The Sun
Our nearest star is the Sun. It is the centre of our solar system and provides heat and light for the planets, asteroids and other objects that circle around it. Despite playing such a critical role in our solar system, the Sun is in fact a fairly average type of star, only one of over 100 billion in the Milky Way galaxy.
The Sun is an enormous ball of gas, mainly hydrogen and helium, with no solid centre or surface. In its very hot centre regions, hydrogen nuclei (hydrogen atoms without the surrounding electrons) come together to form helium nuclei (helium atoms without the surrounding electrons). As each helium nucleus has slightly less mass than the four hydrogen nuclei that formed it, energy is released according to Einstein’s famous formula, E = mc2, where E is energy, m is the mass loss and c is the speed of light.
The energy slowly makes its way outwards from the centre. It is initially transferred in the form of radiation like the heat from an electric bar radiator, while closer to the surface the transfer is by cells of hot gas rising, cooling and then sinking.
On its visible surface the Sun often shows dark spots. These sunspots are regions where strong magnetic fields inhibit the upward flow of heat and are therefore cooler and darker than the rest of the surface. Sunspots are only one example of activity on the Sun; huge explosions called flares release strong bursts of radiation, while coronal mass ejections throw large clouds of fast atomic particles into space.
THE SUN FACTS
Temperature of visible surface: 5500°C
Temperature at centre: 15.7 million °C
Diameter: 1.392 million km
Period to turn around axis: 25 days (equator)
Number of known planets: 8
The large sunspot group number AR2371 near the centre of the Sun on 21 June 2015. Courtesy SDO/HMI
Update: We are past the peak of the current solar cycle, number 24, which has had the lowest amplitude for a hundred years. Scientists plot the number of visible spots on the surface of the Sun against time to see the changes in the activity of the Sun. These plots show cycles that last for an average of 11 years.
The bright patch in this extreme ultraviolet image is a flare or explosion associated with the sunspot group AR2371 that was active between 21 and 25 June 2015. Courtesy Solar Dynamics Observatory, NASA
Some intense activity still occurs occasionally on the Sun despite it being on the decreasing part of its activity cycle. Between 21 and 25 June 2015 sunspot group AR2371 (see image opposite) was associated with flares (see image above) and coronal mass ejections.
Coronal mass ejections consist of large clouds of charged particles that take two or three days to reach the Earth and its magnetic field. When one does it can lead to the light display in the sky called an aurora. Normally only seen near the south and north poles, on 23 June 2015 the faint pink glow of an aurora could be glimpsed from as far north as Sydney; the light show was also reported from Tasmania and parts of Victoria.
Although until recently scientists could only observe flares and coronal mass ejections at large scales, such activity seems to occur on the Sun at all scales. Using satellites, scientists at NASA’s Marshall Space Flight Center in Huntsville, Alabama, made the first observations of explosions from small-scale filaments. Filaments are tubes of relatively cool, electrically charged gas rising above the Sun’s visible surface supported by magnetic fields.
Explosions from large-sized filaments give rise to coronal mass ejections, while the miniature versions now observed are associated with jets of X-rays that expel hot gas into the Sun’s outer atmosphere, the corona.
The planet Mercury is the closest planet to the Sun and is named after the swift-footed messenger to the gods of Roman mythology. Of the planets that are visible with the unaided eye, Mercury is the hardest to see. This is because it always appears near the Sun and is often lost in its glare. We can only see Mercury when it is low on the horizon, just before sunrise or just after sunset.
A strange fact about Mercury is that its day, the time from one sunrise to the next, is twice as long as its year, the time it takes to travel around the Sun. Because Mercury is so close to the Sun, it is extremely hot — up to 430°C on the side facing the Sun. But on the side facing away from the Sun the temperature falls to about –170°C. These temperature extremes are caused by the lack of a proper atmosphere.
In January 2008 NASA’s Messenger spacecraft flew by Mercury for the first time since Mariner 10’s mission in 1975. Data collected by Messenger shows that lava ejected from volcanic vents is likely to have played a role in forming the planet’s surface. Scientists located a number of volcanic vents along the edges of the giant 1500-km-wide Caloris impact basin.
Messenger’s images again confirm that the surface is crossed by hundreds of long cliffs called rupes (the Latin word for cliff). Rupes indicate faults in the crust of the planet caused by the contraction of the surface. Mercury has an iron core that makes up about 60 per cent of its mass. It appears that the planet shrunk due to the cooling of this large core.
MERCURY FACTS
Diameter: 4878 km
Distance from Sun: 58 million km
Period to travel around Sun: 88 days
Period to turn around axis: 59 days
Number of known satellites: 0
A 120-km-wide impact crater is at the centre of this view of Mercury’s horizon, taken by the Messenger spacecraft on 2 October 2013. Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Craters permanently in shadow at Mercury’s north and south poles have been found to contain water ice. Given the closeness of the planet to the Sun, this is a surprising fact, although Earth-based radar observations had given indications of this possibility. Now a number of sensors on the Messenger spacecraft have confirmed the presence of water ice.
A Messenger spacecraft image of a crater on Mercury named after John Lennon of The Beatles. Courtesy NASA/ Johns Hopkins Applied Physics Lab/ Carnegie Institution
A sensor on the spacecraft, called a neutron spectrometer, measured the concentration of hydrogen and thereby water, which is, of course, made up of hydrogen and oxygen. Not only did the sensor find ice in some permanently shaded craters, it also found that, in many cases, the ice was covered by a surface layer 10 to 20 cm in depth. This ‘blanket’ on top cools the ice sufficiently for it to stay frozen.
Scientists believe that both the water and its blanket of insulating material were brought to Mercury by small comets and asteroids, which crashed into the planet in the early days of the solar system.
Update: The Messenger spacecraft that has been circling and observing Mercury since 18 March 2011 crashed onto the surface of the planet on the morning of 1 May 2015. Mission operators allowed the spacecraft to crash as it had exhausted all of the propellant that had helped to maintain it in orbit. After the crash at an estimated velocity of about four kilometres per second, Mercury has a new crater that could be up to 15 metres wide.
During its four years circling Mercury, Messenger sent back over 250,000 images of the planet as well as other data obtained by the spacecraft’s various instruments. Thanks to Messenger, scientists have established the surface composition of the planet and its geological history, and gained an understanding of its magnetic field.
Venus, named after the Roman goddess of love and beauty, is the brightest object in the night sky, after the Moon. People often call Venus the Morning or the Evening star because it is always seen just before sunrise in the east, or just after sunset in the west.
The best time to examine Venus through a telescope is just after sunset at twilight. Later, when the sky becomes darker, Venus appears so bright that sometimes our eyes cannot focus on it properly. No surface features can be seen due to the planet’s clouds, but we can see phases like those on the Moon. This is because Venus is always closer to the Sun than the Earth. Phases on Venus were first discovered by the Italian astronomer Galileo in 1610 and helped to convince him of the then revolutionary idea that the Earth circled the Sun.
Venus has a thick atmosphere of mainly carbon dioxide with clouds of sulphuric acid. This dense atmosphere hides the surface from view and explains why Venus has the highest surface temperature of all the planets, 470°C. The atmosphere acts like a greenhouse, letting the Sun’s radiation in but not letting the heat back out. (Increasing levels of carbon dioxide in our atmosphere are concerning scientists about a ‘greenhouse effect’ on Earth.)
Update: A report published in 2015 indicates that Venus may have active volcanoes. Scientists looking at infrared images from the European Venus Express spacecraft circling the planet have identified temporary hotspots on the surface. They interpret these as volcanic eruptions.
VENUS FACTS
Diameter: 12,104 km
Distance from Sun: 108 million km
Period to travel around Sun: 225 days
Period to turn around axis: 243 days
Number of known satellites: 0
Venus imaged by the Messenger spacecraft on 5 June 2007. Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Earth is the third planet from the Sun. With oxygen in its atmosphere and water in its rivers and oceans, it has ideal conditions for life. Although some scientists think that Mars may have harboured primitive bacteria in the distant past, Earth is still the only planet in the solar system which we know to have life.
Earth moves around the Sun in a giant, nearly circular path 300 million km wide. The path is slightly elongated — Earth is 3 per cent closer to the Sun in the Australian summer than in winter. This does not cause summer or winter! It has a minimal effect compared to the effect of the tilt of the Earth, which is the real cause of the seasons. In summer the southern part of the globe is tilted towards the Sun, while in winter it is tilted away from the Sun.
Although the only part of Earth that we can study directly is its surface, we do have a good idea of what is inside. Earthquakes create vibrations which are picked up at a number of observatories dotted around the surface of the globe, allowing scientists to study the planet’s interior. It seems that at the centre there is a solid core mainly made of iron. Around it is an outer core mainly made of liquid iron. Electric currents in this liquid core give rise to Earth’s magnetic field and make a compass point in the right direction.
On the Earth’s surface are continents and oceans. The continents are slowly shifting relative to each other as they are supported by a system of moving plates. Boundaries between these plates are zones with increased risk of earthquakes. A collision between the Pacific and the Australian plates led to the devastating Christchurch earthquake of 22 February 2011.
EARTH FACTS
Diameter: 12,756 km
Distance from Sun: 150 million km
Period to travel around Sun: 365 days
Period to turn around axis: 24 hours
Number of known satellites: 1
The Earth Polychromatic Imaging Camera (EPIC) on NASA’s DSCOVR spacecraft caught the Moon crossing in front of the Earth on 16 July 2015. Courtesy NASA/NOAA
Because of its orange-red appearance in the night sky, Mars is often called the ‘red planet’. It derives its name from the Roman god of war.
Mars is similar to Earth in a number of ways. A day on Mars is about the same length as a day on Earth. The tilt of Mars’ axis of rotation is also about the same as Earth’s — it therefore has similar seasons. Using a telescope, it is possible to see that the surface of Mars goes through changes in step with the change of seasons. This led people in the past to believe (incorrectly) that Mars had life, perhaps even a thriving civilisation. Today we know these changes are due to gigantic seasonal dust storms, which alternately cover and uncover darker areas of the surface.
There is no liquid water on Mars. All water is frozen, either in the polar caps or in the soil as permafrost. Mars appears red because the iron-rich surface is rusted by the locked-up water. Close-up photographs of Mars have revealed winding valleys and channels that have convinced most scientists that Mars had a thicker atmosphere and running water in the past (see page).
The best way to study Mars would be for geologists to visit the planet and roam its surface. However, it is a long, dangerous and lonely trip to Mars. The astronaut geologists would need to take several years’ supply of food, oxygen and medicines. They would also need huge quantities of rocket fuel for the return trip!
MARS FACTS
Diameter: 6787 km
Distance from Sun: 228 million km
Period to travel around Sun: 687 days
Period to turn around axis: 25 hours
Number of known satellites: 2
The Hubble Space Telescope imaged Mars on 26 August 2003, just 11 hours before the planet was at its closest to Earth in nearly 60,000 years. Courtesy NASA, J Bell (Cornell University) and M Wolff (Space Science Institute)
At present robotic explorers examine Mars on our behalf. Curiosity, the roving Mars Science Laboratory, survived a difficult descent through the planet’s thin atmosphere to land in the Gale Crater on 6 August 2012. Curiosity’s mission is to study the environment and geology to discover if conditions in the crater could allow for the existence of microbial life forms, either today or in the past.
Curiosity achieved its main aim early on when drilling a rock in the Yellowstone area of Gale Crater. In the grey powder extracted from the rock it found clay and sulphate minerals, plus other chemicals that could have provided energy for microbes early in the history of Mars.
Clay is formed from the reaction of water with certain types of rocks, such as those deposited by cooling lava. So finding clay shows that the area must once have been wet. Water could only have existed on the Martian surface if the planet had a substantial atmosphere in the past.
Update: In a major discovery, scientists using observations from NASA’s Mars Reconnaissance satellite claim to have found running water on the surface of Mars. Previously there have been strong indications of frozen water below ground in some regions, as well as water on the surface in the distant past (as discussed above). The new observations, however, suggest running water today in a number of places where dark streaks appear on downward slopes when the temperature rises above –23°C.
Dark streaks on the wall of the Mars Garni Crater. Courtesy NASA/ JPL-Caltech/University of Arizona
An instrument on the satellite called a spectrometer has revealed salts associated with water molecules in these streaks whenever they were relatively wide. Such salts can lower the freezing point of water sufficiently for it to stay liquid on the cold surface. Though finding the water is significant as it could sustain life, it should be noted that such contaminated salty water is dangerous to humans and is unlikely to be favoured by most bacterial organisms, if they exist on the planet.
Jupiter is by far the largest planet in the solar system: 11 times wider than Earth, over 1300 times Earth’s volume and nearly three times as massive as all the other planets put together. It is a huge ball of gas with no solid surface and is the second brightest planet in the night sky. (Venus is the brightest.) Even through a small telescope you can see Jupiter as a disc flanked by its four largest moons: Io, Europa, Ganymede and Callisto. These moons are often called the Galilean Satellites after their discoverer, the Italian astronomer Galileo.
Due to its large size and strong gravity, Jupiter is the most likely planet to be hit by a comet. This was observed for the first time in July 1994 when Jupiter was bombarded by the fragments of Comet Shoemaker-Levy 9. These fragments slammed into Jupiter at 200,000 km per hour, becoming extremely hot and forming fireballs as they plunged into the planet’s atmosphere. These fireballs rose thousands of kilometres above Jupiter and faded, leaving behind dark clouds.
Jupiter’s most conspicuous feature is the Great Red Spot, which was first seen in 1664. This is a giant storm or cyclone that has lasted for over 300 years and has a width of 25,000 km, almost twice the size of Earth. It is spinning in a counter-clockwise direction, indicating that, unlike cyclones on Earth, it is a high-pressure system. To astronomers the Great Red Spot provides a laboratory for the study of weather patterns under unfamiliar conditions.
JUPITER FACTS
Diameter: 142,980 km
Distance from Sun: 778 million km
Period to travel around Sun: 12 years
Period to turn around axis: 10 hours
Number of known satellites: 67
A montage of images of Jupiter and its moon Io, taken by the New Horizons spacecraft. The Jupiter image uses infrared wavelengths that highlight the clouds of the planet, while the Io image is in ordinary light and shows an eruption from the volcano Tvashtar. Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Goddard Space Flight Center
Jupiter’s innermost moon, Io, has at least 400 volcanoes active on its surface, making it the most volcanic body in the solar system. The volcanoes can eject plumes of material up to a height of about 500 km. Recent research has confirmed that all this activity is being driven by an ocean of magma or molten rock under the surface of the moon.
NASA’s Galileo spacecraft, which circled Jupiter from 1995 until 2003, found that Io caused disturbances in Jupiter’s extremely strong magnetic field. Scientists suspected the moon had a conductive layer through which Jupiter’s magnetic field was generating an electric current to create Io’s own magnetic field. However, what and where this conductive layer was remained unknown.
Experiments in the last few years have shown that a volcanic rock, called ‘ultramafic’ rock, has the required conductivity when melted. Scientists now believe that the conductive layer under Io’s surface is composed of this type of molten rock, which has a high content of magnesium, a little silica and some iron.
The layer of magma is likely to be at least 50 km thick. The magma is heated to a temperature of 1200°C by the changing tides raised by Jupiter as Io circles the planet at slightly changing distances. Volcanoes then bring this molten rock to the surface where it spreads out in the form of lava.
Update: After an almost five-year journey NASA’s Juno spacecraft is expected to reach Jupiter on 4 July 2016. The spacecraft will map the magnetic and gravitational fields of the giant planet with a view to gaining insights into its interior. To that end the spacecraft will also measure water vapour in the planet’s atmosphere. It will be a difficult and dangerous mission; in order to make its measurements the spacecraft has to approach the planet’s intense radiation more closely than any other spacecraft has done before.
An artist’s conception of NASA’s Juno spacecraft on a close approach to Jupiter. Courtesy NASA/JPL-Caltech
According to Roman mythology, Saturn was the god of agriculture and father of Jupiter, king of the gods. Nine times the diameter of the Earth and 750 times its volume, Saturn is second in size only to Jupiter. In beauty it is second to none.
Even through a relatively small telescope, Saturn can be seen as a bright disc surrounded by a spectacular ring. This ring was discovered in 1655 by the Dutch astronomer Christiaan Huygens. Later astronomers noticed that the ring is divided into three separate rings. In recent times Voyager spacecraft have shown that each of the rings seen from Earth is itself divided into thousands of narrow rings. The rings are made up of billions of icy chunks, ranging in size from a grain of sand to a large house. Although they stretch over 300,000 km, they are believed to be less than 1 km thick.
Saturn’s largest moon, Titan, is one of the most interesting objects in the solar system, for it is the only moon around any of the planets with a significant atmosphere. This atmosphere is mainly made of nitrogen and some methane. It is so thick that it hid Titan’s surface from the cameras of the Voyager spacecraft.
Saturn has 54 named moons, of which 12 were announced in 2005. Many of these moons interact with the rings and with each other in complex ways. For instance, Prometheus and Pandora act like shepherds to the outer F-ring. The gravitational pull of the two moons, circling Saturn just inside the ring and just outside respectively, prevents the ring from spreading out and so keeps it narrow.
SATURN FACTS
Diameter: 120,540 km
Distance from Sun: 1426 million km
Period to travel around Sun: 29 years
Period to turn around axis: 11 hours
Number of known satellites: 62
The moons Dione on the left and Mimas on the right are dwarfed by giant Saturn in this Cassini spacecraft image taken on 27 May 2015. NASA/JPL-Caltech/Space Science Institute
In 2005 NASA’s Cassini spacecraft discovered jets of icy water vapour being ejected from narrow openings, called ‘tiger stripes’, on Saturn’s moon Enceladus. Cassini has now found that the intensity of the jets varies in a predictable way in relation to the distance of Enceladus from Saturn. The observations suggest that the jets are at their most intense when the moon is furthest from Saturn and reduce to about a quarter of that intensity when the moon is nearest to Saturn.
Cassini took this image of Saturn’s odd sponge-like and potato-shaped moon Hyperion during its final close pass of the moon on 31 May 2015. Courtesy NASA/JPL-Caltech/Space Science Institute
Scientists believe that the jets originate from a sea of salty water underneath Enceladus’ icy surface. They hypothesise that when the moon is close to Saturn its strong gravitational pull squeezes the tiger stripes shut so that only small amounts of water vapour can escape. As the moon moves further away from the planet, its gravity lessens and the tiger stripes open up, releasing a stronger jet of vapour.
Update: Scientists studying two separate chemicals using Cassini observations have found evidence for hot water vents operating in Enceladus’ underground ocean. If the vents exist, they would be of great interest as on Earth underwater hydrothermal vents, especially those known as black smokers, sustain complex ecosystems of basic life forms.
One study relates to tiny grains of silica rock found in the Saturn system. The grains most likely form when hot mineral-rich water from the interior of Enceladus suddenly drops in temperature as it meets the cooler waters of the underground ocean.
The second study focuses on the methane found in the plumes on Enceladus along with ice and water vapour. A likely scenario is that so much methane from hydrothermal vents reaches the underground ocean that the ocean becomes supersaturated; that is, it cannot dissolve all the methane pouring into it.
In 1781 the musician and amateur astronomer William Herschel discovered the first new planet to be found since ancient times. It was named Uranus after the Roman god of the sky, who was the father of Saturn and grandfather of Jupiter.
At the top of Uranus’ atmosphere is a layer mainly of hydrogen, with about 10 per cent helium. Beneath this are clouds of methane ice, and further down, clouds of ammonia and water. The planet lies on its side, with its axis inclined 98°. This means that unlike the other planets, Uranus’ poles face towards the Sun. No-one knows why this is so, but one theory is that an ancient collision knocked Uranus on its side.
Uranus, like Saturn, has rings, but they are much thinner, narrower and darker. Astronomers discovered the main rings in 1977 as they watched Uranus pass in front of a star and saw the star blink on and off due to the rings blocking the starlight. Subsequently, a total of 11 rings were identified; more recently, the Hubble Space Telescope found two more. Both of these are far out from the planet with the span across the larger one twice that of the previously known rings.
As rings continually lose the dust and other particles from which they are made, they can only survive for an extended period of time if they have a supply of new material. A newly discovered moon called Mab seems to be the source of fresh material for the planet’s largest ring. As small rocks moving around the solar system hit Mab, they blast dust from its surface and this dust settles on the ring.
URANUS FACTS
Diameter: 51,120 km
Distance from Sun: 2877 million km
Period to travel around Sun: 84 years
Period to turn around axis: 17 hours
Number of known satellites: 27
On 26 July 2006 the Hubble Space Telescope captured a rare event, an eclipse of the Sun on Uranus. The moon Ariel is seen passing in front of the planet and casting a shadow on its cloud tops. Courtesy NASA, ESA, L Sromovsky (University of Wisconsin, Madison), H Hammel (Space Science Institute), and K Rages (SETI)
The existence of another planet beyond Uranus was first predicted in the 1840s to explain a difference between the observed and the calculated path of Uranus. The planet was found less than one degree from its predicted position, and the discovery was hailed as a brilliant achievement. It was named Neptune after the Roman god of the seas.
Nearly four times the diameter of Earth and 54 times its volume, Neptune is like a blue version of Jupiter. The blue colour is probably caused by the red in the Sun’s light being selectively absorbed by methane in Neptune’s upper atmosphere.
The spacecraft Voyager 2 visited the planet in August 1989. The most obvious feature it found in Neptune’s atmosphere was the Great Dark Spot. This spot is about the size of Earth and similar to the Great Red Spot seen on Jupiter. Scientists were surprised to find that the spot was not visible on recent Hubble Space Telescope photos of Neptune. However, another Great Dark Spot was seen in a different part of the planet. These dark spots may be holes in the cloud layers. Through them we can peer into the dark lower levels of Neptune’s atmosphere.
Triton, with a diameter of 2700 km, is Neptune’s largest moon. It orbits Neptune backwards; that is, in the opposite direction to most of the other moons in the solar system. To astronomers this suggests that Triton was formed elsewhere and later captured by Neptune.
NEPTUNE FACTS
Diameter: 49,530 km
Distance from Sun: 4508 million km
Period to travel around Sun: 165 years
Period to turn around axis: 16 hours
Number of known satellites: 14
A recent Hubble Space Telescope image of Neptune. Courtesy NASA, L Sromovsky, and P Fry (University of Wisconsin-Madison)
Ceres was discovered on 1 January 1801 and named after the Roman goddess of agriculture. At first astronomers considered that it was a planet filling the gap between the orbits of Mars and Jupiter, but soon other objects were found circling the Sun in the same part of the solar system. A British astronomer coined the term ‘asteroids’ to encompass these objects.
Ceres is the largest of the asteroids and its mass is about one-third of the total mass of all the asteroids. In August 2006 the International Astronomical Union promoted it to dwarf planet on the basis that it has sufficient mass for its own gravity to pull it into a near-spherical shape.
Until recently, Ceres appeared as no more than a small blurred dot even through the largest telescopes. That all changed with the arrival of NASA’s Dawn spacecraft at Ceres on 6 March 2015. Dawn has sent back pictures of a heavily cratered surface with collapsed structures and signs of landslides. In the southern hemisphere there is a cone-shaped mountain that reaches a height of six kilometres above the surface. Most intriguingly, there are a number of bright spots on the surface of the dwarf planet that have mystified scientists. At the time of writing, there was speculation that we could be seeing exposed ice or deposits of salt, although other possibilities included volcanoes and out-gassing vents.
The International Astronomical Union has recently approved names for a number of the newly discovered features on Ceres. Among these is Haulani, which is the name of a Hawaiian plant goddess, for a 30-km-wide crater containing one of the bright spots.
CERES FACTS
Diameter: 952 km
Distance from Sun: 414 million km
Period to travel around Sun: 1680 days
Period to turn around axis: 9 hours
Number of known satellites: 0
NASA's Dawn spacecraft imaged Ceres on 4 May 2015, from a distance of 13,600 km. The 80-km-wide crater Occator with its bright spots can be clearly seen. Courtesy NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Pluto was discovered by Clyde Tombaugh of the Lowell Observatory in Arizona in 1930. He used a blink comparator, a device with which he could compare photographs of the same part of the sky taken at different times to look for objects that changed their position. An 11-year-old girl in England suggested the name Pluto, the god of the underworld in Roman mythology. It was readily accepted, especially as the first two letters were the initials of Percival Lowell, the founder of the observatory where it was discovered.
For many years Pluto was seen as the ninth planet of the solar system. The first doubts arose in 1978 when an eagle-eyed astronomer noted it had a satellite. This was to be named Charon; it allowed the mass of Pluto to be measured and it turned out to be surprisingly small, about one-sixth that of the Earth’s Moon.
By August 2006 when astronomers attending a meeting of the International Astronomical Union in Prague were debating the definition of a planet, it was known that there were other objects circling the Sun beyond Neptune in similar paths to Pluto. Pluto was then demoted to dwarf planet as it had not ‘cleared the neighbourhood around its orbit’.
On 14 July 2015, after a nine-year journey, NASA’s New Horizons spacecraft flew past Pluto. Due to the spacecraft’s huge distance from the Earth the images and other data it collected during the flypast will reach Earth over a period of more than a year. Images so far indicate that Pluto is geologically active with some very young surfaces not dotted with impact craters and evidence of flowing ices.
PLUTO FACTS
Diameter: 2370 km
Distance from Sun: 4437 to 7376 million km
Period to travel around Sun: 248 years
Period to turn around axis: 6.4 days
Number of known satellites: 5
Four images from NASA's New Horizons spacecraft were combined to produce this whole-disc view of Pluto from a distance of 450,000 km. Prominent on the visible side of the dwarf planet is a vast heart-shaped region, informally known as Tombaugh Regio. Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
Comets are lumps of frozen gases such as water ice, ammonia and carbon dioxide mixed in with space dust. Apart from rare comets like Halley’s Comet that are trapped in the interior of the solar system, they are generally found at its edges. The Kuiper Belt extends outwards from the orbit of Pluto and is likely to contain tens of thousands of these giant ‘dirty snowballs’. Pluto may, in fact, be just one of the largest and closest of these. Astronomers also think there is a cloud of billions of comets, called the Oort Cloud, surrounding the solar system and extending part of the way to the nearest stars.
Every now and then one of the comets from the Kuiper Belt or the Oort Cloud approaches the Sun. Then spectacular things start to happen. The Sun’s heat turns some of the ice into gas and a huge cloud of released dust envelops the original comet. This is the ‘coma’ and is seen from Earth as a fuzzy blob. The other part of the spectacle is the tail. A combination of sunlight and charged particles emitted by the Sun push dust and gas from the comet into a tail that always points away from the Sun.
Halley’s Comet was discovered when English scientist Edmond Halley recognised a comet he had observed in 1682 was the same as those that had been observed in 1607 and 1531. Halley’s Comet is the most famous of all comets, with the roughly 75 years between its returns punctuating human lifetimes.
The most significant past return was the sighting in 1066 before the Battle of Hastings in which William the Conqueror defeated King Harold of England. During its most recent return in 1986 Halley’s Comet became the first comet visited by spacecraft. These included the European Giotto probe that flew into the comet’s coma and sent back images of its dark nucleus.
Halley’s Comet from an original photographic plate taken at Sydney Observatory in 1910. The print was made by astrophotographer Dr David Malin just before its 1986 return. The streaks in the image indicate the comet’s motion in relation to background stars during the exposure. Courtesy MAAS
Comet 67P/Churyumov–Gerasimenko was first found in 1969 when Klim Churyumov from Kiev State University was examining a photographic plate taken by fellow astronomer Svetlana Gerasimenko and noticed a fuzzy image. Initially, this comet was little known and only studied by a few specialists but it quickly became famous when the European Space Agency chose it as the target for its Rosetta mission.
Comet Churyumov–Gerasimenko circles the Sun every 6.5 years with its closest point roughly halfway between the paths of the Earth and Mars, while its furthest point is just outside the path of Jupiter. Interestingly, the comet’s present orbit only dates from about a decade before its discovery. Astronomers have calculated that a century or so ago its closest distance to the Sun was four times that of the Earth to the Sun. Subsequently, successive encounters with the giant planet Jupiter gradually changed its path into its present one.
After a ten-year journey the Rosetta spacecraft caught up with the comet on 6 August 2014 and went into orbit around it. Images from Rosetta showed a 4-km-wide nucleus with a duck-like shape consisting of two lobes connected by a neck region. The two lobes may have once been separate and then collided; the collision must have been gentle as otherwise each part would have been smashed to pieces.
On 13 August 2015 the comet, with Rosetta along for the ride, reached perihelion, its closest point to the Sun. As the warming from the Sun increased, the comet started to become more active, releasing jets of gas and vapour. An exceptionally bright jet was recorded on 29 July emerging from the neck of the comet. After the outburst instruments on Rosetta detected increased amounts of carbon dioxide, methane and the rotten-egg gas, hydrogen sulphide. Dust hitting Rosetta also increased both in amount and velocity.
Rosetta took a close-up view of the smooth but boulder-strewn neck region of Comet Churyumov–Gerasimenko on 22 January 2015 from a distance of only 28 km. Courtesy ESA/Rosetta/Navcam – CC BY-SA IGO 3.0