin’s third chapter. This chapter resonated with me personally, as it reflects my own metamorphosis from an industrial basic-research physicist to someone concerned with energy and the environment. Determining the direction of the energy enterprise is not like creating the market behind the next iToy, where scientific and technical matters are paramount. With energy, science plays at best a 50% role, the remainder being driven by raw economics skewed by political and social perceptions. Realizing this was an epiphany indeed.

Laughlin gives readers a great example of such an energy epiphany in his chapter on “Carbon fever”. Seven decades ago, Linus Pauling taught us the marvels of the 2s–2p hybridization of carbon’s outer shell, which lie behind all of the element’s subsequent manifestations, from life to locomotion. Laughlin points out that the economics and physics of energy production from loosely bound carbon – in whatever form it is found, whether mineral, gaseous or organic – is overwhelmingly favourable compared with those of other “alternatives”. Because of this simple fact, it is very likely that we humans will continue to oxidize pretty much every atom of available number six we can find.

Fortunately, there is a lot of mineral and organic carbon around, and this is likely to remain true at least for a while. A problem may arise in dealing with the element’s greenhouse-gas form, carbon dioxide, although some economists have argued that the wealth created and banked by using fossil fuels to their limit could underwrite whatever climate-change adaptation technologies may be needed in the next century. Regardless of your views on carbon dioxide and climate change, though, we are likely to run out of the useful forms of cheaply available carbon sometime in the next 40–60 years. Then what? The first half of this book provides some hints of the answers, and for that reason alone, it should be mandatory reading for the next president of the US and their cabinet, and for those who follow – even if one or more of them does possess a Nobel Prize for Physics.

For readers who do not have enough time for the entire book, let alone Laughlin’s extensive endnotes (which, though great for physicists, can be tedious for non-specialists), I strongly recommend at least perusing the chapter “Inspiring mammoths”. The title is a Laughlin-euphemism for nuclear energy of any origin, and in the chapter he explains that the chief economic barrier to a renaissance of nuclear-fission power is the expanding availability of coal and natural-gas reserves worldwide. There are other hurdles too, but they are mostly political and environmental in origin. The political argument against nuclear fission encompasses some sound concerns, such as weapons proliferation, and Laughlin suggests that these should be addressed by international enforcement, not just agreement. The environmental arguments, in contrast, essentially stem from a lack of proper perspective. It is instructive to point out that the death toll from a single commercial airline crash is approximately twice that of the confirmed number of radiation-exposure deaths from every nuclear-plant disaster to date, including Chernobyl and Fukushima. As far as we know, the toll from the latter remains zero (see our May feature article).

Concerns about running out of “burnable” fission material – for example uranium and thorium ore – seem likewise overblown based on Laughlin’s analysis, which also covers issues of waste, reprocessing and “breeding” fissile material in specialist reactors. Given the vast amounts of uranium and thorium in the earth and the sea, the extension of such supplies through deployment of the above technologies, and the economic drivers that will come into play with the soaring costs of exploiting disappearing fossil reserves, my “take home” message from Powering the Future is that uranium and thorium nuclei will probably be the source of the parenthetical “eventually” in the book’s subtitle. What about fusion? Well, judging from Laughlin’s “Inspiring mammoths” chapter, fusion could indeed be the energy of the future – but it will likely remain so for a long time.

Those of us who are personally acquainted with Bob Laughlin know him as a colourful character. In New York, where I grew up, we would call him, warmly, a “wise guy”. It is just this delightful attitude that makes his book so readable, and I can think of no better way to illustrate this than to quote his words in the book’s closing sentences. After thoroughly exploring present and foreseeable energy resources for humanity, Laughlin ends with a prosaic, but most profound, warning that “The most terrible cosmic explosion of all will occur if I show up late again for dinner. It might be a good idea to stop worrying about the universe and hustle home.”

The physics of running

At its best, athletics is about sporting dramas. When leading athletes push their bodies to the limits it can create national heroes and inspire new generations of sports enthusiasts. But behind the stellar sporting performances there is also a lively arena of fascinating science and technology. In this series of videos for Physics World we will take you on a scientific tour of three of the most fundamental and iconic sports: running, cycling and swimming.

This short film focuses on running, as Physics World journalist James Dacey visits the city of Sheffield in the north of England. He takes a jog around Don Valley stadium with sports engineer Steve Haake who talks about how footwear and athletics tracks have evolved over the years to assist runners in their strides towards new world records. Dacey also visits the Centre for Sports Engineering Research (CSER) at Sheffield Hallam University to observe an elite athlete undertake a physiology test designed to gauge his fitness levels.

You can also watch our films about cycling and swimming. These were produced in association with the July issue of Physics World, a special edition that looks at the physics of sport. It includes features on the physical principles underpinning athletics, and the roles technology plays in enabling and enhancing sporting performance.

The physics of cycling

This short film is sure to get your wheels spinning as the sports engineer Steve Haake takes you on a tour de science of professional cycling. Haake breaks cycling down into its physical principles as he talks about how road and track bikes have been optimized in different ways to offer speed and control. Haake – a physicist-turned-engineer – showcases a bike that has been specially designed to cut through the air with minimal resistance.

You can also watch the two other films in this series on the physics and technology of professional sport, which focus on running and swimming. These were produced in association with the July issue of Physics World, a special edition that looks at the physics of sport. It includes features on the physical principles underpinning athletics, and the roles technology plays in enabling and enhancing sporting performance.

The physics of swimming

In this short film, the sports engineer Steve Haake takes you beneath the surface of professional swimming. Haake explains how science and technology have helped top swimmers to get around the fact that humans are not adapted for water. He dives in and addresses some of the big questions in the world of swimming, such as whether there is such a thing as “fast water” and why the international governing body of swimming has decided to ban the full-body swimsuits that led to so many world records falling a few years ago.

You can also watch the two other films in this series on the physics and technology of professional sport, which focus on running and cycling. This was made in conjunction with the July 2012 issue of Physics World, a special edition that looks at the physics of sport. It includes features on the physical principles underpinning athletics, and the roles technology plays in enabling and enhancing sporting performance.

Scientists craft the lightest material in the world

aerographite


(Courtesy: TUHH)

By Tushna Commissariat

Two teams of researchers in Germany have fabricated a material that they say is the lightest in the world. Aerographite – as the researchers have dubbed it – is a 3D network of porous carbon nanotubes and weighs only 0.2 mg per cubic centimetre, making it 75 times lighter than Styrofoam. Nevertheless, the researchers say that it is very strong and can withstand large amounts of compression (up to 95%) and tension loads. This is one of its many unique features, as most lightweight materials can easily be compressed but become weak when exposed to large amounts of stress. Aerographite, on the other hand, becomes more solid (up to a certain point) when compressed, making it stronger.

The researchers at Kiel University and Hamburg University of Technology, both in Germany, say that aerographite is jet-black, stable, electrically conductive, ductile and non-transparent, and has a very low density thanks to the fact that it is composed of hollow carbon nanotubes. Aerographite weights four times less than the hitherto lightest material in the world – a nickel material that was revealed only six months ago. The scanning-electron-microscope image above shows the hollow carbon tubes that form a fine mesh.

The researchers say that aerographite could have innumerable applications – it could be used to make lightweight lithium-ion batteries, to build satellites and even in water-purification systems.

Physicists solve Casimir conundrum

Physicists in the US may have ended a decade-old debate about how the Casimir force – which affects objects separated by tiny distances – should be calculated for two metal objects. They say that the so-called Drude model, which treats metal as a collection of billiard-ball-like positive ions and electrons, wins out over the “plasma model”, which assumes the electrons move in a fixed lattice of positive ions. Understanding how to determine the force could play an important role in the design of micrometre- and nanometre-sized machines.

The Casimir force was first predicted in 1948 by Dutch physicist Hendrik Casimir, who considered what happens when two uncharged, perfectly conducting metal plates are placed opposite one another in a vacuum. According to quantum mechanics, the energy of an electromagnetic field in a vacuum is not zero but continuously fluctuates around a certain mean value. However, resonance means that only certain wavelengths will exist between two plates separated by a particular distance.

What Casimir worked out was that the radiation pressure of the field outside the plates will tend to be slightly greater than that between the plates, which will therefore be attracted to one another. As it is so tiny, the Casimir force proved extremely difficult to measure and it was not until 1997 that Steve Lamoreaux, then at the University of Washington in the US, provided the first firm experimental confirmation of Casimir’s theory. Although Lamoreaux and others have since made better measurements, an important mystery remained regarding how the Casimir force should be calculated for realistic objects.

Larger gap, weaker force

Although successful Casimir measurements have been made between two gold surfaces, the problem is that gold is not a perfect conductor – which means that electromagnetic radiation can penetrate a finite distance into the metal. The gap between the surfaces is effectively greater and the force weaker than if the metal were a perfect conductor, explains Thorsten Emig of the University of Paris Sud, an expert on the Casimir force who was not involved in this latest work.

Both the plasma and Drude models are good at describing how short-wavelength light interacts with the metal surfaces – and can therefore be used to calculate the Casimir force at relatively short separations of less than about 1 µm. At larger separations, however, the models differ. The plasma model predicts that the “static transverse” electric mode of the electromagnetic field within the gap contributes to the Casimir force, whereas the Drude model says that it does not. Unfortunately, physicists had not been able to use one apparatus to measure the Casimir force over a large enough range of distances to decide which model works best at all separations.

Drude works best

Lamoreaux, who is now at Yale University, has joined forces with Hong Tang and colleagues to measure the Casimir force over the widest range of distances to date – from 100 nm to 2 µm. In doing so, the team is the first to show that the Drude model works best at both long and short distances.

While Casimir originally formulated his theory for parallel plates, actually measuring the force in this way is tricky because it is very difficult to align the plates well enough to perform the experiment. Lamoreaux’s breakthrough in 1997 involved measuring the force between a metal plate and a metal sphere – an arrangement that does not require precise alignment. His latest experiment involves measuring the force between a gold-covered sphere of radius 4 mm and an extremely thin membrane of silicon nitride that is also coated with gold. The membrane is just a few hundred nanometres thick and the gold coating is 200 nm think. An important feature of the resulting gold surface is that it is flat to within 3 nm throughout the entire membrane, which is a square with sides measuring 1 mm.

Vibrating membrane

The membrane is stretched drum-like across a silicon frame, which is vibrated using a piezoelectric actuator. A measurement is made by bringing the sphere to within about 1 µm of the gold surface while monitoring the vibrations of the membrane using a fibre interferometer. The presence of the Casimir force can be detected by its effect on how the membrane vibrates, with the force measured by varying the separation from between about 100 nm to 2 µm.

In theory, all points on a metallic surface should be at the same electrical potential, but in practice the molecules adsorbed on the surface make the potential vary such that it can affect force measurements – particularly at relatively large separations. To allow for this effect, the team raster-scanned the sphere across several membranes to measure the surface potential as a function of position. This allowed the researchers to select a membrane with the smallest variation for their Casimir measurements. Information from the scan is also used to correct for spatial variations during the measurement.

As well as showing that the Drude model is best at describing the Casimir force, the research also reveals the important role that variations in potential across the surfaces play in Casimir measurements. Indeed, the team suggests that an important next step in Casimir measurements will be to map the potential variations on the surface of the sphere. If successful, this could allow measurements to be made at even greater separations, which is something that Lamoreaux sees as an important next step in our understanding of the Casimir force.

The experiment is described in Physical Review Letters.

UK to support open access

The UK government has “widely accepted” the recommendations of a major report into open-access publishing that was released in June by a 15-strong working group led by the British sociologist Janet Finch. The Finch report concluded that the UK should lead the way in transforming scientific publishing from a “reader pays” to an “author pays” model, supporting the need for a fee – known as an “article processing charge” – to fund open-access journals. The report has also called for the UK research councils to “establish more effective and flexible arrangements to meet the cost of publishing in open access and hybrid journals”.

In a letter to Finch outlining the government’s support for the report, UK science minister David Willetts says that the UK government recognizes “that while open access means free access to the user and full right of search, it does not follow that open access has no cost”. He adds that publicly funded research institutions will need help in paying for article processing charges with this funding set to come out of “existing research funds”. The only proposal by Finch that will not be implemented is a recommended reduction in value added tax for e-journals, which Willetts says would contravene EU rules.

Willetts states in the letter that the government favours the use of “gold” open-access publishing, whereby authors pay a fee to publish in an open-access journal and the paper is then immediately made available for anyone to read for free. It prefers this to “green” open-access publishing, where the published paper is placed behind a publisher’s paywall but then deposited into a centralized free-to-access repository after a certain embargo period. If a particular journal does not support gold publication, however, researchers will be required to place their papers in a repository within six months of publication. If funding is not provided for gold publication, publishers may extend the embargo period before papers can be made freely available from six months to 12.

Peter Knight, president of the Institute of Physics, which publishes physicsworld.com, welcomes the government’s commitment to support almost all of the recommendations of the Finch report. “The response recognises the importance of the academic publishing sector that delivers jobs and exports for the UK, and of learned societies such as the Institute of Physics, whose educational and outreach activities depend on gift aid from our publishing company,” he says.

Yet Knight says that there will be “complex challenges” that could carry additional costs in the transition to open access. “We are concerned that these transitional costs appear set to fall on the science budget, reducing the funding available for UK researchers to carry on the work that has put this country at the forefront of many fields, including physics,” he warns. One issue in particular is that other countries will get free access to UK-based research while the UK has to still pay for journal subscriptions to access work done at foreign institutions.

Towards 2014

Research Councils UK (RCUK) – an umbrella organization for seven UK research councils – has also announced that, from April 2013, any scientific paper that results from research wholly or partially funded by RCUK must either be put into an open-access journal or in a journal that allows papers to be deposited in a repository. In the latter case, the paper must be put in a repository within six months and must include all changes resulting from peer review. However, RCUK has also announced that any article-processing charges will not be covered in research grant applications, but rather through “block grants” awarded to universities, although how this will work in practice is not yet clear.

In a statement from Nature Publishing Group (NPG), which publishes the Nature suite of journals, they welcome RCUK’s announcement to make centralized funding available to institutions to pay open-access publication charges. NPG also says that its existing self-archiving policy is already fully compliant with RCUK’s new policy of encouraging self-archiving for public access six months after publication. However, the firm is urging RCUK and its funding bodies to “quickly clarify the process for allocating funds to UK institutions, so that they can establish procedures and make the transition towards gold open access as smooth as possible for funded researchers”. 

Open access on the horizon

Meanwhile, the European Commission (EC) announced today that it will make open access “a general principle” of its next funding programme, called Horizon 2020, which runs from 2014 to 2020. In a statement it says that from 2014 all articles produced with funding from Horizon 2020 will have to be either accessible immediately via gold open access (with up-front publication costs eligible for reimbursement by the EC) or available in an open-access repository no later than six months after publication.

The EC also recommends that its members take a similar approach to open-access publishing and sets a goal of 60% of European publicly funded research is available under open access by 2016.

Particle trapped with light and heat

Physicists in Germany have developed a trap for micron-sized particles that relies on both laser light and heat. The “optothermal” trap, which is built around a photonic crystal fibre, puts a new twist on microparticle trapping. The technique could offer a way to make precise measurements of the thermal forces acting on tiny particles and could even be used to sort and concentrate microparticles.

For more than 20 years it has been possible to trap microscopically small objects using light. The key development came in 1986 when Arthur Ashkin, Steven Chu and others at Bell Labs in the US invented what became known as optical tweezers: focused laser light that can hold microparticles in mid-space. The phenomenon works because dielectric particles are attracted to the region of strongest electric field, which is where the laser beam is focused.

Light traps and transports

Optical tweezers are now regularly used to trap and manipulate microparticles – particularly biological microparticles, such as cells and viruses. But light isn’t the only way to trap microparticles – thermal forces, generated by light absorption, can also play a role. For instance, a laser can heat just one side of a particle so that molecules bouncing off it receive different amounts of momentum depending on the side that they strike. The effect can be used to trap or transport particles over centimetre distances in air.

Now, Oliver Schmidt, Tijmen Euser and colleagues of the Max Planck Institute for the Science of Light in Erlangen, Germany, have taken a new approach to trapping. In their device, a microparticle with diameter of about 6 μm is contained within a hollow-core, photonic-crystal fibre. This is a transparent fibre with a 100 μm outer diameter, 12 μm inner diameter and a 2D array of holes in the cladding. Laser light drives the microparticle along the fibre’s air-filled core but, when it approaches a black band drawn around the fibre, it comes to a halt of its own accord.

Thermal creep flow

The trick works because of a phenomenon known as “thermal creep flow”, which exists only very close to a surface that has a temperature gradient – that is, a hot side and a cold side. Hot molecules in the air will impart the surface with more momentum than the cold molecules, which means that the surface receives an overall force in the cold direction. In reaction, the air at the centre of the cores flows in the opposite direction – from cold to hot.

Artist's impression of how a tiny particle is trapped in a photonic-crystal fibre
Trapping with heat and light

In the Max Plank group’s device, the thermal gradient is created by the microparticle’s scattering of light, which is absorbed by the black band. The resulting thermal creep drives a flow of air along the core’s surface towards the hot spot, which is balanced by a flow of air along the middle of the core in the opposite direction – away from the hot spot – creating a structure resembling a convection current. This complex series of actions effectively produces a viscous drag force that counteracts the laser’s pushing and traps the microparticle at the band.

Sorting microparticles

Schmidt and Euser point out that, unlike the previous optothermal traps, their device doesn’t require the microparticles themselves to be light-absorbing. Moreover, the radiation and viscous forces scale linearly with the laser’s power, which means that the trapping is independent of laser power. The researchers say that the trap could have applications in so-called “lab on a chip” systems in which microparticles could be moved round a tiny chip while various analyses are performed. Or, if the drawn-on bands are replaced with actual heating elements, the trap could be used to sort microparticles according to their physical properties, or to measure the thermal forces acting on them.

“Small, electrically controlled heating elements could be placed at specific locations along microfluidic channels,” the researchers say. “When switched on, the induced thermal creep flow can be used to either trap particles or to sort them. Such localized heating elements would be extremely easy to implement, not requiring any optical connections.”

Biophysicist Dieter Braun at the Ludwig Maximilians University Munich in Germany believes that for the trapping and detection of biomolecules, the fibre would need to be tuned for fluorescence imaging. But he adds that the use of hollow fibres is “very original”. Its application potential “is encouraging and should be pursued,” he says.

The research is described in Physical Review Letters.

Galaxy cluster motion seen for the first time

An international team of astronomers and physicists has, for the first time, detected the large-scale motion of galaxy clusters, using an effect that was proposed almost 40 years ago. This is the first direct measurement of the motion of objects at cosmological distances and such observations could lead to a better understanding of how the universe formed and evolved and also help astronomers study dark matter and dark energy.

In 1972 Russian physicists Rashid Sunyaev and Yakov Zel’dovich argued that a moving cluster of galaxies should, in theory, cause a slight temperature shift in the cosmic-microwave-background (CMB) radiation – the leftover thermal radiation from the Big Bang – as it passes through it. This Sunyaev–Zel’dovich (SZ) effect is caused by high-energy electrons distorting the CMB through inverse Compton scattering and can be divided into three categories, or “effects” – thermal, kinematic and polarized. It is the second variety – the kinematic Sunyaev–Zel’dovich (kSZ) – that was used in the new work to detect the cosmic-scale motion. The kSZ is a second-order effect where the CMB photons interact with high-energy electrons in the galaxy clusters, as a result of the electron’s bulk motion. Radiation passing through a galaxy cluster moving toward Earth appears hotter by a few millionths of a degree, while radiation passing through a cluster moving away appears slightly cooler. Although proposed 40 years ago, this is the first time the kSZ effect has been observed.

Come together

To get around the difficulties of detecting such a small temperature change, lead author Nick Hand from University of California, Berkeley in the US, along with 58 collaborators from the Atacama Cosmology Telescope (ACT) in Chile and the Baryon Oscillation Spectroscopic Survey (BOSS) project in New Mexico, compiled signals from several clusters to detect the temperature shift. Data from a catalogue of 27,291 luminous galaxies from BOSS were laid over maps of the same region of sky observed by the ACT between 2008 and 2010. As each galaxy likely resides in a galaxy cluster, their positions were used to determine the locations of clusters that would distort the CMB radiation.

In a galaxy far far away

The teams detected the motion of galaxy clusters that are several billion light-years away and moving at velocities of up to 600 km/s. The velocities of these distant objects are extremely difficult to detect as they require very precise distance measurements.

“One of the main advantages of the kSZ effect is that its magnitude is independent of a galaxy cluster’s distance from us, so we can measure the velocity of an object’s motion toward or away from Earth at much larger distances than was possible,” explains Hand. He also says that the method could serve as an additional statistical check, independent of currently used measuring methods, for future large-scale measurements.

Of the 27,291 galaxies in the BOSS data, the team used 7500 of the brightest galaxies to uncover the kSZ signal. As two galaxy clusters move toward each other as a result of their mutual gravitational attraction, the team found that the kSZ effect becomes more pronounced – a slight cold spot in the CMB data would suggest that a cluster was moving away from us, whereas a slight hot spot would mean the cluster was moving towards us, similar to the Doppler effect. As the temperature shift data is averaged over thousands of the BOSS objects, a clear kSZ signal was seen.

“The kSZ signal is small because the odds of a microwave hitting an electron while passing through a galaxy cluster are low, and the change in the microwave’s energy from this collision is slight,” says ACT collaborator and physicist David Spergel of Princeton University, US. “Including several thousand galaxies in the dataset reduced distortion and we were left with a strong signal.”

Two collaborations are better than one

The researchers point out that if the data from just the ACT or the BOSS project was analysed by itself, the signal would not have been apparent, as neither was originally built to look for it specifically. Both the ACT and the BOSS projects differ in the objects they study, their method of data collection and even the wavelengths in which they operate – microwaves for the ACT, visible-light waves for BOSS. This work highlights the importance of large collaborations, which might even be fundamentally different in their missions, sharing and combining their data to study subtle physical effects that no single survey could detect – says the team.

According to Hand “The [kSZ] signal agrees remarkably well with the typical CMB cosmology that we have developed over the past decade…so well, in fact, that it was not expected! So it was quite exciting to confirm something predicted 40 years ago, when neither the ACT or BOSS teams were planning on it.”

The strength of the kSZ effect’s signal depends on the distribution of electrons in and around galaxies. So the signal could be used to trace the location of atoms in the nearby universe, revealing how galaxies form. In the near future, Hand hopes that increased sensitivity for the ACT, which is due an upgrade, and larger data sets will mean even further improvements of the kSZ signal, which in turn will mean better velocity measurements.

The research is to be published in Physical Review Letters. A preprint of the work is available on arXiv.

Moons galore for dwarf planet

Pluto and its five moons


A Hubble Space Telescope image showing the five moons that orbit Pluto.
(Courtesy: NASA, ESA and M Showalter at the SETI Institute)

By Tushna Commissariat

The dwarf-planet Pluto is back in the news this week, as astronomers have discovered that it has a fifth icy moon orbiting it. The newly discovered moon, which was seen as a speck of light in nine separate sets of images taken by the NASA/ESA Hubble Space Telescope, apparently has a rather irregular shape, and is about 10–25 km across. With its 95,000 km diameter circular orbit around Pluto, the moon should lie within the same plane as Pluto’s other four moons.

“The moons form a series of neatly nested orbits, a bit like Russian dolls,” says Mark Showalter of the SETI Institute in the US, who was also the leader of the scientific team that discovered the new moon. The team was intrigued that a dwarf planet such as Pluto can have such a complex collection of satellites and says that the new moon could provide further clues towards understanding how Pluto’s system has formed and evolved.

Pluto’s largest moon, Charon, was discovered in 1978. Further observations in 2006, again made by Hubble, uncovered two additional small moons, Nix and Hydra, and the fourth moon, known only as P4, was found last year. The new moon has provisionally been called P5.

NASA’s New Horizons space probe, which is currently en route to Pluto, has a high-speed fly-by scheduled for 2015. It will return the first ever detailed images of the Pluto system, which is so small and distant that even Hubble can barely see the largest features on its surface.

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