Seven Earth-like exoplanets discovered orbiting a single nearby star

Astronomers at the University of Liège in Belgium have discovered seven Earth-like exoplanets orbiting a single, nearby star called TRAPPIST-1. The scientists have uncovered these planets with NASA’s Spitzer Space Telescope and several ground-based telescopes, by detecting small decreases in the light intensity of the star as the planets passed in front of it. TRAPPIST-1 is located approximately 40 lightyears from the Earth in the constellation Aquarius and is so small and cool that all seven planets feature temperate conditions, suggesting that liquid water could be present at any of their surfaces. Moreover, three of these planets are located within the habitable zone, the area around a star where conditions are most favorable for life. This discovery, which has been published in Nature, represents a new record for the greatest number of habitable-zone planets found in a single star system and is therefore an important milestone in the search for extraterrestrial life.

For more on the story behind this fascinating discovery, watch the video by NASA below.

Journal reference: Gillon, M. et al. (2017). Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1. Nature, 542(7642), 456–460.

Image: Artist’s impression of the surface of TRAPPIST-1f, one of the newly discovered planets in the TRAPPIST-1 star system. Credit: NASA/JPL-Caltech.

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.

Pillars of Creation

The Pillars of Creation are pillars of interstellar gas and dust in the Eagle Nebula, ~ 5 lightyears tall and at a distance of ~ 7000 lightyears from the Earth. Massive new stars are being formed within the pillars, but at the same time the gas and dust are being eroded by the intense ultraviolet light from nearby young stars. The different colors in the Pillars of Creation result from the electromagnetic emission from different elements: blue is related to oxygen, orange is related to sulfur and green is related to both hydrogen and nitrogen.

Information source: NASA

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