The Dawn of the Solar System

The Sun and the Solar System are believed to have formed about 4.57 billion years ago, more than 9 billion years after the universe came into existence. As with any other star, the formation of the Sun started with the development of an accretion disk from a nebula. The nebula that would form the Solar System contained all of the 92 naturally occurring elements, because it had incorporated remnants of preceding generations of stars. In the swirling accretion disk, matter was therefore not only present in the form of gas, but also as ice or dust. Because these are the raw materials required to form planets, such an accretion disk is also known as a protoplanetary disk.

Over time, the ball of gas at the center of the swirling protoplanetary disk evolved into a proto-Sun, while the remaining materials developed into a series of concentric rings. Because protoplanetary disks are hotter towards their center, particles of refractory materials (dust) concentrated in the inner rings of the disk whereas particles of volatile materials (ice) concentrated in the outer rings of the disk. The materials of surrounding rings subsequently started to coalesce and form progressively larger objects. Following continuous collisions, some of these objects grew into planetesimals, solid chunks of matter that were so large that they exerted enough gravity to attract other surrounding objects. Eventually, the planetesimals that succeeded in attracting the most matter grew into protoplanets. Once these protoplanets had incorporated essentially all of the matter that was present in their orbits, they would become true planets.

The characteristics of the planets that formed depended on their distance from the proto-Sun. Small terrestrial planets (Mercury, Venus, Earth and Mars) formed in the inner rings of the young Solar System, which consisted mostly of dust, while large gaseous planets (Jupiter, Saturn, Uranus and Neptune) formed in the outer rings, which consisted primarily of gas and ice. Because of their massive size, the outer planets attracted so much additional gas and ice that they evolved into gas giants. Towards the end of planetary formation, the proto-Sun became so hot that it ignited and transformed into the true Sun. This generated a stellar wind – or solar wind – that blew away any remaining gases from the inner region of the Solar System.

Altogether, this model for the formation and evolution of the Solar System is referred to as the nebular hypothesis. It is the most widely accepted model because it is able to explain several key characteristics of the Solar System, including why all planets orbit the Sun in the same direction and why their orbits all occur in the same plane. These observations are all consistent with the formation of stars and planets from gas, ice and dust in an accretion disk rotating around a central mass.

Book reference: Marshak, S. (2007). Earth: Portrait of a Planet: Third International Student Edition. WW Norton & Company.

Image: Artists impression of the Solar System showing the Sun, the eight planets and several other celestial objects. Credit: NASA.

The Origin of the Elements

The first stars formed from nebulae that consisted entirely of atoms of the five lightest elements (H until B), which were formed during big bang nucleosynthesis. Atoms of heavier elements, such as carbon (C), oxygen (O), silicon (Si) and iron (Fe), were all formed later during the life cycles of stars in a process known as stellar nucleosynthesis. Through a progressive series of fusion reactions, stars continuously assemble heavier elements out of lighter elements.

The specific elements that may be formed during stellar nucleosynthesis depend on the mass and temperature of the stars. Stars with a low mass, like the Sun, burn slowly and are able to produce elements with an atomic number of up to 6 (C). In comparison, stars with a high mass, for instance 10 – 100 times the mass of the Sun, burn quickly and are able to produce elements with an atomic number of up to 26 (Fe). However, in order to form elements heavier than Fe, even more extreme conditions are required than those that are generally found in very massive stars. The heaviest elements are therefore mostly formed during supernova explosions at the end of a stellar life cycle.

Atoms may either be released into space during the lifetime of stars, or upon their collapse. If atoms move fast enough to overcome the gravitational pull of their stars, they may escape in streams of gas known as stellar winds. Alternatively, they are discharged in large gas clouds and supernovae during the death of stars. In space, atoms may subsequently form new nebulae or may be incorporated into existing nebulae. So, from the remnants of dying stars, successive generations of stars with an increasingly diverse elemental composition are born.

Book reference: Marshak, S. (2007). Earth: Portrait of a Planet: Third International Student Edition. WW Norton & Company.

Image: Galactic center of the Milky Way as seen by the Hubble Space Telescope, Spitzer Space Telescope and Chandra X-ray Observatory. Credit: NASA/JPL-Caltech/ESA/CXC/STScl.

The Formation of the First Stars

Approximately 200 million years after its inception, the universe consisted of massive, slowly swirling nebulae with large voids in between. Because of the effects of gravity, denser regions of these nebulae started to attract gases from their surroundings and thereby started to grow in mass. These regions pulled in progressively more matter and as they became more condensed over time, the initial swirling movement of the gases transformed into a progressively faster rotation around an axis in accretion disks. Eventually, the gravitational attraction of these spinning accretion disks grew strong enough to cause complete inward collapse of the surrounding nebulae. Gravity further moulded the inner portions of these accretion disks into dense balls and consequently large amounts of energy were transformed into heat. These hot balls of gas ultimately became the first precursors of stars, so-called protostars.

Protostars continued growing until their cores became very dense and reached a temperature of approximately 10 million degrees. Under these conditions, hydrogen nuclei joined to form helium nuclei in a series of fusion reactions that released tremendous amounts of energy. The bodies of the protostars began to light up, resulting in the formation of the first true stars approximately 400 million years after the universe was born.

Stars of the first generation were generally very massive (for example, 100 times the mass of the Sun) because of the large amounts of matter present in the young nebulae. These stars burned very hot and bright, but consequently their lifetime was also relatively short – only a few million years. When stars exhaust all of their resources, they die in a dramatic explosion and flash of light known as a supernova.

Book reference: Marshak, S. (2007). Earth: Portrait of a Planet: Third International Student Edition. WW Norton & Company.

Image: The Pillars of Creation in the Eagle Nebula as seen from the Hubble Space Telescope. Credit: NASA/ESA/Hubble Heritage Team.

The Birth of the Universe

Before the universe as we know it came into existence, all matter and energy is believed to have started out in an infinitesimally small point. For reasons yet unknown, this point exploded approximately 13.7 billion years ago in a cataclysmic event known as the big bang, after which the universe started to expand.

In the first moments of its existence, the universe was so dense and hot that it consisted entirely of energy, but already within seconds it had cooled enough for atoms of the lightest element – hydrogen (H) – to form. During the following minutes, new atomic nuclei of other light elements such as helium (He) were formed through the collision and fusion of hydrogen atoms, which is referred to as big bang nucleosynthesis. This process continued until the universe was approximately 5 minutes old, when it had expanded so much that atomic collisions became increasingly rare. At this point in time, all matter was present in a plasma state consisting of atomic nuclei scattered in a dynamic ocean of electrons.

After a few hundred thousand years, temperatures decreased to a few thousand degrees and neutral atoms with a positively charged nucleus orbited by negatively charged electrons were formed. The appearance of chemical bonds between atoms of specific elements subsequently gave rise to the first molecules. Upon further expansion and cooling, atoms and molecules accumulated into clouds of gas called nebulae. The earliest nebulae consisted only of the lightest elements, including hydrogen (74 %), helium (25 %), and trace amounts of lithium (Li), beryllium (Be) and boron (B).

Book reference: Marshak, S. (2007). Earth: Portrait of a Planet: Third International Student Edition. WW Norton & Company.

Image: Timeline of the universe from the big bang until present-day. Credit: NASA/WMAP Science Team.

Lake Baikal, Siberia, Russia

Lake Baikal is located in the southern part of eastern Siberia in Russia and is the oldest and deepest freshwater lake on Earth. Containing approximately 20 % of all fresh water at Earth’s surface, it is also the largest freshwater lake in the world by volume. The lake was formed approximately 25 million years ago in an ancient rift valley in the Baikal Rift Zone and is surrounded by mountains on all sides. Lake Baikal is characterized by a very high biodiversity, hosting many plant and animal species that exist nowhere else in the world, such as the Baikal seal or nerpa. In winter, the surface of the lake becomes completely frozen.

Information source: Encyclopaedia Brittanica, Wikipedia

Image: Shamanka, or Shaman’s Rock, along the shores of Olkhon Island in Lake Baikal in southern Siberia, Russia. Credit: Виктория Шерина, Wikimedia Commons.

Observation of gravitational waves proves Einstein’s general theory of relativity

An international group of more than a thousand physicists and astronomers has proven the existence of gravitational waves, a 100 years after Albert Einstein’s initial predictions that dramatic outbursts of energy could generate ripples in spacetime at the speed of light. On September 14, 2015, a transient gravitational wave signal was simultaneously observed by the two detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States. The recorded signal was studied extensively and is thought to have resulted from the collision and merger of two black holes into a single, massive black hole, approximately 1.5 billion years ago. This gravitational wave signal demonstrates the existence of binary black hole systems and is not only the first direct observation of a binary black hole merger, but, more importantly, it represents the most convincing evidence for Albert Einstein’s general theory of relativity to date. Because gravitational waves contain information about their origins and the nature of gravity itself, their discovery holds great promise for improving our understanding of the universe.

Click here to see the fascinating story behind the discovery of gravitational waves.

Journal reference: Abbott, B. P. et al. (LIGO Scientific Collaboration and Virgo Collaboration) (2016). Observation of Gravitational Waves from a Binary Black Hole Merger. Physical Review Letters, 116(6).

Image: Black hole at the center of the Centaurus A galaxy, 13 million lightyears away from Earth. Credit: ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray).

Black Smokers

Black smokers are hydrothermal vent structures that discharge hot, dark fluids from the ocean floor. They generally form near areas with submarine volcanic activity, such as mid-ocean ridges, where hydrothermal fluids circulate through the oceanic crust and exchange elements with the surrounding rocks. As these hot fluids come into contact with the much colder seawater, the dissolved minerals precipitate into particles and may form spectacular chimney-like structures.

The waters surrounding hydrothermal vents are often very hot and acidic, but nevertheless life is able to thrive under these extreme environmental conditions. Select groups of bacteria, known as extremophiles, can survive in these hostile environments and are capable of harnessing the hydrogen sulfide (H2S) emitted by black smokers as an energy source through chemosynthesis. These bacteria subsequently attract and are eaten by higher organisms, such as crustaceans, worms, molluscs and fish. As a result, hydrothermal vents may form the basis of entire ecosystems that are independent of solar energy. The biodiversity of ecosystems near black smokers is even thought to be higher than in the rest of the deep ocean.

For more information on black smokers and how they form, check out this article on

Information source: National Oceanic and Atmospheric Administration (NOAA).

Image: Black smokers and tube worm communities at Sully Vent in the Main Endeavour Vent Field in the northeast Pacific Ocean. Credit: NOAA.

Dallol, Danakil Depression, Ethiopia

Dallol is a volcanic explosion crater, or maar, in the Danakil Depression in northeast Ethiopia. It is located approximately 50 meters below sea level and has been formed by the intrusion of magma underneath Miocene evaporite deposits in the Afar Triangle of the East African Rift system, followed by several eruptions. Subsequent hydrothermal activity has led to the formation of hot springs and brines in a landscape with striking red, yellow and green colors related to the presence of iron oxide, sulfur and microbes. Dallol is known as one of the hottest places on Earth, with average temperatures of 35 degrees Celsius throughout the year.

Information source: Smithsonian Institution National Museum of National History Global Volcanism Program

Image: Salt and sulfur deposits near the hot springs of Dallol in the Danakil Depression, Ethiopia. Credit: Ji-Elle, Wikimedia Commons.

Zooplankton migrate under influence of moonlight in dark Arctic winter

New research performed by biologists from different universities has shown that zooplankton migrate vertically through the waters of the Arctic Ocean by using moonlight during dark winter nights. By studying acoustic data recorded by moored instruments, the scientists found that the vertical migration patterns of zooplankton in winter are driven by lunar illumination across the entire Arctic, in fjord, shelf, slope and open sea environments. During the Arctic winter, zooplankton shift their known diel vertical migration (DVM) periods from a solar day (24 hours) to a lunar day (24.8 hours) when the moon rises above the horizon. In addition, mass sinking of zooplankton from the surface waters to a depth of approximately 50 meters occurs during periods of full moon (every 29.5 days). The scientists suggest that lunar vertical migration (LVM) may enable zooplankton to avoid visual predators, such as carnivorous zooplankters, fish and birds, which use moonlight to hunt during the polar night. The discovery of LVM in the Arctic indicates that light-mediated patterns of biological migration may occur even without the presence of sunlight and has important implications for the exchange of carbon between the surface waters and deeper waters during the Arctic winter.

Journal reference: Last, K. S., Hobbs, L., Berge, J., Brierley, A. S. & Cottier, F. (2016). Moonlight Drives Ocean-Scale Mass Vertical Migration of Zooplankton during the Arctic Winter. Current Biology, 26, 1-8.

Image: Icebergs in the Arctic near Cape York, Greenland. Credit: Brocken Inaglory, Wikimedia Commons.

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