Topic 2 Astronomy notes GCSE

Página 1

Topic 2-Planetary systems

2.1: Our solar system

There are many different objects orbiting the sun:

8 Planets: Mercury, Venus, Earth and mars (Rocky terrestrial planets), and Jupiter, Saturn, Uranus and Neptune (Gas giants with ring systems).

Satellites (moons) of planets: Over 160 moons and the number is increasing.

Dwarf planets: Ceres (asteroid belt), Pluto, Eris, Makemake and Haumea (Kuiper belt). Must be in orbit around the sun, hydrostatic equilibrium, has not cleared its orbit of other objects, not a satellite.

Asteroids (minor planets): Small solid bodies (bigger than 10 m but less than a few hundred km), with the majority orbiting in the Asteroid belt between Mars and Jupiter (Vesta and Pallas).

Comets: Nuclei of ice, dust and rock that, when close to the sun, form a gaseous coma and tails.

Centaurs: Show similarities to both asteroids and comets-named after mythological half man half horse. Generally orbit Sun between Jupiter and Neptune (Chiron, Hidalgo and Asbolus). They are rocky bodies like asteroids. Like comets, they cross the orbits of gas giants.

Trans Neptunian Objects (TNOs): Objects orbiting the sun beyond Neptune.

Ceres was promoted from an asteroid to a dwarf planet and Pluto from a planet to a dwarf planet because a planet is a body that:

Is in orbit around the sun

Is large enough to be spherical

Has cleared its orbit of other objects.

The object is heavy enough for gravity to overcome its original shape and make it nearly round-Hydrostatic equilibrium.

Many objects have been hard to classify in recent years. Alternative names such as Small solar system bodies and Plutoids have been suggested but what is certain is that our solar system contains a vast variety of bodies worth studying.

1 AU is defined as 150 million km and is the mean distance between the Earth and the sun. Jupiter orbits the sun at a distance of 5.2 AU. Therefore Jupiter is about 5.2 times further from the Sun as Earth.

The ecliptic is the plane that Earth orbits around the Sun. All planets orbit in ellipses.

Characteristics of planets

Mercury: It is heavily cratered and contains highlands and lava filled basins (It's features are similar to the Moon's). Low gravitational field strength. The day side is heated to 470 degrees. The night side is -170 degrees. Mercury has a large metallic core.

Venus: It spins backwards and is similar to Earth in size. It has clouds of sulphuric acid, a surface pressure 90x greater than on Earth and a surface temperature of 470 degrees Celsius. It has a toxic and dense atmosphere containing carbon dioxide: prevents infrared radiation from leaving Venus, making the surface and lower atmosphere so hot (runaway greenhouse effect). Atmosphere composes of: Sulfuric acid, carbon dioxide and nitrogen. It has a cratered and volcanic landscape. It is the only planet in the solar system to spin backwards (retrograde rotation).

Mars (the red planet): It has iron-rich rocks, seasonal ice caps and a 450 km long water carved canyon (Valles Marineris-Mariner Valley). It stretches east to west across the surface. Contains the highest volcano in the solar system (Olympus Mons). Violent dust storms rage across its surface. Atmosphere made up of carbon dioxide, nitrogen and argon. It has a thin atmosphere. Has a metallic core. (mariner 9)

Jupiter: It rotates on its axis in only 10 hours, producing an equatorial bulge and causing dynamic wind systems that split the atmosphere into a series of Red-brown belts and yellow-white zones. The great red spot is an anticyclone weather system larger than Earth. Composed of mainly hydrogen and helium. (Jupiter).

Saturn: It is a gas giant similar to Jupiter but reveals less structure in its atmosphere and shows no evidence of long term features. It is most notable for its rings. (Cassini)

Uranus and Neptune: They are often regarded as twins (cold gas giants of similar size and composition of hydrogen, helium, methane and ammonia). Uranus spins almost on its side and its surface is featureless. Its atmosphere is mainly hydrogen helium and methane. Its composition is of solid water, methane and ammonia. Neptune shows many surface markings (great dart spot) with dark banded features and cirrus like clouds of frozen methane at high altitudes. Its atmosphere is of hydrogen, helium and methane and it has a composition of a solid core, water and methane and ammonia.

A probe is an unmanned space craft that travels through space to collect scientific information and then send the data back to Earth. All have radia telecommunication devices.

The knowledge we have of our solar system has been gathered using scientific instruments such as cameras, spectrometers and magnetometers that are flown on unmanned space probes. Manned space missions have been restricted to the Moon.

there are many problems astronauts would face when venturing further into the solar system. These Include:

Space adaptation syndrome: Astronauts may become disoriented and suffer from headaches, poor concentration and vomiting.

Other Physiological problems: Prolonged exposure to 0 gravity environment can cause astronauts to suffer from brittle bones, muscle fatigue and reduced red blood cell counts.

Communication delays: Radio signals travel at the speed of light so the further away an astronaut is from earth, the longer time taken for a radio wave to travel.

Radiation risk: Astronauts do not have the protection of the atmosphere therefore would be subject to harmful ionising radiation from the sun. Prolonged exposure to solar radiation can increase risks of cancer and radiation from solar flares is life threatening.

Psychological problems: Astronauts are living and working in a confined space with the same people which can cause irritability, fatigue and low motivation.

It is argued as to whether manned or unmanned space exploration is better. On the grounds of cost and safety , robotic exploration will complete the job equally as well. However, future manned exploration would attract media interest world wide and would inspire many potential astronauts and scientists in the future.

Moons

Astronomical objects that orbit a bigger object.

Mars has two small moons: Deimos (smaller and more distant) and Phobos. They are both irregularly shaped, heavily cratered, and only tens of km in size. Their densities are half of Mars and their composition is similar to an asteroid. Therefore astronomers believe that these moons were captured from the asteroid belt.

Neptune's moons are interesting. Triton is the largest moon (slightly smaller than Earth's), has a highly inclined orbit and revolves around Neptune in the opposite direction to which Neptune spins (only large moon in Solar system to do so). Triton is said to be a captured body, but as it has a large size and mass, in is believed that Triton's capture might be from a collision with Neptune or another moon. It is possible that some of Neptune's smaller moons were formed from leftover debris.

Dark Proteus is Neptune's second largest moon and largest irregular satellite in the solar system. Discovered in 1989 by Voyager 2 space probe due to its close proximity to Neptune (making it invisible from Earth). Proteus orbits in the same direction as Neptune spins and orbits by its equator. This suggests that it was formed at the same time as Neptune (however some think that Proteus and Saturn's moon Phoebe were formed in a different region of the Solar system and were captured by these planets).

Nereid has a highly eccentric orbit and takes 360 days to orbit Neptune. It is said to be captured from the Kuiper belt.

Over 170 moons known.

Ring systems

Even though Saturn's rings are clearly visible and most prominent, all four gas giants have ring systems above their equators. Each system is divided into several distinct rings, each containing billions of individual particles of ice, rock and dust that range from very small (micrometers) to several metres in size. The composition varies between planets: Jupiter's rings are dusty and dark whereas Saturn's are icy and highly reflective.

All ring systems are extremely wide (hundreds of thousands of km across) but also extremely thin (less than 1.5 km thick). There are notable gaps in the rings (Cassini division) and some are incomplete (Uranus and Neptune have a number of poorly understood ring arcs). Most believe that ring systems are relative short-lived features.

The origin of the rings is uncertain, but it is thought that they were formed from debris that was left over after the formation of the planet, through large impacts between moons, from a moon that was torn apart through tidal gravitational forces or from material ejected from the surfaces of moons by metioritic impacts.

Orbits of planets

Planets orbit the sun in slightly squashed circles called ellipses. The plane of the Earth's orbit is called the ecliptic and the planes of the other planets are inclined by only a few degrees to this.

Therefore, the planets appear to move through a narrow region of the sky called the Zodiacal band.

Relative positions of a planet and Earth determine how easily a planet can be observed. There are different for the two inferior planets and the superior planets.

Because Mercury is so close to the sun, it is very hard to spot. The inferior planets never appear very far from the sun in the sky.

Mercury and Venus are best placed for observation when they are furthest from the sun in the sky. The positions in their orbits are known as greatest eastern elongation (GEE) and greatest western Elongation (GWE) and at these positions the angle between the lines planet-Earth and planet-sun is 90 degrees.

At GEE, a planet may be visible shortly after sunset, setting in the West. At GWE, the planet is visible in the east, shortly before sunrise. Venus has a highly reflective surface and is referred to as the morning or evening star.

An inferior planet lies at inferior conjunction when its angle of elongation is 0 degrees. At this time the planet may pass in front of the solar disc, undergoing a transit. These events are quite rare and the Transit of Venus occurred in 2004 and was the first since 1882. Transits of Mercury are much more common as Mercury is closer to the sun.

When an inferior planet is close to superior conjunction, it may disappear for a short while behind the sun. This is known as occultation.

An Astronomical body is occulted when it passes behind another and is temporarily hidden from view (total solar eclipse when sun is hidden by the moon).

Superior planets such as Mars and Jupiter are more favourably placed for observation when they are at opposition. At this position, they are closest to Earth (offering best resolution), directly opposite the Sun in the sky and so fully illuminated, and visible throughout the night.

The orbital motion of the planets around the sun makes their position change from night to night in the sky with respect to the fixed stars. They appear to move from West to East (more distant planets moving less than closer planets at a given time), but if observed for long periods, their direct motion occasionally stops, goes backwards in a loop or zig zag path (retrograde motion) before stopping again and resuming its original direction.

This is because the planets orbit the Sun at different speeds-both actual and angular-with speed decreasing as distance from the Sun increases. Therefore planets are seen from different viewpoints, sometimes overtaking and undertaking the inner and outer planets respectively and making them appear to move backwards.

2.2: Comets and Meteors

Comets

Comets are balls of rock and ice that form tails in the course of their highly elliptical orbits around the sun. as they approach the sun, they develop tails that point in the opposite direction of the Sun, and it is the tail that makes the comet visible. Bright comets appear in the night about once every 10 years. It has a coma that is its atmosphere caused by it being nearer to the sun.

They can be divided into two categories depending on their orbital period. Short period comets have a period of less than 200 years and originate from the Kuiper belt (thin disc-shaped region of icy bodies that lies beyond Neptune-30-50 AU from sun). The most famous is Comet Halley with a period of 76 years. It's last encounter was in 1986 and was studied by many space crafts, most notably the Giotto probe. Observations showed Halley's nucleus to be peanut shaped and much darker than originally thought. This led scientists to conclude that the nucleus was formed 4.5 billion years ago by ices condensing onto dust particles.

Short period comets orbit in less than 200 years.

Long period comets originate from the Oort cloud (a spherical distribution of cometary nuclei at the outer reaches of the solar system-50,000 AU from the Sun). This was proposed by dutch astronomer Jan Oort in 1950. There is no evidence for the Oort cloud, there is strong support for its existence.

Evidence of the Oort cloud includes:

Directions: the comets come from all directions, not from the same plane.

Orbits: The orbits of comets suggest that they originate from 50,000 AU from the Sun.

Observations of long period comets indicate that:

Paths around the Sun are in either sense (clockwise or anticlockwise).

orbits are often highly inclined to the ecliptic.

A high percentage of them originate at vast distances.

The most spectacular long period comet observed was Comet Hale-Bopp in 1997.

As a comet approaches the Sun, its temperature rises and gas and dust form an extremely rarefied spherical coma that surround the nucleus, up to 100,000 km across. the comet can develop 2 or more tails:

The ion tail: Blue coloured and straight. Consisting of atoms and molecules of gas (carbon monoxide) that have been ionised by solar wind. When they de-excite, atoms emit light by fluorescence.

The dust tail: Light coloured, shorter and slightly curved tail produced by radiation pressure that pushes particles out of the nucleus. The tail consists of dust and grit that shines by reflecting sunlight and its curvature is due to individual dust particles following their own orbit.

Comets' tails can be several millions of km long.

Meteors

Meteoroids are small rocky irregular lumps of debris (with iron-nickel content) in the solar system that range in size from micrometres to several meters. some are broken fragments of colliding asteroids whereas others were formed through impacts with the surface of the Moon or Mars. they orbit the sun at speeds of up to 40 km/s. A number of meteoroids originate from dust tails of comets, forming groups called meteoroid streams and are the sources of meteor showers.

Many meteoroids intersect the Earth's orbit. When they enter our atmosphere, friction causes the meteoroid and surrounding air to heat up, producing a short streak of incandescent light (meteor or shooting star). Meteors with a magnitude -3 or brighter are called fireballs.

If a meteor lands on the Earth's surface, it is called a meteorite. Geo-chemical analysis allows classification of meteorites into three categories: Stones, irons and stony-irons. This depends on the ratio of rock to metal. A more detailed analysis can propose their likely origin.

Depending on the time of the year, there is a dramatic increase in the number of meteors that are observed from a given location. This happens when Earth passes through a meteor stream left in the wake of a comet. The increased number of dust particles entering and burning up causes a meteor shower.

Actual dust particles are orbiting the Sun parallel to one another. The meteors in a shower appear to diverge from a point in the sky called the radiant. This is due to perspective.

The shower will be named after the constellation in which the radiant is found. The Perseid shower lies on the constellation of Perseus. Others include Quandrantids, Leonids and Geminids. Meteor showers are associated with the orbit of a comet. They leave particles of dust and rock.

PHOs

Near Earth Objects are asteroids and comets whose trajectory might bring them closer to Earth than 0.3 AU (45 million km). In October 2008, 5600 NEOs were know.

A significant proportion of NEOs have projected orbits that may bring them closer to our planet. These Potentially Hazardous Objects have orbits closer than 0.05 AU (7.5 million km) and in October 2008 1000 were identified.

There are many examples of collisions within the solar system:

The Moon, several planets, Moons and asteroids are covered in impact craters. Many have been imaged at close quarters by instruments on space probes.

Venus and Uranus rotate on their axes in unusual ways (backwards and sideways). These are thought to be a consequence of gigantic collisions early on.

Geological evidence from the moon says it was once part of Earth and formed through a giant impact.

Several fragments of Comet Shoemaker-Levy 9 collided with Jupiter in 1994. This was observed all over the world.

There is evidence of previous impacts between PHOs and Earth such as the Barringer Crater in Arizona, the Cicxulub crater on the coast of Mexico (believed to have caused mass extinction of dinosaurs) and the Tunguska Event.

Most PHOs have very little chance of inflicting damage on Earth. They have remained virtually the same since 4.6 billion years ago. They offer vital clues to the mixture of chemicals from which the planets formed. Successful space missions have collected and analysed samples from comets (Giotto) and journeys to asteroids are being planned.

PHOs with a diameter larger than 1 km pose a threat. These are considered to have enough impact energy to cause a catastrophe on a global scale. In 1999, scientists adopted the Torino scale to categorise impact hazards and communicate potential risks to the public. It numbers from 0-10 based on the likelihood of a collision and estimated Kinetic energy of the impactor. 0-1 would indicate collisions with objects so small that there would be no damage. 8-10 refer to what would amount to certain collisions, capable of inflicting localised, regional or worldwide devastation.

2.3: Solar System Discoveries

Nicolaus Copernicus developed the theory of the Heliocentric model (that all orbited the sun). He published a book in 1543 which suggested that all planets orbited the sun instead of the Earth as previously thought. The geocentric universe was developed by Egyptian philosopher Ptolemy in the 1st century.

Copernicus explained the occasional retrograde movement of Mars, Jupiter and Saturn (only superior planets). by placing the Sun in the center, his model explained the motion as being due to the Earth 'overtaking the outer planets on the inside' as it moved faster in its orbit.

The Heliocentric model was gaining acceptance in the late 16th century. There was still disagreement over where the planets were located. Danish astronomer, Tycho Brahe undertook meticulous observations of the skies with unprecedented precision. He later died and his assistant Johannes Kepler used observations of Mars which Tycho had made to formulate his laws of planetary motion.

Kepler's first law states that planets move in elliptical orbits around the sun, with the sun at one focus at each ellipse.

the second law relates a planet's speed to its distance from the sun.

His 3rd Law is more complex. There is a simplified equation relating the orbital period of a planet around the sun in years (T) and its mean distance from the sun in AU (r). T²=r³

Italian mathematician and astronomer Galileo Galilei used the recently invented 'optic tube' to view the sky and make sketches. Among his discoveries were:

The Moon was not spherical but contained mountains and craters.

Venus exhibited phases like the moon and changed its apparent size on a timescale of weeks.

Four satellites were orbiting Jupiter: Callisto, Europa, Ganymede and Io.

Thomas Harriot had discovered the moons prior to Galileo but had no interest for fame so did not publish his findings.

The phases of Venus and the moons of Jupiter gave strong evidence in favour of the heliocentric universe. This gave him problems with the church of Rome and he was placed under house arrest. But this was later proven by Isaac newton by his theory of gravitation to explain planetary orbits. After that heliocentric cosmology was almost universally accepted.

Newton then published a book stating that the gravitational force of attraction between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between them

Force ∝ 1/ Distance² This is called the inverse square law. Since if the distance between two objects doubles, the force is one quarter of its previous value (one quarter is 2 squared). Newton showed that a planet moving under the inverse square law of gravity would move in an ellipse, and would later show that Kepler's second and third laws could be explained quantitatively by his theory of gravitation.

The discoveries of Uranus, Ceres, Neptune and Pluto

Uranus was discovered in 1781 by William Herschel. He was using a home-made telescope in bath, with a particular emphasis on faint naked eye stars. He found that one star appeared as a small disc. He first mistook it for a comet, but observations allowed him to determine an orbit from which he found the object was a planet.

Ceres was the first asteroid to be discovered in 1801 by Italian astronomer Giuseppe Piazzi. Several predicted that an undiscovered planet lay between Mars and Jupiter. It has recently been elevated to that of a dwarf planet. Piazzi was observing the skies at the Palermo observatory and noted that a faint star had moved in position on several successive nights. Later in 1801 the orbit was determined by mathematician Carl Friedrich Gauss and Von Zach. They were able to confirm the existence of a new 'planet'.

In 1846, the existence of a planet beyond Uranus was predicted by English undergraduate John Couch Adams and French astronomer Urbain Le Verrier, to accounts for wobbles or perturbations in the motion of Uranus. These could only be explained by the existence of another planet that pulled Uranus outward at times. Both were unsuccessful in finding it but Le Verrier confronted Johann Encke. Two of his assistants discovered the new planet on the first night of their telescopic search and it was named Neptune.

Uranus was seen to have further irregularities in its orbit. American astronomers Edward Pickering and Percival Lowell suggested (incorrectly) that a second undiscovered planet may be affecting the motion of Uranus gravitationally. American student, Clyde Tombaugh, obtained predictions of its position and discovered the planet Pluto in 1930.

2.4: Exoplanets

The first exoplanet was discovered in 1995 by Michel Mayor and Didier Queloz. Astronomers have now discovered many known examples of planetary systems around stars apart from the sun.

There are three ways to detect or infer the existence of extra solar planetary systems. All are indirect as they rely on observing the effect that exoplanets have on a host star and require extreme precision:

Astrometry-The gravitational pull of a large exoplanet can cause their star to move, or wobble, in a small orbit. These wobbles can be detected by measuring the exact position of a star.

Transit method-An exoplanet may move in front of the disc of their host star. This will cause a small dip in the brightness that is temporary and by measuring the amount of light received from that star over a period of time, these small winks can be revealed.

Radial Velocity method (Doppler shifts)-The wobbling of a star causes the wavelengths of its spectral lines to be slightly red-shifted and blue shifted on a regular time scale as the star moves away from us and then towards us. Spectroscopy can reveal these small shifts in wavelength and this technique is most successful at discovering exoplanets.

When these methods are combined, astronomers can estimate the masses and relative sizes of exoplanets. Lots are believed to be large planets orbiting at extremely close distances to their parent star.

Most exoplanets are the size of Jupiter but no earth-like worlds have been found. The current methods do not allow the discovery of small, rocky exoplanets such as Earth. This is due to atmospheric turbulence and small planets with a smaller mass have less of an effect on the host star.

Many planned space missions will hopefully, provide evidence that there are more Earth-like worlds.

The conditions for life

Two things are essential for life:

Carbon-This makes interesting compounds that form the basis of more complex organic molecules, the chemical building blocks of life.

Liquid water-an excellent solvent and transport mechanism for many nutrients. Necessary for photosynthesis and for braking down large molecules into small ones (hydrolysis).

Water is thought to have two possible origins:

Out-gassing of hydrogen and oxygen from volcanoes that combined to produce steam that condensed to water.

Deposited by icy comets striking Earth.

The Rosetta space probe went on a 10 year journey to Comet 67P/Churyumov-Cerasimenko where, on arrival, it will drop a small lander onto the surface of the nucleus. One scientific instrument, Ptolemy, will analyse the comet's water content and examine whether it has the same relative abundances of isotopes as water. This will help to understand where water came from.

Mercury and Venus are too hot to sustain life and Mars and the outer planets are too cold.

For life to develop on an extra-solar planet, it must lie within the Star's habitable zone. This is a narrow range of distances from the star in which the temperature allows liquid water to exist (sometimes called Goldilocks zone).

Are we Alone?

One attempt to quantify the likelihood of life was developed by American astronomer Frank Drake in 1961. The Drake equation has slightly different forms, but all combine estimated probabilities of a number of factors, such as the fraction of Earth-like planets per planetary system, in order to arrive at an approximate figure for the number of civilizations within our galaxy that would be able to communicate with us.

N=R* x Fp x Ne x Fe x Fi x Fc x L

R*=Average rate of star formation in our galaxy.

Fp=Fraction of those stars that have planets.

Ne= Average number of planets that can potentially support life per star that has planets.

Fe=Fraction of the above that actually go to develop life at some point.

Fi=Fraction of above that go on to develop intelligent life.

Fc=Fraction of civilizations that develops technology to release detectable signs of their existence into space.

Li=Length of time such civilizations release detectable signals to space.

There may be more candidates in our solar system that might possess the correct ingredients for life to exist, including Mars and Jupiter's large moon Europa.

Astronomers use techniques to search for signs of life on other planets:

Space probes-Several have searched for microbes or their bi-products (includes Viking probes to mars and Huygens probe to Titan, Saturn's largest moon).

Spectral analysis of planetary atmospheres above rocky exoplanets to search for gases such as oxygen or methane that are produced by living organisms.

Radio wave analysis from space to detect signals that may have originated from extra terrestrial intelligent forms in our galaxy.

There are some questions regarding potential dangers of finding it:

Would it be wise to move organisms from one environment into a different one in which they might become extinct or adapt too well and begin to flourish.

Could live on Earth be threatened as nothing is known about alien life.

Is it wanted to discover that we may not be alone.