Knowledge in a Nutshell: Astrophysics

Chapter 7 The Solar System

For millennia, the solar system was known to contain eight major objects: the Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn and of course our Earth. Little was known about them and it was speculated that they were balls of luminous gas orbiting Earth while affixed to rotating nested spherical shells of a crystalline or other material. It wasn’t until the detailed observations by Tycho Brahe in the late 1500s and the detailed studies of Tycho’s data by Johannes Kepler that these ancient ideas were finally abandoned. The movement of the Great Comet of 1577 showed a path that intersected the crystalline spheres, destroying them, while Tycho’s Nova of 1572 had previously showed no change of sky position as viewed by Tycho and observers in Europe, so it must be well beyond the orbit of the moon and a part of the presumably changeless celestial sphere.

The work by Kepler to test Tycho’s high-precision planetary data against the prevailing models for planetary orbits led to three remarkable conclusions, which are known as Kepler’s Laws of Planetary Motion. First, the planets orbited the sun on elliptical, not circular paths. Second, the speeds of the planets are such that they sweep out equal areas in equal times. Finally, Kepler’s Third Law states that the orbital distance from the sun (a) and the period of the orbit (T) are equal to each other according to T2 = a3 when the planet distances are in units of the Earth-Sun distance, and the orbit period is given in Earth years. For example, Jupiter’s period is T = 11.9 Earth years so if T2 = a3, then a = 5.2 AUs. Sir Isaac Newton later explained these laws by using his theory of universal gravitation, and went on to show how Kepler’s Third Law could be used to measure the mass of a planet once the orbit of its satellite was known. This would turn out to be a powerful method that could even be extended to measuring the masses of stars and whole galaxies.

The basic landscape of our solar system has been known for over a century, but has steadily been improved in detail and inventory thanks to robotic spacecraft observations and Earth-based investigations with optical, radio and infrared telescopes largely begun in the early 1960s. This chapter only touches upon the highlights and implications of this knowledge.

Large Bodies

There are eight major planets in the solar system, including four rocky ‘terrestrial’ planets (Mercury, Venus, Earth and Mars), two gas giants (Jupiter and Saturn) and two ice giants (Uranus and Neptune). There are also six identified dwarf planets: Pluto, Makemake, Ceres, Charon, Eris and Haumea, although this list will continue to grow as more large objects are detected in the outermost regions of the solar system beyond the orbit of Neptune. The distinction between a planet and a dwarf planet was established by the International Astronomical Union in 2005, with the result that Pluto – previously regarded as the ninth planet – was demoted to dwarf planet status. There is now a generation of children born after 2005 who will never know Pluto as the ninth planet but who may eventually hear about a true ninth planet as astronomers search for a massive Earth-sized world orbiting far beyond Pluto in a region called the Kuiper Belt (see opposite).

By 2019, all the major bodies in the solar system had been visited by spacecraft that made on-the-spot imaging studies of their accessible surfaces and measurements of local particle and magnetic field conditions. Several bodies, such as Venus, the Moon, Mars and Titan (the largest moon of Saturn), have been the direct subjects of surface studies by landers or rovers. The four giant outer planets do not have physical surfaces or conditions that would allow landings, but the atmospheres of Jupiter and Saturn have been examined by spacecraft plunging into them and beaming back data before the enormous planetary pressures destroy them. The properties of the planets accessible from external observations can be fully understood through remote imaging and on-the-spot spacecraft observations. But the details of their internal structures remain mostly speculation until they can be studied using seismic stations for the inner, rocky worlds, or with robotic probes for the gas and ice giants.

Smaller Bodies and Solar System Rubble

There are two belts of material left over from our solar system’s planet-building era. The first is the asteroid belt between the orbits of Mars and Jupiter, which has over 100,000 identified asteroids ranging in size from minute dust grains – viewed from Earth as the Zodiacal Light – to the dwarf planet Ceres at over 950 km (590 miles) in diameter. The total mass of the asteroid belt is about 4 per cent that of our Moon. The second belt of material is the Kuiper Belt, which includes objects with orbits beginning near the region of Neptune and likely extending over 150 billion km (93 billion miles) from the Sun. About 1,000 bodies larger than a few kilometres have been discovered in the Kuiper Belt so far. The Kuiper Belt extends into an area called the Oort Cloud, which is believed to be a large reservoir of comet nuclei ejected from the inner solar system by Jupiter. It extends about 0.5 light years from the Sun in a roughly spherical cloud, hence the name.

The Minor Planets Center is the central reporting agency that holds the official sightings and orbital records for all objects in the solar system from metre-sized asteroids to the satellites of each planet. Currently, over 650,000 small bodies are known, and thousands of new ones are discovered every year. Of particular concern are the 18,000 near Earth objects (NEOs) whose orbits come within 30 million km (18 million miles) of Earth. Among the NEO population are the potentially hazardous objects (PHOs) numbering 1,400 identified objects whose current orbits are within 20 lunar distances (6 million km = 4 million miles) of Earth. Based on the current orbital data and computation limits, PHOs may pose a high risk of collision with Earth. The vast majority of these objects, so far, are less than a few hundred metres across, but it is believed they represent only 20 per cent of a much larger population of small bodies yet to be discovered. Based on discovery rates, it is believed that 93 per cent of all PHOs larger than 1 km (⅝ mile) have been identified. Since 2015, more than 1500 previously unknown NEOs have been discovered each year. The estimated population of NEOs larger than 140 metres (459 feet) is about 25,000, so only six per cent of these potentially devastating asteroids have as yet been discovered.

About 50,000 tons of material strikes Earth every year, but the majority of this mass is dust and small objects below one metre across. The frequency of terrestrial impacts is such that every 30 seconds an object about 1 mm (⅜ in) in diameter enters Earth’s atmosphere, while metre-sized objects arrive once a year. On the larger scale, 100 m (328 ft) objects capable of creating craters like the Barringer Crater in Arizona are once in 50,000-year events, while 10 km (6¼ mile) extinction-level events occur about once every 100 million years. The most dramatic recent impact was the February 2013 Chelyabinsk Event by a 20 m (65½ ft), once-a-century, body. This caused over 3,000 injuries and considerable damage to this small Russian town. The air-burst energy of this blast was equal to 500,000 tonnes of TNT or a small atomic bomb.

Although photographic detection remains one of the most effective ways for identifying and cataloguing interplanetary bodies, other approaches also provide insight into how vulnerable we are.

As objects enter the atmosphere, they produce intense pulses of infrasound energy at frequencies too low for humans to detect. Sensitive infrasound detectors across the globe have been installed by military observers to detect violations of the Nuclear Test Ban Treaty. In an instant, small nuclear devices exploded above ground can be detected anywhere on the surface of Earth, but over the years there have been an increasing list of false alarms due to large asteroids entering the atmosphere. For example, on October 7, 2008, a small 80 tonne asteroid named 2008 TC3 was detected in space about 19 hours prior to its impact on the Earth. It detonated 37 km (23 miles) above the ground with an energy equivalent to 2000 tonnes of TNT. Infrasound data have been used to estimate that 30 objects with detonation yields of more than 100 tonnes of TNT explode in the atmosphere every year. So in addition to the larger 100 m (328 ft) class objects that can be detected while still in space, there is a significant population of still smaller objects that cannot be detected before they make impact with Earth. Objects of this size tend to explode in the atmosphere and pose a substantially reduced human hazard.

Surface Exploration

The list of objects in the solar system for which we now have kilometre-scale images or better is an impressive one. Including Mercury, Venus, Mars and our Moon, the total surface area of the 22 moons, six asteroids and six comets amounts to 285 million square km (110 million square miles) or about twice the land area of Earth. Our Moon’s entire surface has been mapped to 2 m (6½ ft) resolution by NASA’s Lunar Reconnaissance Orbiter satellite, and several robotic rovers are in operation on the surface of Mars, with landings on Venus and Titan having been accomplished in the late 20th century.

Surface exploration remains one of the most challenging and costly means for direct studies of the geology and minerology of planetary, asteroidal and cometary surfaces. However, progress in this research continues to advance every decade. Rovers equipped with sophisticated telerobotics and instrumentation can perform on-the-spot chemical assays of minerals, as demonstrated by the Mars Curiosity Rover. Lunar rovers have now been fielded by US, Russian, Chinese and Indian space agencies. Surface comet and asteroid samplers and rovers (hoppers!) are now steadily increasing in numbers, and in the 2020s the first rover mission to Europa will be launched, with eventual plans to return to Mercury and Venus. The principle driver for rover technology is to assess surface minerology for use by future astronauts as rocket fuel, water, and building materials, but also in the search for the conditions required for life. In the case of Europa and Mars, these locations are receiving the highest level of scrutiny and investment.

The Search for Life

For decades it was thought that liquid water, an essential ingredient for life, was only found on Earth. Since the 1980s, and with the help of the Voyager, Galileo and Cassini spacecraft, evidence for liquid water has been discovered below the surface of the Jovian moons Europa and Ganymede, and apparently beneath the icy crust of Saturn’s moon Enceladus, whose water geysers were discovered by Cassini in 2006.

Among the rocky terrestrial planets, water ice has been detected in the permanently shadowed craters on the poles of Mercury and the Moon. On Mars, many different geological signs suggest running water has existed in the recent past, and in some cases is present today, as well as the discovery of a vast subsurface lake of liquid water detected in 2018 beneath the south polar cap. Traces of organic molecules have also been detected on the red planet, including an annual increase and decrease in atmospheric methane.

The possibility that life may exist on other bodies in the solar system has been given a boost by studies of extremophile bacteria on Earth, which have revealed that life is far more robust than previously thought. Signs of bacterial life have been found many miles below the surface of Earth. Extremophile bacteria have been found living under the near-boiling conditions of hot springs, geysers and submarine hydrothermal vents. Anaerobic bacteria not only survive well without oxygen to provide energy, but have been found respiring a whole host of other chemicals such as hydrogen sulphide and formate. Some species called endoliths are perfectly happy living inside solid rock. These enormous possibilities for redefining life have spurred attempts by NASA to actively explore the solar system bodies with the presumption that life may abound under many conditions previously thought to be hostile to life. Even radiation is no longer considered a severe problem for organism survival. In 1969, Apollo 12 astronauts returned samples from the Surveyor 3 spacecraft, which landed in 1967, and found surviving Streptococcus mitis bacteria. This finding dramatically increased NASA’s protocols for spacecraft decontamination prior to launch so that terrestrial life doesn’t accidentally hitchhike to other solar system locations and contaminate the life-signs found there. Most of the search for signs of life on Mars now involve subsurface studies where the biota would be shielded from surface radiation and ultraviolet light, and have access to known sub-surface aquifers.

Key Points

• The solar system contains eight major planets and millions of smaller bodies called asteroids and comets, which all orbit our sun.

• Earth is literally situated within an interplanetary ‘shooting gallery’ and is bombarded by thousands of meteorites every year from dust grain ‘shooting stars’ to objects tens of meters across. The larger of these, though infrequent, can cause severe property damage and even death.

• The exploration and mapping of all planetary and minor-body surfaces is an ongoing process to complete a full assay of the minerology and evolution of our solar system over billions of years.

• The discovery of extremophile bacteria on Earth, and liquid water or solid ice among the planetary and asteroidal surfaces, has ignited a search for signs of life, fossil or living, beyond Earth.

• Surface exploration using robotic rovers such as Curiosity on Mars is the most economical means for exploring planetary surfaces for purposes of determining basic surface chemistry and the conditions for living systems.