The solar system – what's in our locality?

We live on a planet called the Earth that orbits the Sun once every 365 days. The Earth is one of nine known planets, while the Sun is a very ordinary star about halfway through its lifetime with another 5000 million years to go. The only reason the Sun does not look like the other stars to us is because it is much nearer to us. Even so, at 147 million kilometres (93 million miles) away, it still takes about 8 minutes for light to reach us from the Sun. All the planets orbit the Sun in more or less the same plane. This is called the plane of the ecliptic.

The planets are not evenly spaced but are in three groups: the inner planets, Mercury, Venus, the Earth and Mars ; the gas giants, Jupiter and Saturn; the outer planets, Uranus, Neptune and Pluto.

The inner planets, comparatively speaking, are very close to the Sun and are solid. Since most of the visible objects in the universe consist of the gases hydrogen and helium, these planets are oddities. Much farther out are the two giants of the solar system, the gaseous planets Jupiter and Saturn. Beyond them are the outer planets, Uranus, Neptune and Pluto. As far as we know, Uranus and Neptune have a rocky core covered by a liquid mantle of ammonia and a surrounding outer gaseous layer.

Currently the textbook view is that there are only nine planets in our solar system although Pluto lost it status as a planet in 2006 and analysis of the orbits of comets (see later in this episode) has suggested that there may be a tenth planet, between 1 and 10 times the size of Jupiter. This planet, if it exists, is about three trillion miles out from the Sun and is invisible to telescopes. Expect such claims and counter claims to arise from time to time.

Knowledge about the planets, ideas based on evidence

Much of the information about the planets in the solar system has been determined by observation. The planets Venus, Mars, Jupiter and Saturn can all be seen with the naked eye.

However much better information can be gathered with a telescope and, better still, by satellite and space probes.

The name planets comes from the Greek word planetos, which means wanderer. This is because, unlike the stars whose position relative to each other is fixed, the planets appear to wander across the sky, first going ahead of the fixed stars and then appearing to stop and fall behind. As you can imagine, trying to devise theories to explain why this happened was a major preoccupation for many astronomers for much of history.

Much of our knowledge of planets has come from simple observations with telescopes.

For example, telescopes have revealed the rings of Saturn, the great red spot of Jupiter, the polar ice caps of Mars, the meteorite craters on the Moon and the clouds on Venus.

In the field of astronomy, scientists are limited in the extent to which they can carry out direct experiments on planets (as they have done on Mars and Venus). They are mainly limited to observation, so they must develop theories that are consistent with observational data.

For example:

• The fact that Venus has clouds means that it must have an atmosphere.
• The fact that the Moon has large craters over its surface and no visible signs of water suggests that it does not have an atmosphere. The craters are produced by meteors hitting the surface. In the cases of the Earth and Venus, the meteors are burnt up as a result of friction with the atmosphere. Those that collide with the Moon (and with Mercury, which also has no atmosphere) go flying in with nothing to slow them down.
• The observation that Mars has polar caps, that shrink and grow with the seasons, suggests that it must have an atmosphere in which snow and ice can vaporise and then condense.

In the past 50 years, enormous advances have been made in our knowledge of the planets. The improvements began with radar observations but the big leap forward came with the ability to fly space probes to the planets and their moons or in the case of our own Moon, to visit it.

Most of our recent knowledge about Jupiter and Saturn has come from a succession of space probes sent to them in the 1970s and 1980s.

The outer planets, Uranus and Neptune, have been visited by a single space probe, Voyager 2, but Pluto has not. Consequently we know little about Pluto other than what we can calculate from its period of orbit and observe through telescopes.

Thales and early models

The history of how our view of the heavens changed is fascinating. This is a reconstruction, identifying some salient points useful for science lessons.

Astronomy is one of the oldest sciences. Ever since the first person had time to stop and gaze at the night sky, people have wondered about the nature of the cosmos and developed models to explain what they observed.

The first models were based on the idea that the Earth is flat and fixed. For instance, Thales, the Greek philosopher (born 624 BCE), believed that the Earth was a flat disc floating on an infinite ocean. The importance of Thales' contribution is that he was the first recorded person who tried to explain phenomena by rational means based on observation and evidence rather than by appealing to the supernatural.

The Celestial Sphere

Early Greek ideas placed all the stars on a large sphere, known as the Celestial Sphere, that surrounded the Earth. There were only seven objects, in addition to the stars, visible to the ancients.

These were the Sun and the Moon, plus the five planets, Mercury, Venus, Mars, Jupiter and Saturn. It was obvious that the planets were not on the Celestial Sphere since the Moon clearly passes in front of the Sun and planets. In addition, Mercury and Venus can be seen to cross in front of the Sun.

Plato proposed that the planets follow perfectly circular orbits around the Earth in what is now called the geocentric solar system model. Later, in about 330 BCE, Heraclides developed that model, apparently placing the planets in order from the Earth (although some historians claim that Heraclides believed the Sun to orbit the Earth with the planets orbiting the Sun).

In 270 BCE, Aristarchus of Samos proposed an alternative system to the geocentric model, placing the Sun at the centre, in the heliocentric system.

While today we know that the Sun is at the centre of the solar system, this did not become at all apparent until the 16th century. In particular, the philosophers of the day ruled out Aristarchus' model for two reasons:

• If the Earth is in orbit around the Sun then the Earth is moving. Before the discovery and recognition of Newton's laws of motion, it was difficult to imagine motion without being able to feel it. As people cannot sense the Earth moving, it was hard for them to believe that it was. If, for example, the wind blew constantly in a given direction, then people might have been more likely to be convinced.
• A geocentric model seemed more natural to the philosophers of the day. Having the Earth at the centre of the universe is a highly egocentric point of view with a strong aesthetic appeal.

The Ptolemaic system

The Ptolemaic system was based on the idea that the Earth is fixed and immovable at the centre. It was developed by a Greek, Ptolemy, who lived in Alexandria between 87–150 CE. Ptolemy set out his ideas in 13 books called the Almagest. The basis of his theory was that everything is fixed on celestial spheres, which were set out like the layers of an onion. These were not intended as theoretical entities but as real crystalline spheres, the outermost of which was the primum mobile (prime mover) that drove the whole system.

Each of the 13 books of the Almagest deals with different aspects of the objects in the solar system and the stars. What made Ptolemy's model so successful, and the feature that led to it dominating all thinking about the solar system for the next 13 centuries, was its comprehensive nature.

The other reason why Ptolemy's theory survived for so long is that it worked. It could be used to predict the motions of the stars and the planets pretty accurately. What Ptolemy's theory lacked though, was simplicity or elegance as it needed 28 epicycles to account for all of the observed motions. From a scientific perspective, this makes the theory less attractive as scientists look for simple solutions to problems.

The Ptolemaic theory was very accurate at predicting the motion of the planets, so why did the Sun-at-centre theory displace it? Confronted with competing theories, science draws on some basic values to make a judgement between them. Despite a general view that science is value free, this is not true.

One of those values is parsimony or simplicity. Given two theories that can explain things equally well, scientists tend to go for the simpler one. Another value in science is explanatory power, such that theories that explain more are generally reckoned to be better.

Those who saw Galileo's theory recognised its inherent simplicity and explanatory power, but unfortunately the church did not. Only 359 years later did the Vatican finally publish an edict admitting that Galileo was right after all!

This is a very good example of the way in which science always operates in a social and cultural setting and is inevitably influenced (and in this case constrained) by that setting.

Retrograde motion

The major challenge for Ptolemy's model was explaining the retrograde motion of the planets. This is the fact (as mentioned earlier) that the planets appear, when observed over a period of time, to loop or wander backwards against the backdrop of the stars.

Ptolemy solved this problem by suggesting that the planets moved in circles about a point on the circumference of a circle, technically known as an epicycle.

The combination of the moving centre and the planet moving around a circle produces a motion that almost exactly models the motion of the planets.

The downfall of the Earth-at-centre world view

It was Galileo's observations (made around 1609) of Jupiter and the orbit of its moons that were of enormous significance in leading to the downfall of the geocentric worldview developed by the Greeks.

These observations were in accordance with, and built upon, the Sun-at-centre model proposed by Copernicus.

Galileo did not invent the telescope but he was perhaps the first person to improve on the original Dutch design and use it to make astronomical observations. Here is a reproduction of his original notes and sketches.

The Church and the society of the time believed that everything went around the Earth. Yet the only way to explain why sometimes there were two, sometimes, three and sometimes four stars besides Jupiter was that these were not stars but moons going around Jupiter and not around the Earth!

If the Sun is at the centre, the explanation of the retrograde motion of the planets becomes simpler. It is the line of sight from the Earth to the planet that appears to move against the background of fixed stars.

Three biographical sketches

Copernicus was born in 1473 and went to university at Cracow before becoming a canon in a cathedral in Germany. One of his passions was astronomy and using his careful unaided observations of the night sky, he developed the idea that the Sun was at the centre of the universe (pre-dating the work of Galileo). However, he was reluctant to publish his work fearing the opposition of the Church. Copernicus' De Revolutionibus Orbium Coelestium, published a few days after his death, gave new currency to the ancient Pythagorean hypothesis that the Sun was at the centre of the universe and that the planets (including the Earth) orbited the Sun. The work was, however, published with a preface by Andreas Osiander, that declared that the theory of the Polish astronomer was a mere hypothesis whose value lay in the way that it simplified astronomical calculations.

Giordano Bruno went well beyond Copernicus (and the available evidence), suggesting that space was boundless and that the Sun and its planets were but one of any number of similar systems; why, there may be other inhabited worlds with rational beings equal or possibly superior to ourselves. For such blasphemy, Bruno was tried before the Inquisition, condemned and burned at the stake in 1600. There is a statue of him now in the Campo Del Fiore (the main square of Rome).

Galileo Galilei was born in Pisa in 1564, the son of Vincenzo Galilei, well known for his studies of music, and Giulia Ammannati. He was exceptionally clever from a young age, gaining a chair in mathematics at the University of Pisa at the age of 27. He was then appointed to the chair of mathematics at Padua where he remained until 1610. He built a telescope with which he made celestial observations, the most spectacular of which was his discovery of the satellites of Jupiter. In 1610 he was nominated the foremost mathematician of the University of Pisa and given the title of Mathematician to the Grand Duke of Tuscany. He studied Saturn and observed that Venus has phases like the Moon. The only explanation for these had to be that Venus was orbiting the Sun. In 1611 he became a member of the Accademia dei Lincei, and observed sunspots for the first time. In 1612 he began to encounter serious opposition to his theory of the motion of the Earth that he taught after Copernicus.

In 1614, Father Tommaso Caccini denounced the opinions of Galileo on the motion of the Earth from the pulpit of Santa Maria Novella, judging them to be incorrect. Galileo therefore went to Rome where he defended himself against these charges, but in 1616 he was admonished by Cardinal Bellarmino and told that he must not uphold Copernican astronomy because it went against the doctrine of the Church. In 1622 he wrote the Saggiatore (The Assayer) which was approved and published in 1623. In 1630 he returned to Rome from Florence to obtain the right to publish his dialogue on the two chief world systems, which was eventually published in Florence in 1632.

In October of 1632 he was summoned by the Holy Office to Rome. The tribunal passed a sentence condemning him and compelled Galileo to solemnly denounce his theory. He was sent to exile in Siena and finally, in December of 1633, he was allowed to retire to his villa in Arcetri. His health condition was steadily declining. By 1638 he was completely blind, and also by now bereft of the support of his daughter, Sister Maria Celeste, who died in 1634. Galileo died in Arcetri on 8 January 1642.

Comets

Comets appear to be relics from the formation of the solar system. Unlike the planets, they are on highly elliptical orbits around the Sun. That means they come very close to the Sun for a short period of time, typically a month or two, and then spend much longer at a vast distance from the Sun. Halley's comet, for example, returns every 76 years.

Comets consist of a solid nucleus surrounded by a mass of frozen ice and gases. As the comet approaches the Sun, the outer layers warm up, producing a stream of vapour particles. These are swept away from the Sun to form a tail by the solar wind of particles coming from the Sun. The tail is long but not very substantial and shines as a result of reflected sunlight.

In 1950 Jan Oort noticed that no comet had been observed with an orbit indicating that it came from interstellar space and that most of the orbits went way beyond Pluto. He therefore proposed that comets reside in a vast cloud at the outer reaches of the solar system and this has come to be known as the Oort Cloud. The statistics imply that it may contain as many as a trillion comets. Unfortunately, since the individual comets are so small and at such large distances, we have no direct evidence about the existence of the Oort Cloud.

The Kuiper Belt is a disk-shaped region beyond the orbit of Neptune containing many small icy bodies. It is now considered to be the source of short-period comets. Occasionally the orbit of a Kuiper Belt object will be disturbed by interactions with the giant planets in such a way as to cause the object to cross the orbit of Neptune. It may then have a close encounter with Neptune sending it out of the solar system, or into an orbit crossing those of the other giant planets, or even into the inner solar system.

Asteroids

On the first day of January 1801, Giuseppe Piazzi discovered an object that he first thought was a new comet. However, after its orbit was better determined it was clear that it was not a comet but more like a small planet. Piazzi named it Ceres, after the Sicilian goddess of grain. Three other small bodies were discovered in the next few years (Pallas, Vesta, and Juno) and by the end of the 19^th century several hundred had been identified.

In total, several hundred thousand asteroids have been discovered and given provisional designations so far, with thousands more being discovered each year. There are undoubtedly hundreds of thousands more that are too small to be seen from the Earth. There are 26 known asteroids larger than 200 km in diameter.

The census of the largest ones is now reasonably complete and probably 99 % of the asteroids larger than 100 km in diameter are known. Of those in the 10–100 kilometre range, about half have been catalogued. Very few of the smaller ones have been identified and there are probably considerably more than a million asteroids in the 1 km range. Asteroids are generally assumed to be fragments of larger bodies (such as planets) and although they are large in number, their total mass is less than that of the Moon.

Between the orbits of Mars and Jupiter there is a band of rocks in orbit around the Sun that is the main band of asteroids. There is also a body of near-Earth asteroids, which tend to be on more elliptical orbits than the planets. The result is that some of these occasionally come close to the Earth (within a million kilometres), the equivalent of an astronomical near miss. If one collided with the Earth, the results would be catastrophic.

Meteors and meteorites

Meteors are small lumps of rock or grains of dust travelling around the Sun. The Earth goes through streams of these at particular times of the year. When it does, the grains burn up in the Earth's atmosphere as they may be moving very fast relative to the Earth. The burning material appears as a glowing object in the sky – what we commonly call a shooting star. It is worth noting that approximately 40 to 50 tons of meteorite material falls to the Earth each day.

A meteorite is a meteor that reaches the surface of the Earth without completely burning up. Most of them consist of material left over from the formation of the solar system. Large meteorites can do considerable damage as they are moving very fast and have a lot of energy associated with them.

A child spending between 10 and 15 minutes in a school playground is likely to have two or three meteoritic dust grains fall on their head!

The collision of a meteorite with the Earth in the Gulf of Mexico, 65 million years ago, is one possible cause for the demise of the dinosaurs.

Our nearest star

Beyond our solar system there are millions and millions of stars, but the nearest star to us is our own Sun.

It may not look like a star but this is simply because it is relatively close to us. Light from the Sun only takes about 8 minutes to reach us, but light from the next nearest star takes 4 years to arrive.

The Sun is a very ordinary star about halfway through its lifetime of 10 000 million years. At the end of its life, the Sun will expand into a red giant star, swallowing the Earth. It is always a bit sad to realise that the Earth will not be around forever. Stars are different than all the other elements of the solar system mentioned so far because they are a source of light. That light is emitted as a result of nuclear reactions.

Getting started

Stars are not unchanging objects – they don't last for ever. They are born, evolve and die.

The life of a typical star starts when a giant gas cloud begins to collapse under its own gravitational attraction. As the particles and atoms fall towards each other, they speed up and their temperature rises.

Eventually, the temperature becomes sufficient for the forming star to begin radiating visible light at the red end of the spectrum, and so the new star appears as a large, bright, red object. This phase is relatively short (in astronomical terms) and generally lasts less than 1000 million years.

The star soon settles into a stable part of its life during which it converts hydrogen to helium by nuclear fusion. What happens next depends on the mass of the star, being different for low mass stars (like our Sun) as compared with more massive stars like Sirius, the brightest star in the sky, or Betelgeuse, a super giant star in the constellation Orion.

Low and high mass stars

Low mass stars use up all of their hydrogen over several thousand million years, converting it to helium. This results in the core of the star collapsing in on itself, the internal temperature increases and the star burns the helium. When this cycle is finished, it is followed by a similar cycle with the heavier elements such as carbon and oxygen. The incredibly high temperatures which are generated at the core of the star (of the order of 120 million degrees Celsius) cause the outer layers to expand and cool and the star becomes a red giant. In the case of the Sun, this will lead to it swallowing up Mercury and Venus, and the Earth will become much too hot to live on. Having now used up all of its fuel, the star collapses to become a very hot, but small, white dwarf star.

yellow dwarf  →  red giant  →  white dwarf

Stars of greater mass go through the same cycle initially as smaller stars, apart from the fact that they run at higher internal temperatures leading to a more rapid exhaustion of hydrogen. They then begin to go through the stages of burning helium and carbon to form a red supergiant. Ultimately, the star's nuclear fuel is exhausted and it becomes unstable as there is no energy source to prevent the core collapsing under the force of gravity.

As it does so, a huge amount of energy is released and the star explodes. This is called a supernova and happens very rapidly (in a matter of weeks). The explosion blasts the outer layers of the star off into interstellar space.

The inner remnants of the supernova continue to collapse in on themselves. Protons are converted into neutrons, causing the matter to become increasingly neutron rich, and in some cases a neutron star is formed (a neutron star is thought not to consist of neutrons alone, even though they are believed to account for most of the mass). While this is happening the density increases to such an extent that a matchbox-full of the matter would have a mass of about one million tons.

Rapidly rotating neutron stars can be observed many years later as radio pulsars. The outer remnants of the supernova explosion ultimately produce nebulae – masses of cloud and dust found in the Milky Way.

In the case of stars with a mass greater than about eight solar masses (this is not a definite figure), the collapse continues to the point where the matter is so dense that even light cannot escape and a black hole is created.

blue supergiant  →  red supergiant  →  supernova  →  neutron star or black hole

Why will the Sun not last forever?

The Sun is a finite source of energy. It is depleting its energy stores at the rate of approximately 3.8 × 10²⁶ joule / second.

To put this figure into context, this is equivalent to the output from 380 000 million, million power stations.

Clearly the energy source is not some large coal fire; the Sun produces this energy by the fusion of hydrogen nuclei to make deuterium, followed by further reactions to make helium nuclei. During this process there is a reduction in mass and the release of an equivalent amount of energy as the nuclear store is depleted.

As a result, the Sun is losing around 4 million tons of mass a second. Of course, this is not sustainable in the long term, and models of the Sun's behaviour suggest that it is halfway through its lifetime of 10 000 million years.

Building atoms in stars

Most of the atoms you are made of were themselves made in stars. Read more about this interesting idea here.

If most of the matter in the universe, at its formation, was hydrogen and helium, where have all the other elements come from?

The estimated age of the universe is 13.7 billion years. The Sun is only about 5000 million years old and was formed out of the remnants of other stars that blew up. Before the formation of the first stars, the matter in the universe consisted only of hydrogen (74 %) and helium (26 %). All of the elements since then have literally been manufactured in stars by nuclear synthesis.

Either lighter nuclei have been sufficiently energetic to undergo nuclear fusion and form heavier nuclei or, alternatively, they have acquired an extra neutron, which has then decayed into a proton, raising the atomic number by one.

What this means is that every one of the heavier elements found in our bodies, beyond hydrogen in water and some of the helium found in our atmosphere, was made in a star millions of years ago.

This idea is captured elegantly by Marcus Chown, a popular science writer:

But if all these examples of our cosmic connectedness fail to impress you, hold up your hand. You are looking at star-dust made flesh. The iron in your blood, the calcium in your bones, the oxygen that fills your lungs each time you take a breath – all were baked in the fiery ovens deep within stars and blown into space when those stars grew old and perished. Every one of us was, quite literally, made in heaven.

Clusters of stars

Observations show that the stars are not uniformly distributed across the night sky, but are clustered in large collections which are called galaxies. Each cluster contains anything from 1 million to millions of millions of stars. It has long been known that our own star, the Sun, belongs to one such galaxy, but it was only in 1925 that it was realised that the universe consists of millions of galaxies. This discovery was made possible by the building of the 200 inch telescope at Mount Palomar in California.

Turning this telescope to look at what, until then, had been thought to be nebulae (the mass of gas and dust left behind as the remnants of a supernova explosion) Edwin Hubble realised that they were, in fact, individual stars. These so called nebulae were galaxies in their own right. Overnight the known universe had become a much, much bigger place.

The Sun is just one of 200 000 million stars that form our galaxy, which is called the Milky Way. The Milky Way looks like a couple of fried eggs placed together with one upside down.

From the Earth, you can see a band of stars, about 5 degrees across, right across the night sky, where there are many more stars than elsewhere. This is the view we get of the Milky Way, looking into the disc of our galaxy.

Measuring distances in the universe: getting to grips with the scales

Distances in the universe are so big that we use a rather unusual unit, the light year, to measure them.

One light year is the distance travelled by light (in a vacuum) in one year.

Given that light travels at about 300 × 10⁶ metre / second you can calculate that:

one light year is 300 000 000 metre / second  ×  (365  ×  24  ×  60  ×  60 second), which is 9,460,800,000,000,000 metre.

The Milky Way galaxy (ours) is about 100 000 light years across, with the Sun about 33 000 light years out from the centre, so we are long way from the centre.

Pupils are invariably fascinated by the links that can be made from the speed of light to distances across space, and travel times through space. It is really helpful to have some relevant facts and figures at the ready.

For example, Supernova 1987a (a supernova is the explosion at the end of a massive star's lifetime) occurred in a nearby galaxy called the Large Magellanic Cloud. Light from this supernova was observed on Earth in 1987, but the distance to the Large Magellanic Cloud is about 190 000 light years. Thus, we normally say that Supernova 1987a occurred in 1987, but it really happened about 190 000 years earlier. Only in 1987 did the light of the explosion reach the Earth. If we want to know what the Large Magellanic Cloud looks like now, we will have to wait 190 000 years.

In comparison, the Sun is only about 8 light minutes away. That is, it takes 8 minutes for light to travel from the Sun to the Earth. So the light we see from the Sun represents what the Sun looked like 8 minutes ago, and we must wait another 8 minutes to see what it looks like now. It is an interesting fact that if the Sun went out we would not know about it until 8 minutes after the event. Reflected light from the Moon travels to the Earth in about 1 second.

The most distant things that astronomers can see are about 12 000 000 000 light years away. Thus, the light that we presently see from these objects began its journey to us about 12 thousand million years ago.

Since that is close to the estimated age of the universe, this light is a kind of fossil record of the universe not long after its birth! So the observation of very distant objects is in a very real sense equivalent to looking backwards in time. (Closer to home light travels about 30 centimetre in one thousand millionth of a second – a foot per nanosecond, so even on the scale of a room, you only know about the past.)

Some ideas about the universe

The universe is the sum total of everything that exists – the aggregate of existing matter, radiation, time and space. The most easily observable element is the matter that exists as stars, most of which is collected in galaxies. There are millions and millions of galaxies in the universe.

James Jeans once said, in answer to the question: How many stars are there?

There are as many stars in the universe as there are grains of sand on all the beaches in the world.

Currently, it's estimated that there are 100 000 000 000 galaxies in the universe. They are not evenly distributed, but appear in clusters. However, as well as matter, there is a large amount of energy and invisible matter. Some of this is in black holes, which do not emit light and cannot be seen.

The age of the universe is calculated from the discovery made by Edwin Hubble that the universe is expanding. Hubble observed that the light from all stars appears to be shifted towards the red end of the spectrum. An analogous effect is the drop in the note of a police siren as it passes you and moves away.

As the universe is expanding, the frequency of light becomes lower, that is light becomes more red. From the change in frequency, you can infer how fast the object is moving away.

By plotting a graph of the velocity of recession of stars against distance, it is possible to work backwards to find how long ago the process must have started. The current answer is about 13 billion years.

Is there likely to be life elsewhere in the universe? There is a good chance that there is, although there's no certainty.

When a star is created there is thought to be about a 1 in 3 chance that it will form with an orbiting planet or planets – that's a prediction from computer modelling. However, through improved technology, astronomers are now beginning to detect the presence of planets around other stars and these findings support the models. What is known is that in our own solar system 1 in 9 of the planets has life on it. Assuming this is typical, in itself a major assumption, you might think that there are an awful lot of stars with planets that might have life on them.

The next big question is how long is it before intelligent life appears? In our own case this was roughly 3000 million years. And, when it does appear how long is it likely to last? Even if intelligent life lasts for only 100 000 years, the calculations suggest that there should be many other planets out there with intelligent life.

Since the late 1960s astronomers have been searching for radio signals that have a non-random pattern. Signals of this nature might be emitted by civilisations that have evolved to a sufficient level to produce the technology to transmit radio waves. None has been found so far, but this does not necessarily mean that we are alone. There are a host of other explanations, with the most obvious one being that our instruments are not sensitive enough to detect the signals.

In the words of Carl Sagan, an American astronomer:

If we are alone in the universe, then it is an awful waste of space.

Knowledge and evidence about the solar system

Different kinds of objects in the solar system are known to exist. How we know that they exist is often due to a long chain of reasoning, as many of them are inhospitable and rather too far off to be inspected at close range.