Life in the Solar System and Beyond

One of the most difficult problems that science still needs to solve is the question of the origin of life. We have very strong evidence that life emerged on Earth around 3.7 billion years ago, when our planet was less than 1 billion years old. It is very exciting to notice that in the first billion year, the Earth resembled very closely what might be considered hell, with a molten surface, lava and very active volcanoes. Indeed that geologic era is known as “the Hadean” which takes its name from Hades, the Greek god of the underworld. Life emerged almost immediately after the surface of the Earth solidified and once the heavy asteroid and comet bombardment of the Earth ended.


The Hadean Earth, 4 billion years ago. The surface is still partially molten and constantly hit by a heavy flow of comets and asteroids.

The heavy bombardment, which was produced by the migration of the giants planets in the Solar System, is also thought to have brought most of the water that exists on our planet (the asteroids being the primary source rather than comets). If it took such a relatively short time for life to emerge, does it mean that we were just lucky but life is a very complex and unlikely event or that life does indeed create itself very easily? Or perhaps life was brought to Earth from space (the so-called panspermia)?

It is well known that several kinds of microbes (mostly named “extremophiles”) can survive in acidic environments which are more corrosive than sulfuric acid. Some others can survive extreme temperatures well above 100 degrees Celsius or below -200 degrees Celsius. The same goes for high pressures, with bacteria found thriving at 400 times the pressure we’re used to (which would crush you in an instant).

The Tardigrade is a micro-animal that can withstand extreme conditions (although it is not an extremophile). It can survive in space, radioactive environments, boiling water, freezing temperatures close to absolute zero and can survive without a food/water source for more than 10 years.

The Tardigrade is a micro-animal that can withstand extreme conditions (although it is not an extremophile). It can survive in space, radioactive environments, boiling water, freezing temperatures close to absolute zero and can survive without a food/water source for more than 10 years.

However, one common mistake that is often made is to confuse the fact that life has spread basically everywhere on Earth, even in the most extreme environments, with the fact that life can form everywhere. Indeed the fact that we find microbes in such extreme environments on Earth means that life has an immense power to adapt to those conditions. Adaptation is an evolutionary process and has nothing to do with the origin of life which is an entirely different question. Indeed the theory of evolution is a well known and established scientific theory whereas the origin of life still lacks a complete explanation and there are only a number of hypotheses that still need experimental confirmation. There are more than 20 proposed scenarios which attempt to explain the origin of life on Earth. Many of those build upon the “primordial soup” idea of Haldane and Oparin, two scientists that in the 1920s proposed that the atmosphere of the early Earth, when exposed to an energy source (e.g., lightnings), can produce organic molecules that accumulate in the sea forming a “soup”. These molecules then start a chain of chemical reactions which ultimately lead to the formation of life. A famous attempt to verify this idea was made by Miller (and Urey) in 1953 who created several amino-acids (the building blocks of proteins and thus life) in a “soup” when injecting energy in a simulated early atmosphere of the Earth.

A scheme of the apparatus used by Miller to produce organic compounds and amino-acids in his experiment.

A scheme of the apparatus used by Miller to produce organic compounds and amino-acids in his experiment.

The Miller experiment has been criticized since the simulated conditions of the early atmosphere were probably not the correct ones as it emerged in later studies. Thus if Miller had used a simulated atmosphere with the right composition, his experiment might have failed to produce amino-acids, although more recent experiments have succeeded in producing some. Another interesting fact is that when Miller performed his experiment  he reported that about 10% of the carbon present in the simulated atmosphere was transformed into organic compounds. About 1 milligram of a few different types of amino-acids were created. After the death of Miller in 2007, the sealed vials of the experiment were opened and re-examined. It was found that there were way more amino-acids than those reported by Miller in the his work in 1953, so this experiment was even more successful than originally reported.  In any case, amino-acids are just organic compounds and in the past few years they have also been found in space and even on meteorites landed on the Earth’s surface. Such meteorites had experienced extreme temperatures of more than 1000 degrees Celsius while burning during their descent in the Earth’s atmosphere. Therefore amino-acids might possibly be quite widespread and resist to extreme physical conditions, at least in the Solar System. Could the amino-acids present in the “primordial soup” on Earth (provided there was really a primordial soup!) have been brought here by asteroids/comets/meteorites?  Whatever the answer is, it is worth noticing that there’s a gigantic leap between creating organic molecules and creating life. But what is life? The most elementary forms of life are very very simple and it is difficult to establish whether such systems, viruses for example, are really alive. Today science defines a living system as a complex with a body, a metabolism (i.e., life can take resources from the environments and transform them into useful energy) and the property of having inheritable information (e.g., RNA and DNA). However, the latest scientific evidence suggests that the distinction between life and non-life is blurred and continuous rather than discrete.

How can astrophysics help to shed new light on the problem of the origin of life? In the last decade there has been an increasing interest in planetary exploration with a particular emphasis on the search for life in the Solar System. In the past, a lot of research has focused on Mars, for the reason that it is a rather close planet with a possible similar history as the Earth. We do know that large seas and a thick atmosphere were present on the ancient Mars, whereas today water is found in relatively small quantities and only in form of ice. Since Mars had liquid water and a favorable position in the Solar System, i.e., it lies within the so-called “habitable zone”, it was and still is obvious to look for the presence of life on that planet.

An artistic impression of how the ancient Mars might have looked like with its large oceans and thicker atmosphere. The close resemblance with the Earth is striking. (

An artistic impression of how the ancient Mars might have looked like with its large oceans and thicker atmosphere. The close resemblance with the Earth is striking. (“AncientMars” by Ittiz – Own work. Licensed under CC BY-SA 3.0 via Commons)

However, the whole concept of “habitable zone” is rather debated in astronomy. This zone is defined as that region in a solar system where a planet with sufficient atmospheric pressure can support liquid water on its surface. A few decades ago it was pretty clear that only rocky planets like the Earth (and Mars) can satisfy this condition, since gas giants (e.g., Jupiter and Saturn) have a huge atmosphere that sits on a thick layer of liquid hydrogen and helium. There are then the icy dwarf planets like Pluto, which, however, are too far to receive enough energy from the Sun and are too small to have a dense atmosphere so they do not have any liquid water on their surface.

However, with further planetary exploration there is now mounting evidence that not only rocky planets might be able to host life, but also large moons around gas giants, even if those are well outside the “habitable zone”. There are at least three examples of such moons in the Solar System: Titan (the largest moon of Saturn), Europa and Callisto (two of the four large moons of Jupiter).

The three gas giant moons Europa and Callisto (orbiting Jupiter) and Titan (Saturn).

The three moons Titan (orbiting Saturn),  Europa and Callisto (orbiting Jupiter).

Titan was recently explored by the Cassini-Huygens mission (NASA and ESA) with the Huygens probe that landed on its surface in 2005. The data collected revealed the presence of large amounts of organic material (organic aerosol) and the existence of large lakes of methane, ethane and propane. Europa and Callisto instead are increasingly becoming the two most striking examples of potentially habitable worlds that we know. In the late ’90s Jupiter was visited by the Galileo orbiter (an ESA mission) and Callisto was found to behave like a perfect conductor. This implies that Callisto is a highly conductive sphere which means that Jupiter’s magnetic field  – which is 10x larger than Earth’s –  cannot penetrate inside Callisto which is able to deflect it, as perfect conductors do. This was interpreted as a sign that beneath its solid surface there must be a thick layer of more than 10 km made of highly conductive liquid. Since water ice is widespread on the surface of this moon and salty water is a highly conductive material, it has been proposed that a large ocean can be enclosed within Callisto’s solid crust.

Current model of Callisto, with an outer solid crust, an inner ocean of liquid salted water (in blue) and a solid core.

Current model of Callisto, with an outer solid crust, an inner ocean of liquid salted water (in blue) and a solid core.

Even more striking is the case of Europa, another moon of Jupiter. The current idea is that a large ocean exists here too underneath the solid surface. Astronomers have estimated the content of liquid water on this moon and have found that there should be more liquid water on Europa than on the whole Earth. This water is liquid instead of frozen because the tides that Jupiter exerts on these moons are huge and the internal friction heats up the water and keeps it in a liquid form.

Estimated total amount of salted water on Europa compared to Earth

Estimated total amount of salted water on Europa compared to Earth

Unfortunately we have no probes that can visit directly these alien oceans because the liquid water is enclosed within a thick shell of solid material which would be impossible to penetrate. Some water vapor plumes are, however, regularly emitted from the surface of this moon. Therefore analyzing the chemical content of the plumes might be a smart way around penetrating the solid surface. The plumes can be used as probes of the internal ocean and help to understand how the underneath environment might look like. Indeed one recently approved mission (the JUICE mission of ESA and NASA) will explore these plumes of Europa and the whole Jupiter system to assess potential habitability of those worlds and determine whether our current ideas are sound and can be applied to other gas giants (especially the many known beyond the Solar System).

But how can life form on Europa or other moons? There is an interesting hypothesis on the origin of life that proposes hydrothermal vents in the deep oceans of the Earth as the possible location for the creation of life. Deep oceanic vents release energy and heat up the environment with water that can reach up to 450 degrees Celsius. Such water creates a gradient of temperature that slowly decreases towards the deep ocean which has an average temperature of just a few degrees Celsius. There should be a shell around hydrothermal vents which has just the right temperature and composition to trigger chemical reactions that might lead to life. Something similar might be happening on Europa and other moons too? It’s still too early to say, but it is probable that we will find out pretty soon.

I just notice that when searching info on abiogenesis and related problems on google I was literally flooded by tens of websites of creationists, christian fundamentalists and so on. They seem to have figured all out: it must have been god. Right.

Welcome to Pluto


After a long journey of 9 years in the Solar System, the New Horizons spacecraft has conducted the first in-situ reconnaissance of Pluto. Our Solar System is made by rocky planets like the Earth, Mars, Venus and Mercury that populate its inner regions, by gas giants like Jupiter, Saturn, Uranus and Neptune in the outer regions and several smaller icy dwarf planets that populate the Solar System beyond Neptune’s orbit. Pluto represents the biggest icy dwarf planet known and its surface has remained a big mystery since its discovery in 1930 by US astronomer Clyde W. Tombaugh.

As of today several icy dwarf planets have been discovered but their characteristics are all very poorly known since it is extremely difficult to observe them from Earth given their large distance and small size. Pluto is the biggest (and among the closest) icy dwarf planets known. Pluto is a special planet also because it is a double-planet system together with its (smaller) companion Charon. All icy dwarf planets populate a region of the outer Solar System known as Kuiper belt (see image below) which is a thick torus made of minor bodies and dwarf planets among which Pluto-Charon.


Today several thousands objects are known to populate the Kuiper belt and up to several hundred thousands bodies are theorized to be present in this enigmatic region. The importance of the Kuiper belt objects and dwarf planets like Pluto is that they are believed to be the remnants of the so-called “protoplanetary disc” which existed before the formation of the Solar System and that contained the material that later aggregated into all planets we know (including the Earth). The objects in the Kuiper belt are a sort of “failed planets” which formed at the very early stages of the Solar System. Observing the composition of Pluto, its physical characteristics and the complex structure of the Kuiper belt can therefore shed new light on the formation process of our Solar System.

I’ve heard some people (as usual) questioning the usefulness of such missions especially nowadays where financial and economic crises are widespread . Why did we have to spend $700 million (the cost of the New Horizons mission) to achieve such a result? First of all the mission cost is not so high as it might seem. I have prepared a simple chart where I compare the cost of the New Horizons with some other human
endeavors which, in my humble opinion, are not as spirit lifting as discovering about Pluto, the Kuiper belt and the origin of our Solar System. You can see that the cost of the New Horizons mission is equivalent to the production cost of two movies of the “Pirates of the Caribbeans” series (those with Johnny Depp just to be clear). The
same amount of money has been spent by the company Gillette to produce the Mach 3 Razor blades, a truly leap forward for humanity (especially for its most hairy members). The cost of the football player Cristiano Ronaldo is around 1 billion euros and the building of the “Creation Museum” in Kentucky where people can finally be indoctrinated about the World and the Universe being only 6,000 years old has costed just a bit over $300 millions. Not shown in the chart is the cost of some very important investment made by former boxer Mike Tyson who had the great idea to build a solid gold bathtub for just over $2 millions.


I’ve also prepared a second chart, which this time compares the cost of the New Horizons (total) with the expenditure (total per year!!!) of a few very important human activities like war, lottery sales, recreational drugs, smoking and even the cost of religion tax exemptions. All things we humans can’t truly live without. You judge
who’s flushing money down the toilet here.


Of course beside the scientific value that I explained above, this mission is also about curiosity and being pioneers, two features that truly define our species. It is about knowledge and exploration of the unknown, it is about our place in the Universe. Few months ago we have heard of the spectacular results of the ESA Rosetta mission and the Philae lander, the first man made object to orbit a comet (see picture below) and land on it.


Then we have seen the staggering pictures of the Vesta proto-planet from the NASA Dawn mission. Now let’s enjoy this first exploration of the Pluto-Charon system and the Kuiper Belt and let’s get inspired by the truly astonishing beauty of knowledge.

Update: Latest image of Pluto shows water ice mountains of 3,500 m. 



  1. Health cost of consumption of tobacco worldwide;
  2. Research and development costs of Mach 3 razor; 
  3. Global military expenditure; 
  4. Recreational drug market;
  5. Cost of religious tax exemption; 
  6. Cost of “Creation Museum”, a creationist museum in Kentucky;
  7. Cost of Cristiano Ronaldo, football player of Real Madrid;
  8. Cost for producing two Hollywood movies “Pirates of the Caribbean: On Stranger Tides” and “Pirates of the Caribbean: At World’s End”;
  9. Cost of sales of lottery in just the United States; 
  10. Mike Tyson gold bathtub.

2014 in review

The stats helper monkeys prepared a 2014 annual report for this blog.

Here's an excerpt:

The concert hall at the Sydney Opera House holds 2,700 people. This blog was viewed about 38,000 times in 2014. If it were a concert at Sydney Opera House, it would take about 14 sold-out performances for that many people to see it.

Click here to see the complete report.

Lightweight and Supermassive Black Holes Hidden in Galaxies

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In the past few days there have been two interesting astrophysics news about black holes. The first one is about a super-massive black hole discovered in an ultra-compact dwarf galaxy. Such galaxies are tiny (both in mass and size) when compared to our Milky Way. The galaxy M60-UCD1 (where UCD is the acronym of Ultra-Compact-Dwarf) is a lightweight object made by “only” 150 million solar masses, to be compared with our Milky Way with a mass of 1,250 billion solar masses. The galaxy has also a small radius, a few hundred light years across, whereas our galaxy extends for tens of thousands light years.

Despite its tiny size, this ultra-compact dwarf galaxy contains a super-massive black hole in its center, and a quite peculiar one: its mass is about 20 million solar masses, about 5 times bigger than the super-massive black hole in the center of the Milky Way. The mass of this black hole is not particularly remarkable, as we already know many supermassive black holes well more massive than this one (even a thousand times more massive). However, the discovery is quite extraordinary for two reasons. The first is that the black hole mass constitutes about 15% of the total galaxy mass, which is an absolute record-breaker. Again, for comparison, the super-massive black hole in our galaxy contains slightly more than 0.0003% of the total mass of the Milky Way, whereas in other galaxies the typical value is about 0.5%, still way below the 15% of the new record-holder. Second, this is the first ultra-compact dwarf galaxy discovered to contain a supermassive black hole, meaning that many other similar galaxies might contain one as well. This discovery doubles the current total estimate of supermassive black holes in the Universe.

The second news is also about black holes but not of the supermassive kind. The black hole in question was born after a supernova explosion of a massive star. An international team of researches has proposed that a previously known galactic X-ray binary, named Swift J1753.5–0127, might contain the lightest black hole known to date. According to the theory of General Relativity there is no true lower limit to the mass of a black hole. Indeed anything can turn into such an object when sufficiently squeezed (e.g., your body can turn into a black hole if you compress it down to a radius of, well… 0.0000000000000000000000001 meters). However, since astrophysical black holes form in massive stellar explosions, there is a specific channel of formation that allows only a certain mass range. E.g., a light black hole cannot be created because the gravitational pull will not be sufficient to overcome the nuclear forces emerging during the compression process. When this happens the gravitational collapse is stopped and a neutron star is formed in place of the black hole.

Theoretical calculations show that the minimum astrophysical black hole mass is about 2-3 solar masses, but the minimum mass ever measured for a black hole is approximately 5 solar masses. The reason for the existence of this mass gap between about 2 and 5 solar masses is not established yet, but it might have important implications for our understanding of supernova explosions. Indeed, in the past decade it has been proposed that the energy liberated in a supernova depends on the mass of the exploding star. Such energy might suddenly diminish when the star becomes sufficiently massive to form a black hole remnant. If the energy of the supernova is too low in massive stars, as this model suggests, then a large fraction of the stellar mass will not be expelled during the explosion and a minimum mass budget will always be present during the gravitational collapse. Such mass budget is set at the observed value of about 5 solar masses.

New observations of Swift J1753.5-0127, performed at various optical observatories, among which the Hubble Space Telescope, show that the stellar companion in the binary wobbles as if the black hole weight is below 5 solar masses. The estimated most probable mass is about 4 solar masses. If confirmed this might be the lightest black hole mass measured so far and would induce astronomers to reconsider the existence of a mass gap. This in turn might mean that either the energy of supernovae does not change much with the mass of the exploding star or that black holes in the mass-gap form in a way different than the gravitational collapse that follows supernovae explosions.

Detecting the Sound of Neutron Stars

Post - September 2013 (11)

If a sound propagates and bounces off an object, its wiggly echo brings signatures of the physical properties of that object . Bats, dolphins and other cetaceans are famous for using this principle, named echolocation, to chase prey and identify objects in the environment. Something similar happens also during an earthquake, when a large number of seismic waves are produced in the Earth’s crust. Seismologists can infer the location and depth of the earthquake by looking at the physical properties of seismic waves: their speed and shape depends on the elasticity and depth of the medium through which they propagate.

Something similar can be done also for the Sun, in a branch of astronomy known as Helioseismology. By looking at small vibrations that appear on the surface of the Sun, astronomers can infer many precious information about the inner regions of our star. For example it is thanks to these studies that we know the location of the outer convective layer of the Sun (a fundamental quantity to understand the Sun’s structure and evolution).

Astronomers have long speculated that oscillations of this kind might also be present in exotic objects like neutron stars. Despite intense studies carried over the last 50 years, the internal structure of neutron stars remains today a big astrophysical puzzle. Detecting neutron star oscillations would open a window towards these mysterious inner dense regions, where atoms dissolve and exotic states of matter, never observed on Earth,  appear.

In a recent paper, scientists at the University of Maryland and NASA have discovered one such possible vibration from a neutron star (named 4U 1636-536). This system is an X-ray binary where the outer layers of a small (companion) star are being eaten by the neutron star. During the eating process, the neutron star accumulates hydrogen, helium (which are the two most common elements that compose stars) and other heavy elements stripped from the companion. Carbon, which is heavier than hydrogen and helium, sinks deep down in the bottom layer of the accumulated gas and at some point it can ignite, i.e., carbon nuclei start nuclear fusion and release a tremendous amount of energy as a huge flow of X-ray radiation. This energy flash is called a superburst,  and its name derives from the fact that other X-ray bursts, with lower energy, are seen often in these binary systems.

Since during superbursts the luminosity of the neutron star increases tremendously, it has been possible to detect the stellar oscillation in the X-ray flow recorded by the space telescope Rossi X-Ray Timing Explorer.  According to scientists, it is also possible that the superburst itself has triggered the observed stellar oscillations. An analogous case was reported a few months ago by the same team on a different source (named XTE J1751-305), which is a neutron star that is devouring its white dwarf companion. In that case the detection was less significant, and the newly reported observation of a second oscillation from a second source makes that first detection much more compelling. In that case the oscillation was not associated or observed during a superburst, but it is most likely associated with hot and dense plasma regions located at the magnetic poles of the neutron star. If these oscillations will be confirmed with further measurements and will be observed in more sources, we will have a new and truly spectacular way of determining how matter behaves inside neutron stars.