The Solar System

• The solar system consists of the Sun, the eight planets and their satellites (or moons), and thousands of other smaller heavenly bodies such as asteroids, comets, and meteoroids.
• The Sun is at the centre of the solar system, and all these bodies revolve around it.
• The gravitational pull of the Sun keeps all the planets and other objects in orbit around it. Thus, the motion of all the members of the solar system is governed mainly by the gravitational force of the Sun.
• Planets revolve around the Sun in elliptical orbits.
• In the solar system, the planet nearest to the Sun is Mercury, and the planet farthest from the Sun is Neptune (not Pluto).
• The size of the solar system has been estimated to be about 100,000 A.U. (not 105 A.U., which only reaches Neptune).
• The solar system is dominated by the Sun, which accounts for almost 99.86% of the mass in the entire solar system.
• The Sun is also the source of all the energy in the solar system.
• Pluto is classified as a dwarf planet by the IAU (International Astronomical Union) since 2006.
• Mercury, Venus, Earth, and Mars are called terrestrial planets, and Jupiter, Saturn, Uranus, and Neptune are called gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune).

The Sun

• The Sun was formed when a swirling cloud of dust and gas contracted, pulling the matter into its centre. When the temperature at the centre rose to 10 million°C, nuclear fusion—the fusing of hydrogen into helium, creating energy—occurred, releasing a constant stream of heat and light. The Sun has the following layers:

The Core

• The innermost layer of the Sun is the core. With a density of 150 g/cm³, about 150 times that of water, the core might be expected to be solid. However, the core's temperature of 15 million kelvins (27 million degrees Fahrenheit) keeps it in a plasma state.
• In the core, fusion reactions produce energy in the form of gamma rays and neutrinos. Gamma rays are photons with high energy and high frequency. The gamma rays are absorbed and re-emitted by many atoms on their journey from the core to the surface. When the gamma rays leave atoms, their average energy is reduced. However, due to the first law of thermodynamics (which states that energy can neither be created nor destroyed), the energy is conserved but redistributed among a larger number of lower-energy photons.
• A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with half-integer spin. Neutrinos are not affected by the electromagnetic force and pass through most matter undisturbed. Neutrinos are created as a result of nuclear fusion reactions in the Sun. About 65 billion (6.5×10¹⁰) solar neutrinos per second pass through every square centimetre perpendicular to the direction of the Sun on Earth.
• Neutrinos are extremely nonreactive. To stop a typical neutrino, it would require a wall of lead about one light-year thick! Experiments are conducted using large tanks filled with chemicals like perchloroethylene, which contains isotopes such as chlorine-37. These isotopes can interact with neutrinos to form argon-37, allowing scientists to detect and count the neutrino flux from the Sun.

Photosphere

• The photosphere is the bright outer layer of the Sun that emits most of the radiation, particularly visible light. It consists of a zone of burning gases about 500 km thick (not 300 km). The effective temperature on the outer side of the photosphere is about 5778 K (5505°C or ~9900°F).
• The photosphere is a comparatively thin layer of low-pressure gases. The composition, temperature, and pressure of the photosphere are revealed by the spectrum of sunlight. In fact, helium was first discovered in 1868 (not 1896) by Jules Janssen and Joseph Norman Lockyer, not William Ramsay, through spectral analysis of sunlight. The gas was named helium, after Helios, the Greek sun god.

Chromosphere

• The chromosphere is the second of the three main layers in the Sun’s atmosphere and is approximately 2000–3000 km thick. Its density is only about 1/10,000 that of the photosphere.
• The temperature first drops from 6000 K to about 3800 K, but then increases with altitude, rising to over 25,000–35,000 K near the top. This temperature rise leads into the transition region, which separates the chromosphere from the corona.

Sunspots

• Sunspots are temporary dark spots on the photosphere, typically with diameters up to 50,000 km, roughly comparable to Earth's diameter. They are cooler than surrounding areas due to magnetic activity that inhibits convection.
• The center of a spot, the umbra, appears dark and has a temperature around 4500 K. Surrounding it is the penumbra, which is lighter and slightly warmer.
• Sunspots follow an approximately 11-year solar cycle, during which the number of sunspots increases and decreases.
• The Sun's magnetic field is responsible for sunspots. Strong magnetic loops rise from the solar interior and suppress convection in localized areas, reducing surface temperature and causing darkening. The individual sunspot may last from a few days to a few months.
• It is estimated that the Sun is slightly brighter (not dimmer) during sunspot maximum due to associated faculae, which are brighter areas that accompany sunspots and increase total solar irradiance.

Corona

• The corona is the outermost layer of the Sun’s atmosphere. It is visible only during a total solar eclipse or through specialized instruments. It is composed of low-density plasma and extends millions of kilometres into space.
• The average temperature of the corona is about 1 million K, though some regions can reach 3 million K.
• This high temperature, despite the distance from the core, is still not fully understood but may be explained by magnetic reconnection and wave heating theories.
• The corona emits X-rays and extreme ultraviolet radiation, despite being much fainter in visible light than the inner layers.

Solar Flares

• Solar flares are intense bursts of radiation originating in the Sun's atmosphere, often near sunspots and regions of complex magnetic fields. These flares can be as large as Earth and may last for minutes to hours.
• The temperature of solar flares can reach up to 10 million to 20 million K, emitting X-rays and gamma rays. These high-energy emissions interact with the corona’s gases, producing enhanced radiation and sometimes impacting Earth's ionosphere, causing radio blackouts and geomagnetic storms.

Planets of the Solar System

A celestial body moving in an elliptical orbit around a star is known as a planet. Planets are generally divided into: (i) the Inner Planets (Mercury, Venus, Earth, and Mars), and (ii) the Outer Planets (Jupiter, Saturn, Uranus, Neptune, and Pluto – a dwarf planet). Of course, icy giants are also a category within (ii).

The four inner or terrestrial planets have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of refractory minerals, such as silicates, which form their crusts and mantles, and metals such as iron and nickel, which form their cores. Three of the four inner planets (Venus, Earth, and Mars) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features such as rift valleys and volcanoes. The term "inner planet" should not be confused with "inferior planet," which designates those planets that are closer to the Sun than Earth is (i.e., Mercury and Venus).

The four outer planets, or gas giants (sometimes called Jovian planets), collectively make up 99% of the mass known to orbit the Sun. Jupiter and Saturn are each many tens of times the mass of Earth and consist overwhelmingly of hydrogen and helium; Uranus and Neptune are far less massive (<20 Earth masses) and possess more ices in their makeup. For these reasons, some astronomers suggest they belong in their own category, "ice giants." All four gas giants have rings, although only Saturn's ring system is easily observed from Earth. The term "superior planet" designates planets outside Earth's orbit and thus includes both the outer planets and Mars.