Joanne Baker - 50 Ideas You Really Need to Know: Universe

you really need to know

universe

Joanne Baker

Astronomy is one of the oldest and most profound of the sciences. Since our ancestors tracked the motions of the Sun and stars, what we have learned has radically altered our view of the place of humans in the universe. Each breakthrough has had social repercussions: Galileo were arrested in the 17th century for teaching the controversy that the Earth goes round the Sun. Demonstrations that our solar system is displaced from the heart of the Milky Way caused similar gasps of disbelief. And Edwin Hubble in the 1920s silenced the debaters when he discovered that the Milky Way is one of billions of galaxies scattered throughout a vast and swelling universe, 14 billion years old.

During the twentieth century technologies upped the pace of discovery. The century opened with gains in our understanding of stars and their fusion engines, paralleling our knowledge of nuclear power, radiation and the building of the atomic bomb. The years during and after the Second World War brought the development of radio astronomy, and the identification of pulsars, quasars and black holes. New windows on the universe were thrown open, from the cosmic microwave background radiation to the X-ray and gamma-ray sky, each frequency bringing its own discoveries.

This book takes a tour of astrophysics from a modern research perspective. The first chapters describe the great philosophical leaps in our understanding of the scale of the universe, whilst introducing the basics, from gravity to how a telescope works. The next set asks what we have learned about cosmology, the study of the universe as a whole its constituent parts, history and evolution. Theoretical aspects of the universe, including relativity theory, black holes and multiverses, are then introduced. The last sections dissect in detail what we know about galaxies, stars and the solar system, from quasars and galaxy evolution to exoplanets and astrobiology. The pace of discovery is still high: perhaps in the next decades we will witness the next great paradigm shift the detection of life beyond the Earth.

How many planets are there? A few years ago it was an easy question that anyone could answer nine. Today, it is contentious. Astronomers have thrown a spanner in the works by discovering rocky bodies in the deep freeze of the outer solar system that rival Pluto, and by finding hundreds of planets around distant stars. Forced to rethink the definition of a planet, they now suggest there are eight bona fide planets in our solar system, plus a few dwarf planets like Pluto.

Since pre-history weve known that planets differ from stars. Planets, named after the Greek word for wanderer, migrate across the night sky through the unchanging backdrop of stars. From night to night the stars form the same patterns. Their constellations all slowly spin together about the north and south poles, each star etching a circle daily on the sky. But planets positions shift a little relative to the stars each day, following a tilted path across the sky that is called the plane of the ecliptic. All the planets move within the same plane as they orbit the Sun, which is projected as a line on the sky.

The major planets other than Earth Mercury, Venus, Mars, Jupiter and Saturn have been known for millennia. They are easily visible to the naked eye, often outshining their stellar neighbours, and their contrary motions lent them mythical status. The arrival of telescopes in the 17th century generated more awe: Saturn was skirted by beautiful rings; Jupiter boasted a coterie of moons and Marss surface was flecked by dark channels.

This heavenly certainty was shaken by the discovery of the planet Uranus in 1781 by the British astronomer William Herschel. Fainter and slower-moving than the other known planets, Uranus was originally thought to be a rogue star. It was Herschels careful tracking that proved conclusively that it orbited the Sun, thus bestowing its planetary status. Herschel basked in fame because of his discovery, even courting King George IIIs favour by briefly naming it for the English monarch.

Definition of planet

A planet is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a round shape, and (c) has cleared the neighbourhood around its orbit.

More discoveries were to come. Slight imperfections in Uranuss orbit led to predictions that it was being disturbed by another celestial body that lay beyond it. Several astronomers scoured the expected location, looking for a wandering interloper, and in 1846 Neptune was discovered by the Frenchman Urbain Jean Joseph Le Verrier, narrowly beating British astronomer John Couch Adams to establish the find.

Then, in 1930, Pluto was confirmed. As was the case with Neptune, slight deviations in the expected movements of the outer planets suggested the presence of a further body at the time called Planet X. Clyde Tombaugh at Lowell Observatory in the US spotted the object when comparing photographs of the sky taken at different times: the planet had given itself away by its motion. But it fell to a schoolgirl to name it. Venetia Burney from Oxford, in the UK, won a naming competition with her classics-inspired suggestion for Pluto, god of the underworld. Planet Pluto inspired a host of popular culture at the time, from the cartoon dog to the newly discovered element plutonium.

Like continents, planets are defined more by how we think of them than by someones after-the-fact pronouncement.
Michael Brown,2006

Our nine-planet solar system stood for another 75 years until Michael Brown of Caltech and his collaborators discovered that Pluto was not alone. Having found a handful of sizeable objects not far from Plutos orbit at the cold edge of the solar system, they happened upon one that was even larger than Pluto itself. They called it Eris. The astronomical community had a conundrum. Should Browns discovery be recognised as a tenth planet?

And what about the other icy bodies near Pluto and Eris? Plutos status as a planet was called into question. The outer reaches of the solar system were littered with ice-smothered objects, of which Pluto and Eris were simply the largest. Moreover, rocky asteroids of similar size were known elsewhere, including Ceres, a 950-km-diameter asteroid that was found in 1801 between Mars and Jupiter during the search for Neptune.

In 2005 a committee of the International Astronomical Union, the professional organization of astronomers, met to decide Plutos fate. Brown and some others wanted to protect the status of Pluto as culturally defined. In their view, Eris should also be considered a planet. Others felt that all the icy bodies beyond Neptune were not real planets. It came to a vote at a conference in 2006. What was decided was a new definition of a planet. Until then the concept was not pinned down. Some were bemused, saying that this was like asking for the precise definition of, say, a continent: if Australia is a continent, then what about Greenland? Where does Europe end and Asia begin? But the astrophysicists agreed a set of rules.