Life, But Not As We Know It!

by Nicholas Mee on August 18, 2017

I remember many years ago watching an episode of Star Trek called The Devil in the Dark. The Starship Enterprise visits a mining community on the planet Janus VI where an unknown lifeform is playing havoc with the mining operations.

A silicon-based lifeform surrounded by its rock eggs, as envisaged in Star Trek.

The crew eventually track down the source of the disruption – a strange creature that looks like a rather angry pizza. It is actually a bizarre lifeform unlike anything encountered during their previous adventures. Spock performs a Vulcan mind-meld with the creature and is fascinated to discover that it is a sentient giant amoeboid being that burrows its way around the mountains living off solid rock. The reason for this unusual and rather crunchy diet is that, unlike the familiar carbon-based life on Earth, this is an organism whose fundamental chemistry is based on silicon.

Star Trek’s rock munching amoeba is far from the strangest lifeform dreamt up in science fiction. Here are a couple of my other favourite aliens.

The Black Cloud

The astrophysicist Fred Hoyle wrote a story in 1957 called The Black Cloud in which astronomers discover a vast gas cloud passing through the solar system. On further investigation this turns out to be a super-intelligent being that is wandering through the galaxy. When scientists work out how to communicate with the cloud, it expresses surprise that there are sentient beings inhabiting a solid planet.

Here Be Dragons

Robert L. Forward went even further in his remarkable novel Dragon’s Egg, describing the evolution of a civilization on a neutron star. There is no chemistry in the extreme environment of a neutron star, so these organisms are composed of hypothetical complex nuclear structures. This really is life in the ultra fast line, as their biology revolves around nuclear physics instead of biochemistry. These fictional nanoscale creatures live their lives a million times faster than us.

Is There Life Out There?

Like the other macromolecules vital for life on Earth, DNA has a backbone of carbon atoms.

What do we know about the possible existence of life elsewhere? We have a very small sample with which to work, just one planet on which life is known.

Life on Earth is built around a biochemistry based on macromolecules that have a backbone formed of chains of carbon atoms. Our understanding of chemistry suggests that silicon is not a suitable building block for the complex chemistry required by life. Carbon seems to be uniquely suited to this role, so unfortunately silicon-based lifeforms will probably always remain fiction, for naturally occurring lifeforms anyway. It might be argued that artificial silicon chip based organisms will some day be created. Sentient clouds and nuclear organisms are also probably just entertaining fantasies, but who knows?

Living Is Easy

In all likelihood it is only possible for living organisms to evolve with a carbon-based biochemistry and a liquid water environment is essential. What we do know is that simple lifeforms arose early in the history of our planet. There is evidence for the existence of simple bacterial life as far back as 4 billion years ago, which is around 400 million years after the formation of the Earth, and perhaps as little as 100 million years after the formation of the oceans.

The interval between the arrival of conditions suitable for life and its appearance on Earth seems to have been a remarkably short geological time span. This suggests that life at the bacterial level arises easily and therefore should be common. If this is true, then we could expect to find life elsewhere in the solar system. There are a number of locations that might be good places to look.

Mars has been on the list for many years. In more recent times attention has focused on satellites of the gas giants, in particular Jupiter’s Europa and Saturn’s Enceladus.

The God of War

Illustration by Alvim Corrêa from a 1906 edition of The War of the Worlds by H.G. Wells.

The glaring red eye of Mars has long been associated with war, so it is no surprise that its supposed inhabitants should have been imagined as hostile towards us Earthlings, eager to get their tentacles around our necks.

The prospects of finding little green octopuses on Mars disappeared long ago, but there is still some hope of eventually finding microbial lifeforms. Mars is a cold and hostile world with little atmosphere today, but it may have been much more hospitable in the distant past. There is evidence that water once flowed on Mars, so it is conceivable that life emerged there. Bacteria survive in even the most challenging environments on Earth, so it is just possible that bacterial life has clung on for billions of years deep within the rock below the surface.


Europa is the fourth largest of the Galilean satellites of Jupiter, and is a little smaller than our Moon. It is thought to be a good candidate for extraterrestrial life as it appears to have a surface composed of ice, which seems to form a crust over a liquid water ocean. Europa is subjected to a continual flexing and straining by the gravitational pull of the giant planet Jupiter. This generates sufficient heat to maintain the liquid ocean. The ice sheets can be seen in the image below.

The surface of Europa is covered in ice-sheet. A liquid water ocean is believed to life beneath the surface. Credit: NASA/JPL/University of Arizona.

NASA is currently planning the Europa Clipper Mission. We will learn much more about this icy moon and it habitability when it arrives at Europa in the mid 2020s.


An artist’s conception of the plumes of Enceladus. Credit: NASA/JPL-Caltech.

Enceladus is a moon of Saturn with a diameter of 500 kilometres
It is covered in fresh uncratered ice. In 2005 the Cassini probe discovered plumes of water venting from the south polar region of Enceladus, raising hopes that like Europa it may have a liquid water ocean beneath the surface.

Several other bodies in the solar system have been suggested as possible homes for living organisms. These include Jupiter’s giant moons Ganymede and Callisto, which are also believed to have subsurface oceans. Ceres, the largest of the asteroids. Neptune’s large moon Triton. Even distant Pluto. Perhaps the most intriguing possibility is Titan.


The Cassini mission to Saturn will soon come to an end after 13 years orbiting the gas giant. Cassini will take its final plunge into the planet next month on 15 September. The highlight of Cassini’s incredible mission came in 2005 when it released the Huygens probe to land on Titan, Saturn’s largest moon. Titan is slightly larger than the planet Mercury. It is one of the few bodies in the solar system where a probe has landed. This was feasible because unlike all the other moons in the solar system Titan has a thick atmosphere composed mainly of nitrogen with clouds of methane and ethane.

Artist’s conception of the Huygens probe parachuting through the atmosphere of Titan. Copyright: Emile-Raphael Franco.

The temperature on Titan is a rather chilly 180° Centigrade below zero. This is far too cold for liquid water to exist on the surface. Nonetheless, Titan has seas, lakes and rivers, but they are compoased of methane and ethane, hydrocarbons that play a similar role on Titan to water on Earth. This has led to speculation that Titan might be home to some form of life. It is perhaps rather unlikely, but just about conceivable, that that some sort of life could have evolved in this hydrocarbon rich environment. If so, it would have to be based on a novel biochemistry. Last month, NASA scientists announced that a chemical known as acrylonitrile has been detected in the atmosphere of Titan. It has been suggested that acrylonitrile might be suitable for forming a novel kind of cell membrane.

Finding any sort of life elsewhere in the solar system would be sensational. It would suggest that Earth is not just an incredibly lucky one-off and that life is abundant throughout the universe.


Further Information

There is more here about NASA’s planned Europa Clipper Mission.

Click here for NASA’s announcement of acrylonitrile on Titan.

You can find updates on the final days of the Cassini mission here: The Grand Finale.



Lovely LISA

by Nicholas Mee on August 15, 2017

One of the amazing ideas to emerge from Einstein’s theory of general relativity was the possibility of gravitational waves rippling their way across the cosmos. It took a century to verify this prediction. Their existence was finally confirmed by LIGO (the Laser Interferometer Gravitational-wave Observatory) in September 2015, as described in this post: Cosmic Ripples.

What’s in a Name?

LIGO has so far detected three gravitational wave signals, all of which are due to black hole mergers in the distant universe. The image below shows an artist’s impression of the system that produced the third of these signals, detected on 4 January 2017. The black holes were 32 and 19 times the mass of the sun and were spinning in different planes, as depicted in the illustration which shows them just before their merger. The signal has been named GW170104. Guess why?

Artist’s impression of a close binary black hole system based on GW170104. Credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)

Lovely LISA

The detection of gravitational waves by LIGO was an incredible technological achievement. The European Space Agency (ESA) is planning to go one better by putting a gravitational wave detector in space. It is known as LISA (Laser Interferometer Space Antenna). There will be three spacecraft orbiting the sun in a triangular formation, consisting of a mother and two daughter craft each separated by a distance of 2.5 million kilometres. They will form a precision interferometer with lasers monitoring the distances between the mother and daughter craft. A passing gravitational wave will change these distances very slightly and this will be detected by the interferometer.

Schematic representation of LISA. Credit: NASA.

Paving the Way for LISA

The LISA Pathfinder mission was launched in December 2015 as a stepping stone to the LISA mission. It was devised to test the technology that will be used in LISA and demonstrate the feasibility of constructing an interferometer in space. LISA Pathfinder released two cubic test masses to float freely within the spacecraft and used its laser interferometer to measure their separation, as shown in the illustration below. It then monitored their positions to an unprecedented accuracy of less than one hundredth of a nanometre.

The goal was to show that test masses can be shielded from any stray internal and external forces and maintained in a state of almost perfect free-fall. Remarkably, the spacecraft avoids any contact with the test masses contained within its structure by sensing their motion and adjusting its own position using micro-thrusters to compensate. This is essential if the sensitivity of the interferometer is not to be destroyed by inevitable perturbations that the spacecraft will suffer. These arise from a number of sources including stray gas molecules within the craft, the solar wind and micro-meteoroid impacts.

The LISA Pathfinder mission released two cubic test masses to float freely in solar orbit while their separation was monitored by a precision interferometer. Credit: ESA/ATG medialab.

In the Pathfinder mission the test masses are located 40 centimetres apart, whereas the three LISA craft will be separated by millions of kilometres. The LISA interferometer will measure their separation just as accurately as the Pathfinder mission, so its sensitivity will scale up in proportion to its increased size. ESA announced in June this year that the technology trialled by LISA Pathfinder has performed beyond expectations, which means it will certainly be sensitive enough to detect gravitational waves when deployed by the LISA mission.

What Will We See?

LISA will greatly enhance our ability to study gravitational waves. It will detect signals invisible to LIGO and other ground based gravitational wave detectors, as it will be sensitive to gravitational waves with much longer wavelengths that are produced by much larger systems. Although the black holes that merged during the GW170104 event were very massive they were only 190 and 115 kilometres in diameter, with the merged black hole around 280 kilometres in diameter. These are very small objects by cosmic standards.

LISA will detect gravitational wave signals emanating from tightly bound binary systems containing two compact objects that may be white dwarfs, neutron stars or black holes orbiting each other prior to their merger. For instance, a binary black hole system such as GW170104 would be detected weeks or even months before the merger event. This will enable the position of the binary system to be located in the sky and the time of merger to be accurately predicted, which will greatly aid in visual identification of the merger event.

Supermassive Black Holes

There is a supermassive black hole of four million solar masses at the centre of our galaxy. Most, if not all, galaxies are thought to harbour a monster such as this within their core. LISA will be able to detect these beasts devouring nearby stars. It will also detect mergers of supermassive black holes. Such extremely violent and spectacular events must occasionally happen somewhere in the universe. We can look forward to finding out much more about them.

An artist’s impression of a supermassive black hole.
Credit: ESO/L. Calçada.

The creation and early growth of supermassive black holes is still not well understood. LISA should detect their birth pangs and provide important clues to how they formed and their relationship to quasars in the early universe. LISA will also help to improve models of the immediate aftermath of the Big Bang and the very early universe. As an important bonus, LISA will add to our knowledge of fundamental physics by providing stringent new tests for general relativity.

LISA is scheduled for launch in 2034 as part of ESA’s Cosmic Vision programme.

Further Information

There is more about the LISA mission on the website of the LISA consortium at:

There is also information on the ESA website at:



We’re Having a Field Day!

August 12, 2017

René Descartes proposed an imaginative explanation for the motion of the planets in the 1630s. He argued in a work known as The World that the existence of a void or vacuum is impossible and therefore space must be filled with some sort of fluid. He suggested that the planets are carried around the sun by […]

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Cosmic Order and the Higgs Force

August 9, 2017

Tracking down the Higgs boson took almost fifty years, so the announcement of its discovery by the Large Hadron Collider (LHC) in 2012 was a momentous occasion. Strange as it may seem, the theory that predicted the existence of this elusive particle was devised to explain one of the great mysteries of the world around us — […]

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From Genesis to Revelation

August 1, 2017

Modern cosmology is one hundred years old this year. Of course, poets, seers and sages have contemplated the origin of the universe for millennia and arrived at various conclusions. There aren’t really that many distinct possibilities, however. The universe is either finite, eternal or cyclic, and the third of these possibilities is like a combination […]

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The Physical World

July 26, 2017

The new book that I have written with Nick Manton The Physical World: An Inspirational Tour of Fundamental Physics has just been published by Oxford University Press. There are more details about the book here: The Physical World: An Inspirational Tour of Fundamental Physics. OUP have designed a very appealing cover for the book.   The […]

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The Ultimate Heavy Metal Space Rock

July 24, 2017

It is fifty years since the Summer of Love. During that summer a young graduate student in Cambridge working on a newly developed radio telescope designed by Martin Ryle and Anthony Hewish noticed a bit of ‘scruff’ in her read-out. Jocelyn Bell looked further and realised that there was something strange going on. The signal was […]

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The Crab and the Jellyfish

July 22, 2017

Fritz Zwicky was a Swiss astronomer who worked for most of his career at Caltech (California Institute of Technology) and the Wilson and Palomar Observatories. Zwicky was a creative and original thinker who ignored fashionable trends and pursued his own ideas.  He referred to himself as a lone wolf. He certainly had a rather misanthropic […]

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Cosmic Ripples

July 19, 2017

Michael Faraday transformed our understanding of the physical world when he realised that electromagnetic forces are carried by a field permeating the whole of space. This idea was formalized by James Clerk-Maxwell who constructed a unified theory of electromagnetism in which beams of light are undulations in the electromagnetic field. Maxwell’s theory implies that visible light […]

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Somewhere Over the Rainbow

July 15, 2017

In 1960, the astronomer Frank Drake weighed up the options for his first attempt to find extraterrestrial intelligence. Two nearby stars Epsilon Eridani and Tau Ceti, 10 and 12 light years distant, seemed like promising candidates. Both are similar to the sun with around four fifths of the sun’s mass. They were ideal targets for Drake’s quest […]

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