The blue-green planet Earth that we live on is a marvel like no other. It harbours life.
It is of the correct size, at the correct distance from the sun, in the correct trajectory, with the correct tilt in its axis of rotation. To help it further, it has the correct sized moon and a whole host of external planets that shield it from the impact of meteors and other deadly objects.
It also has an extremely smooth surface with very little variation – the mountains are less than 9 km on a globe whose diameter is more than 12,000 km. Over the last 4,500 million years, wind, sand, sun and rain have flattened the earth's surface to the extent that it is one of the smoothest objects we know. It is just that we are so small, and the planet so large, that every little deformity becomes a big geographical feature for us.
The uniqueness of the Earth can be gauged by looking at the other planets around it. Most of them are either too small or too large to be Earth-like. Only Venus is of a comparable size to the Earth. Yet, its surface chemistry is all wrong. It has so much sulphur dioxide in its atmosphere that it forms a thick cloud covering the entire surface of the planet, making it hot and acidic. And uninhabitable in every way.
Though uninhabitable, each planet is unique in its own way. Let us look at them individually.
Mercury (called Budha in Indian astronomy) is the smallest planet, and the closest one to the Sun. Its orbital period is about 88 Earth days. It shows the greatest temperature variation of all the planets, from -173° Celsius at night to over 400° Celsius during the day in some equatorial regions. It has no seasons because its rotation axis is perpendicular to its plane of rotation. As seen relative to fixed stars, it rotates exactly three times for every two revolutions it makes around its orbit. As seen from the Sun, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see one day over two years. Mercury is so close to the Sun that it does not go more than 28° away from it, and is very difficult to see.
Venus (known as Shukra in Indian astronomy) is the third smallest planet in the Solar System. The length of a year on this planet is 224.7 Earth days, and one day on Venus equals 243 Earth days. In the Earth's sky, it is bright enough to cast shadows. It is different from the Earth because it has the densest atmosphere of the four terrestrial planets, comprising over 96% carbon dioxide. The atmospheric pressure on the surface of Venus is 92 times that of the Earth's. With a surface temperature of 462°C, it is by far the hottest planet in the Solar System. It is shrouded by an opaque layer of highly reflective clouds of sulphuric acid. It does not go farther than 47° from the Sun and has historically been called the Evening Star or the Morning Star, depending on where it was with respect to the Sun and Earth.
Earth (called Pruthvi in Indian astronomy) is the densest and fifth-largest (or fourth smallest) of the eight planets in the Solar System. It will be able to continue to support life from 500 million years to as long as 2.3 billion years. About 71% of its surface is covered by oceans. The planet's interior remains active, with a solid iron inner core, a liquid outer core that generates the magnetic field (0.3 gauss at the poles), and a thick layer of relatively solid mantle. The Moon is the Earth's only natural satellite. It began orbiting the Earth about 4.53 billion years ago. The Moon's gravitational interaction with the Earth stimulates ocean tides, stabilizes the axial tilt, and gradually slows the planet's rotation.
Earth is able to support evolving life because it is at correct distance from the Sun, large enough to have a good inert atmosphere, warm enough to have liquid water, and a good inclination that ensures seasons. It is in a nearly circular orbit, which means seasonal changes are few, it has a magnetic field strong enough to prevent damage from radiation, it has an ozone layer to protect from UV light, and it has an active geology to keep churning out material. Moreover, it has the Moon to stabilise it and outer planets protect against meteors.
India's maiden Moon Mission, Chandrayaan 1 found water on the Moon. Chandrayaan 1 observed water evaporating from its surface. This water comes from three distinct sources: comets, solar wind reactions and some primordial water. Water on the Moon is said to move at a subterranean level, depending on porosity and pressure gradients.
Mars (known as Mangal in Indian astronomy) is the second smallest planet in the Solar System after Mercury. It has impact craters, volcanoes, valleys, deserts, and polar ice caps. Like the Earth, its axis of rotation is tilted and hence it has seasons. On Mars, you find Mount Olympus Mons, the second highest known mountain within the Solar System (the tallest and largest one is on a minor planet called Rheasilvia), and of Valles Marineris, one of the largest canyons. Mars has two known moons, Phobos and Deimos, which are small and irregularly shaped.
The surface of Mars was once covered on a large scale by water, along with large quantities of the liquid frozen as ice at the poles. At mid-latitudes, chemical compounds containing water molecules could be found. It probably once had a reasonable atmosphere, flowing water and good active volcanoes which could have made it habitable, but it is no longer so. If it did have life, its environment was comfortable for such a short period of time that life would not have evolved beyond the most primitive forms, and even those either died or went underground as the planet gradually lost its atmosphere and internal heat and became uninhabitable.
Jupiter (called Bruhaspati or Guru in Indian astronomy) is the largest planet in the Solar System. Its mass is one-thousandth that of the Sun, but two-and-a-half times the mass of all the other planets combined. Jupiter is bright enough in our skies to cast shadows. It is composed of hydrogen, with a quarter of its mass being helium, although helium only comprises about a tenth of the number of molecules. It may also have a rocky core of heavier elements, but like the other gas giants, Jupiter lacks a well-defined solid surface.
The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries. A prominent result of this is the Great Red Spot, a giant storm that is known since the 17th century. Surrounding Jupiter is a faint planetary ring system and a powerful magnetosphere. It also has at least 67 moons, including the four large ones called the Galilean moons that were first discovered by Galileo Galilei in 1610. Ganymede, the largest of these, has a diameter greater than Mercury's.
Saturn (known as Shani in Indian astronomy), is the second largest planet, and famous for its rings. It's interior is composed of a core of iron, nickel and rock (compounds of silicon and oxygen), surrounded by a deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium, and an outer gaseous layer. The planet exhibits a pale yellow hue due to the ammonia crystals in its upper atmosphere. An electrical current within the metallic hydrogen layer is thought to be the cause of Saturn's planetary magnetic field, which is slightly weaker than the Earth's and around one-twentieth the strength of Jupiter's.
Saturn's outer atmosphere is bland and lacking in contrast, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h, faster than on Jupiter, but not as fast as those on Neptune. Saturn has a prominent ring system that consists of nine continuous main rings and three discontinuous arcs, composed mostly of ice particles with a smaller amount of rocky debris and dust. Sixty-two known moons orbit the planet; fifty-three are officially named. This does not include the hundreds of "moonlets" within the rings. Titan, Saturn's largest and the Solar System's second largest moon, is larger than the planet Mercury and is the only moon in the Solar System to retain a substantial atmosphere. It is also so light that if we could imagine a sea large enough to put Saturn in, it would float on water.
Uranus is the fourth largest planet in the Solar System and is similar in composition to Neptune, and both are of different chemical composition from the larger gas giants Jupiter and Saturn. Uranus's atmosphere, although similar to Jupiter's and Saturn's in its primary composition of hydrogen and helium, contains more "ices" such as water, ammonia, and methane, along with traces of hydrocarbons. It has the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (-224.2° Celsius), and has a complex, layered cloud structure, with water lower down, and methane in the uppermost layer of clouds.
The interior of Uranus is mainly composed of ices and rock. Uranus has a ring system, a magnetosphere, and numerous moons. Its axis of rotation is tilted sideways, nearly into the plane of its revolution about the Sun. Its north and south poles, therefore, lie where most other planets have their equators. Uranus is a virtually featureless planet in visible light, without the cloud bands or storms associated with the other giants. Terrestrial observers have seen signs of seasonal change and increased weather activity in recent years as Uranus approaches its equinox. The wind speeds on Uranus can reach 250 meters per second (900 km/h, 560 mph)
Neptune is the third largest planet of the Solar System. It was the first planet found by mathematical prediction by its influence on Uranus. Its largest moon, Triton, was discovered shortly thereafter, though none of the planet's remaining 13 moons were located telescopically until the 20th century. Neptune's atmosphere is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen. It contains a higher proportion of "ices" such as water, ammonia, and methane. The interior of Neptune is primarily composed of ices and rock. It is possible that the core has a solid surface, but the temperature would be thousands of degrees and the atmospheric pressure crushing. Traces of methane in the outermost regions in part account for the planet's blue appearance.
Neptune's atmosphere is notable for its active and visible weather patterns. The planet's southern hemisphere has a Great Dark Spot comparable to the Great Red Spot on Jupiter, with recorded wind speeds as high as 2,100 km per hour. Neptune's outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching 55 K (-218° Celsius). Temperatures at the planet's centre are approximately 5,400 K (5,000° Celsius).
While Uranus and Neptune were discovered only after the invention of the telescope, they have also been given Indian names, Uranus is called Indra or Aruna, and Neptune is called Varuna. For Pluto, too, the name Yama has been adopted.
For modern astronomers, Neptune is the last planet in the Solar System. Pluto, which lay beyond it, was recently downgraded to a dwarf planet, since we now know there are several dozen objects of the same size as Pluto, and it was easier to have eight planets than over 20! Scientists distinguish between these different objects by classifying them into:
A planet must meet the following conditions:
It must have sufficient mass for its gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly spherical) shape
It must be in orbit around a star, and must be neither a star nor the satellite of a planet.
It should be the dominant object in its region
There are two kinds of planets: gas giants (Jupiter, Saturn, Uranus, Neptune) and classical (Mercury, Venus, Earth, Mars)
Only eight objects belong to this class
Dwarfs or Pluton:
Dwarf planets are all medium sized objects.
They satisfy the first two requirements to be planets, but are not the most dominant objects in their environments.
Those beyond Neptune, near Pluto are called Plutons.
Plutons include Pluto and "Xena" (2003 UB313 – now renamed Eris) and its moon is called Dysnomia
Three dwarf planets are now known, another 12 or 13 can be added to the list
Anything orbiting a planet
Its centre of gravity must not fall outside the planet.
This category includes several bodies much larger than many planets, such as Jupiter's moon Ganymede (diameter: 5262 kilometres). Over 150 of these are known.
Small Solar System body (SSSBs): Anything orbiting the Sun that's not a planet or a satellite. Most asteroids and comets would be SSSBs. More than 130,000 are known.
As regards planets other than Earth and Mars, there is so much wrong with them as far as sustaining life is concerned, that one does not even talk of the possibility of life there. The exceptions are some of the moons of the planets. Moons such as Jupiter's Io, Europa and Ganymede, and Saturn's Titan and Euceladus are very interesting. They seem to be reasonably warm, have flowing water and good internal heat. So maybe, just maybe, some form of primitive life may exist there and scientists are already dreaming of missions to these moons.
Having said that, there is no denying that if the Solar System comprised only the Earth and the Sun, life as we know it would not have existed. The outer planets in particular are crucial to our survival. They work in several ways to provide a stable environment to the Earth. They ensure we remain in a nearly circular orbit around the Sun, and that not too many comets or meteors come and hit our planet. They act like shepherds and vacuum cleaners, making sure that the inner planets remain stable.
But there are other aspects of the Solar System that are equally exciting. See for example this recent article in Nature magazine. First, it seems that when the Solar System was born, it was certainly not organised the way we see it today. Larger planets were originally much closer to the Sun than in present times – Jupiter was as close to the Sun as the Earth is.
But what about planets beyond our Solar System? How common are planets? Twenty years ago, the answer was simple: "We don't know, but they may possibly exist". Ten years ago it was, "There are some cases where planets may exist around other stars". But the NASA's Kepler mission about 6 years ago changed it all; now our answer is: "Certainly. Almost 80% of all stars that we can study in detail have not just planets, but even families of planets".
This has fundamentally altered our perspective of the universe. We now know of 1750 planets around roughly as many stars in our neighbourhood.
But is there life elsewhere? That is a different question.
Dr Mayank Vahia is a scientist working at the Tata Institute of Fundamental Research since 1979. His main fields of interest are high-energy astrophysics, mainly Cosmic Rays, X-rays and Gamma Rays. He is currently looking at the area of archeo-astronomy and learning about the way our ancestors saw the stars, and thereby developed intellectually. He has, in particular, been working on the Indus Valley Civilisation and taking a deeper look at their script.