The formation of the solar system took millions of years. During this time the temperature and pressure of the protosun continued to increase. Finally the centre of the protosun became hot enough for nuclear fusion reactions to begin and the Sun was born.
Stars like our Sun can take 100 million years to form from a nebula. Radioactive data of the oldest material in our solar system suggests it is about 4.6 million years old.
Much of the debris leftover from the formation of the solar system is in orbit around the Sun in two regions—the Asteroid belt and the Kuiper belt. The Asteroid belt lies between Mars and Jupiter while the Kuiper belt is a region beyond Neptune.
The Asteroid Belt
Asteroids are small rocky boulders left over after the solar system formed. Most of them orbit the Sun in a region between the planets Mars and Jupiter. This region is called the Asteroid belt. The Asteroid belt is not uniformly thick, but is instead tapered. On its outer skirts the thickness is about 1 AU, but on it’s inside edge the belt is only one-third as deep. Most asteroids have very elliptical orbits, some of which actually cross Earth’s orbit. Small stony fragments ejected during asteroid collisions are called meteoroids. Some of these meteoroids enter the Earth’s atmosphere and burn up releasing light—such bodies are called meteors. Large meteors that impact with the ground form craters like those seen on the surface of the Moon. More in Chap. 8.
The Kuiper Belt
The Kuiper belt is a bit like the Asteroid belt, except that it is much farther from the Sun and it contains thousands of very cold bodies made of ice and rock. Objects in this outer region take up to 200 years to orbit the Sun. The Kuiper belt is a remnant of the original solar nebula out of which our planetary system formed, but it is made up of material that could not coalesce into a planet due to the large volume of space and low density of matter so far from the Sun. Although the main Kuiper belt ends around 48 AU from the Sun, there is another large reservoir of objects called the Oort cloud, in the distant solar system (see Chap. 13).
The Oort Cloud
There are other smaller objects on the outer edge of solar system, in a region called the Oort cloud. This roughly spherical cloud also contains many objects left over from the formation of the solar system. This cloud extends over one-third of the way to the nearest star system, Alpha centauri, or about 100,000 AU. It likely contains around a trillion objects larger than 1 km across, with orbital periods of a few million years.
There are three competing theories for how the inner Oort cloud might have formed. One theory is that a rogue planet could have been tossed out of the giant planet region and could have perturbed objects out of the Kuiper belt to the inner Oort cloud on its way out. This planet could have been ejected or still be in the distant solar system today. The second theory is that a close stellar encounter could put objects into the inner Oort cloud region. A third theory suggests inner Oort cloud objects are captured extrasolar planets from other stars that were near our Sun in its birth cluster. More in Chap. 13.
Comets
Comets are icy bodies that originate from the outer regions of the solar system. Many comets have highly elongated orbits that occasionally bring them close to the Sun. When this happens the Sun’s radiation vaporizes some of comet’s icy material, and a long tail is seen extending from the comet’s head. Each time they pass the Sun, comets lose about 1 % of their mass. Thus comets do not last forever. Comets eventually break apart, and their fragments often give rise to many of the meteor showers we see from Earth (see Chap. 13, Fig. 1.10).
Fig. 1.10Comet ISON shows off its tail in this 3-min exposure taken on 19 Nov 2013, using a 14-inch telescope located at the Marshall Space Flight Center. The star images are trailed because the telescope is tracking on the comet. At the time of this image, Comet ISON was some 70 million km from the Sun—and 128 million km from Earth—moving at a speed of 217,000 km/h (Credit: NASA/MSFC/Aaron Kingery).
The Modern Nebula Theory
The Modern Nebula theory is supported by observations of dense dusty discs around very young stars in certain nebulas in the universe. It is believed that planets originate in these dense discs. The density of these discs has to be sufficient to allow the formation of the planets and yet be thin enough for the residual matter to be blown away by the star as its energy output increases. In 1992, the Hubble Space Telescope (HST) obtained the first images of these proto-planetary discs (called ‘proplyds’) in the Orion nebula. The Orion nebula star-birth region is 1500 light-years away, in the direction of the constellation Orion the Hunter. Some of the Orion proplyds are visible as silhouettes against a background of hot, bright interstellar gas, while others are seen to shine brightly. They are roughly on the same scale as our solar system and lend strong support to the nebular theory of their origin (see Fig. 1.11).
Fig. 1.11Protoplanetary discs or proplyds in the Orion nebula as seen by the Hubble Space Telescope (Credit: NASA/ESA).
The Modern Laplacian Theory
Planetary mathematician Dr. Andrew Prentice of Monash University, Australia, has found widespread acclaim following the success of his many key predictions about the physical and chemical structure of the solar system. These predictions are linked to his Modern Laplacian theory of how the solar system was formed some 4.5 billion years ago. The controversial theory, first presented by Dr. Prentice in 1976, is based on a hypothesis advanced by French mathematician Pierre de Laplace in 1796.
Laplacian theory proposes that when the Sun first formed, it was a huge swirling cloud of gas and dust. When this cloud contracted inwards to form the present Sun, it cast off a concentric family of orbiting gas rings. The planets later condensed from these rings, starting with Neptune and finishing with Mercury. For the Sun to shed gas rings, Dr. Prentice introduced a new physical concept of ‘supersonic turbulence’. It is this phenomenon that causes the primitive Sun to shed individual gas rings, one at the orbit of each planet.
Prentice has made a long list of controversial predictions about the nature of our solar system. To the surprise of many of his colleagues, recent NASA missions have confirmed that many of his hypotheses are remarkably accurate. Lets look at some of these predictions.In 1977, Prentice hypothesized that a rocky moon belt existed at four planetary radii from Jupiter’s centre. Two years later, such a rocky ring was discovered, though closer to Jupiter than Prentice had predicted. He also predicted that Uranus had two more moons or moonlet streams than commonly thought. Nine years later, a new moon (Puck), was discovered to be orbiting Uranus, in addition to a family of nine moonlets.
In 1980, Prentice predicted that Titan was not a native moon of Saturn but instead had been captured soon after Saturn had formed.
In 1981, Prentice theorized that the mass of Saturn’s moon Tethys was in fact 20–25 % larger than the generally predicted level. Three months later, it was confirmed to be 21 % larger than previously thought.
In 1989, Prentice predicted that Neptune had four additional dark moons, at 5, 3.5, 2.5 and 1.8 radii in Neptune’s equatorial plane. By the end of the year, four dark moons were discovered in Neptune’s equatorial plane at 7, 3, 2.5 and 2.1 radii. He also predicted that dry ice would be the main carbon-bearing chemical on Triton. Three years later, infrared devices confirmed this.
Prentice also correctly predicted Jupiter’s outermost Galilean satellite, Callisto, was a cold, magnetically inert body of rock and ice, and that the smallest Galilean moon, Europa, would have a 150 km deep mantle of ice. The sulphur content of Jupiter’s atmosphere was also found to have exactly matched Prentice’s prediction.
Data from NASA’s space probe missions also proved Prentice’s 25-year old theory that Jupiter’s fifth largest moon, Amalthea, discovered in 1892, was actually a ‘captured’ asteroid and not a native satellite or moon of Jupiter.
The Nice Model
The Nice model of the solar system is a set of theories in which the orbits of the giant planets changed long after the planets formed. This relatively recent theory (2005)
proposes that the planets Jupiter, Saturn, Uranus and Neptune originally had near circular orbits and were closer together than in the present. Objects (called planetesimals) in the Asteroid belt and Kuiper belt were also in slightly different positions—these objects slowly leaked out of their original positions and many were gravitationally scattered by the giant planets. The orbits of the giant planets altered quickly and dramatically—Jupiter was moved slightly inwards while Saturn, Uranus and Neptune were moved outwards. The resulting planetary rearrangement unleashed a flood of comets and asteroids throughout the solar system. Some of the planetesimals were thrown into the inner solar system, producing a sudden influx of impacts on the terrestrial planets including Earth’s Moon (a time called the Late Heavy Bombardment). Eventually, the giant planets reached their current positions, and dynamical friction with the remaining planetesimal disc dampened their eccentricities and made the orbits of Uranus and Neptune circular again.
The Nice model is favored for its ability to explain the movement and position of objects in the Kuiper belt. As Neptune migrated outward, it came closer to the objects in the proto-Kuiper belt, capturing some of them and sending others into chaotic orbits. The objects in the Scattered disc and Oort cloud are believed to have been placed in their current positions by interactions with Neptune and Jupiter.
The Grand Tack Hypothesis
The Grand Tack hypothesis (published 2011) proposes that strong currents of flowing gas moved Jupiter inwards in the earliest 1–10 million years of the solar system, fundamentally altering the orbits of the asteroids and other planets. At one point it was positioned close to where the orbit of Mars is now. The planet’s movements changed the nature of the asteroid belt and caused Mars to be smaller than it should have been. Like Jupiter, Saturn also got drawn towards the Sun. Gradually all the gas and dust in between Jupiter and Saturn got expelled, bringing their sun-bound death spiral to a halt and eventually reversing the direction of their motion. The two planets moved outwards over millions of years until they reached their current positions.
Juipter’s movement helps explain why the asteroid belt is made up of both dry, rocky objects and icy objects. Astronomers think that the asteroid belt exists because Jupiter’s gravity prevented the rocky material there from coming together to form a planet, instead remaining as a loose collection of objects. As Jupiter moved away from the Sun, the planet nudged the asteroid belt back inward and into its present position. Jupiter also deflected some of the icy objects in the outer regions of the solar system, into the inner parts.
In Conclusion
There have been many attempts to develop theories for the origin of the solar system. None of them can be described as totally satisfactory and it is possible that there will further developments which may better explain the known facts. However the majority of astronomers do believe that the Sun and the planets formed from the contraction of part of a gas/dust cloud under its own gravitational pull and that the small net rotation of the cloud was responsible for the formation of a disc around the central condensation. The central condensation eventually formed the Sun while small condensations in the disc formed the planets and their satellites. The energy from the young Sun blew away the remaining gas and dust leaving the solar system as we see it today.
Further Information
https://solarsystem.nasa.gov (click on What is a planet? and Our solar system)
www2.ess.ucla.edu/~jewitt/kb/nice.html (The Nice model)
www.solarsystem.nasa.gov/scitach/display.cfm?ST_ID2429 (Grand Tack model)
www.boulder.swri.edu/~kwalsh/GrandTack.html (Grand Tack model)
© Springer International Publishing Switzerland 2016
John WilkinsonThe Solar System in Close-UpAstronomers' Universe10.1007/978-3-319-27629-8_2
2. Space Probes and Telescopes
John Wilkinson1
(1)Castlemaine, Victoria, Australia
Highlights
The Hubble Space Telescope is one of the most significant instruments ever used to explore the solar system and universe.
The Chandra space probe discovered X-rays coming from Jupiter’s poles.
The Spitzer space probe discovered the dwarf planet Makemake in the outer solar system.
In 2005 the Deep Impact probe intercepted comet Tempel 1 and fired a 370 kg copper slug into the surface of the comet at 10 km/s.
The New Horizons probe visited the dwarf planet Pluto in July 2015.
In 2014, the Rosetta spacecraft encountered the comet Churyumov–Gerasimenko and placed a lander on it.
The Dawn space probe is the first spacecraft ever to orbit two worlds beyond Earth (visiting the asteroid Vesta in 2011 and the dwarf planet Ceres in 2015).
Humans have long been interested in observing the night sky and in particular the planets and Moon. Early humans made observations with their unaided eyes. The invention of the telescope in 1610 opened up a new method to observe these objects. Telescope technology has improved dramatically since 1610 and we now have highly advanced optical telescopes as well as radio telescopes to find out more about the objects in the solar system.
The race to explore space advanced even more in the 1960s when the first artificial satellites where placed in Earth orbit. Since then we have seen advances in unmanned space probes and manned spacecraft through the USA’s Mercury, Gemini and Apollo programs and the Russian Vostok, Voskhod, and Soyuz projects. It was largely as a result of these programs that the first human was able to walk on the Moon in 1969. This was perhaps the most significant step in the exploration of the Solar System, since the Moon was the first object outside Earth that humans ventured onto.
Since landing on the Moon, humans have turned their attention to improving methods of space travel and to studying the long-term impact of space travel on humans. As a result of this new focus, manned space stations like Salyut, Mir, Skylab and ultimately the International Space Station were built. Future manned missions into space will require new technologies and the current space stations are providing a pathway for this to occur.
Fig. 2.1The International Space Station is a joint venture between several countries. The first part of it was put in orbit in 1998 and several modules have since been added. It now measures 108 m across and 88 m long, and has almost half a hectare of solar panels to power it. It orbits Earth at an altitude of 400 km (Credit: NASA/ISS).
Space Telescopes
In the past decade most observation of objects in the solar system has occurred through the use of unmanned space probes and space telescopes (telescopes that orbit the Earth as a satellite). Significant space telescopes include the Hubble Space Telescope (HST), the Chandra Space Telescope, the XMM-Newton Telescope and the Spitzer Space Telescope. The scopes are placed in orbit above Earth’s surface to avoid the dusty and light affected atmosphere. Each of these telescopes observes objects in the solar system or universe in a particular wavelength of light. Some of the wavelengths are actually absorbed by the atmosphere and so we can’t observe in these wavelengths from the surface (see Table 2.1).Table 2.1Significant space telescopes (still operating in 2015)
Name
Operator
Main wavelength
Launch date
Orbits…
Hubble ST
NASA/ESA
UV/VIS
24 Apr 1990
Earth
Chandra
NASA
X-ray
23 Jul 1999
Earth
XMM-Newton
ESA
X-ray
10 Dec 1999
Earth
Odin
Swedish SC
Microwave
20 Feb 2001
Earth
Intern Astro. Lab
ESA
Gamma γ
17 Oct 2002
Earth
Spitzer
NASA
IR
25
Aug 2003
Sun
Swift Burst Expl.
NASA
UV, VIS, X, γ
20 Nov 2004
Earth
AGILE
ISA
X-ray
23 Apr 2007
Earth
Fermi
NASA
Gamma
11 Jun 2008
Earth
Kepler
NASA
UV, VIS
6 Mar 2009
Earth
The Hubble Space Telescope
The Hubble Space Telescope (HST) is one of the most significant instruments ever used to explore the solar system and universe. It was launched from the cargo bay of the shuttle Discovery on 25 April 1990 as a joint venture between NASA and the European Space Agency (ESA). The telescope cost $2.5 billion, weighs nearly 12 tonne and orbits 600 km above the Earth at a speed of 28,000 km/h. It consists of a 2.4-m-diameter mirror mounted in large tube, three cameras and two spectrographs and a number of guidance sensors. Hubble can detect objects about a billion times fainter than the human eye. Scientists are using the telescope to learn about the nature of stars, planets and black holes, the evolution of the universe, and distant objects never seen before (see Fig. 2.2).
The Solar System in Close-Up Page 3
