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Water in Trappist-1 system, illustration. Trappist-1 is a red-dwarf star – the most common variety – located some 40 light-years away in Aquarius. In 2015, astronomers discovered that Trappist-1 was host to three earth-sized planets. Then it came under the spotlight again in 2017 when NASA scientists found an additional four planets, taking the total up to seven. This is the most terrestrial planets that have ever been found to orbit a single star, including our own Solar System. Trappist-1 is only fractionally larger than Jupiter in diameter. This image shows the star and six of the planets as they would appear from the vantage point of the fifth outermost planet, Trappist-1f, which is depicted as being covered in water. All of the planets and the Sun are to scale. One of the worlds is seen transiting in front of the star.
Illustration of a view of Pluto seen from the surface of its largest moon Charon. Because Pluto and Charon are tidally locked, they keep the same face towards each other at all times, as the Moon does to the Earth. So if one stood on Charon (or Pluto) the other world would stay fixed in the sky - never setting or rising, but still cycling through its phases. And if one were on the wrong hemisphere of Pluto (or Charon) one would never see the other world.
Illustration of the Earth, Moon and Sun showing a passing comet. Cities are seen glistening, defining the edges of the Earth's continents. Comets are balls of loosely packed 'dirty ice'. As they near the Sun, their gases sublimate and form long tails blustering away from the star. The tails can stretch for tens of thousands of kilometres, dwarfing even the Earth-Moon separation.
Illustration of a sungrazing comet. These are comets that pass very close to the Sun at perihelion. Sometimes they can skirt above the photosphere at distances of just a few thousand kilometres - the mere diameter of a small planet. Occasionally comets are completely evaporated in this process, but some can last several passes before either falling into the Sun or disintegrating because of tidal forces.
Evolution of Mercury, illustration. The innermost planet, Mercury, has a very substantial iron core, occupying much of the planet's interior. It is far larger compared to the radius of the planet than the iron core of Earth, for example. Astronomers think this is because, shortly after the planet formed, more than four billion years ago, it was hit by another planet, as depicted in this illustration. (1) The impacting planet approaches the proto-Mercury. (2) The objects collide, an impact which removes Proto-Mercury's outer, rocky mantle, but leaves its iron core largely intact. (3) The rocky ejecta from the impact is blown into space, away from Mercury. It does not reaccrete. (4) After impact, Mercury cools and reforms. It is now smaller than before and, having lost much of its mantle, is left with an oversized iron core.
Illustration of jets (upper and lower centre) emanating from the poles of a young star. A dark circumstellar disc of dust is seen nearly edge-on across centre. Discs such as this are thought to be the precursors of planetary systems, with planets forming as the dust coalesces. The jets (bipolar molecular outflows) are thought to form as material ejected from the star is forced into two jets by the rotation of the disc.
Diagram showing the theoretical interior of the ice giant planet Neptune. It probably looks very similar to the interior of Uranus. At the very centre is a rocky and icy core, similar and size and mass to the Earth. This is encased in a thick inner mantle, a slushy mixture of various ices including methane, ammonia and water. Above this is the outer mantle, which is made up of a mixture of liquid hydrogen and other elements. And finally there is a thick atmosphere, composed chiefly of hydrogen, helium and methane.
Like Jupiter, Saturn is surrounded by a large system of varied satellites. This composite shows the nine largest on the same scale. From left to right, in order of increasing distance from Saturn, they are: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus and Phoebe. For comparison, our Moon is about 67 per cent the size of Titan. Saturn is shown at the bottom, on the same scale.
Diagram showing the interior of the Sun. The solar interior is composed of a core (central 30%) a radiative zone outside this, and finally a convective layer occupying the outermost 30% or so. Solar astronomers, using a branch of astronomy called helioseismology, have established that the Sun vibrates in several different modes, somewhat like the skin of a drum, as indicated in this image by the colours on the surface. The blue regions indicate where the surface is moving outwards, while the red ones show where the movement is downwards
Diagram showing the theoretical interior of the ice giant planet Neptune. It probably looks very similar to the interior of Uranus. At the very centre is a rocky and icy core, similar and size and mass to the Earth. This is encased in a thick inner mantle, a slushy mixture of various ices including methane, ammonia and water. Above this is the outer mantle, which is made up of a mixture of liquid hydrogen and other elements. And finally there is a thick atmosphere, composed chiefly of hydrogen, helium and methane.
Image comparing the size of Earth with its single Moon. The Moon is 27% the size of its parent, which it orbits once every 27.3 days. Despite its diminitive size compared to Earth, the Moon is actually very much larger, compared to ts parent, than any other natural satellite in the Solar System, with the exception of Pluto's largest moon, Charon.
Image comparing the size of Earth (left) with the planet Uranus. Uranus is the seventh planet from the Sun, with an average distance from it of 19.2 times the Earth-Sun distance. A fluid world of mostly hydrogen and helium, it is rich in ices of methane, water and ammonia, causing some astronomers to label it (along with Neptune) an ice giant. With a diameter of four times that of the Earth, Uranus is the Solar System's third largest planet (after Jupiter and Saturn).
Image comparing the size of Earth (left) with the planet Saturn. Saturn is the sixth planet from the Sun, with an average distance from it of 9.4 times the Earth-Sun distance. Being a fluid world of mostly hydrogen and helium, astronomers label it (along with Jupiter) a gas giant. With a diameter of 9.4 times that of the Earth, Saturn is the Solar System's second largest planet (after Jupiter).
Illustration of a black dwarf. A black dwarf is the fate that awaits all white dwarfs. The latter are the remains of the cores of dead stars, such as the Sun, after the stars shed their outer layers during the red giant and planetary nebula phases of evolution. White dwarfs only shine by stored energy - they produce no new light. They are destined to cool down forever, eventually disappearing entirely from the electromagnetic spectrum. They are then called black dwarves. However, the process takes so long - longer than the present age of the universe - that no black dwarves will form for perhaps trillions of years.
Diagram showing the interior of the Earth's Moon. The outermost layer, the crust, is about 45 miles (70 km) thick. This is thicker than the Earth's crust, which cooled down at a much slower rate. Beneath the crust is a thick silicate mantle, then a zone of partial melt with a radius of 480 km. This is probably where moonquakes occur. Current thinking, based on a re-examination of Apollo lunar seismometer data, is that the core, once thought solid, is now composed of a liquid outer component (330 km radius) and a solid inner one (240 km).
Diagram showing the interior of the Earth's Moon. The outermost layer, the crust, is about 45 miles (70 km) thick. This is thicker than the Earth's crust, which cooled down at a much slower rate. Beneath the crust is a thick silicate mantle, then a zone of partial melt with a radius of 480 km. This is probably where moonquakes occur. Current thinking, based on a re-examination of Apollo lunar seismometer data, is that the core, once thought solid, is now composed of a liquid outer component (330 km radius) and a solid inner one (240 km).
Artwork of alien technological planet. The planet is in orbit around a red dwarf star, the most common type. The red dwarf is relatively sedate, making the environment of its habitable zone conducive to life. The planet is shown with its night side brilliantly lit by major cites and technology.