Posts Tagged ‘Earth’

History of life on Earth

Pangea supercontinent from space, as it may have looked 300 million years ago.
Pangea Supercontinent  from space, as it may have looked 300 million years ago.

The Earth is a little over 4.5 billion years old, its oldest materials being 4.3 billion-year-old zircon crystals. Its earliest times were geologically violent, and it suffered constant bombardment from meteorites. When this ended, the Earth cooled and its surface solidified to a crust – the first solid rocks. There were no continents as yet, just a global ocean peppered with small islands. Erosion, sedimentation and volcanic activity – possibly assisted by more meteor impacts – eventually created small proto-continents which grew until they reached roughly their current size 2.5 billion years ago. The continents have since repeatedly collided and been torn apart, so maps of Earth in the distant past are quite different to today’s.

The history of life on Earth began about 3.8 billion years ago, initially with single-celled prokaryotic cells, such as bacteria. Multicellular life evolved over a billion years later and it’s only in the last 570 million years that the kind of life forms we are familiar with began to evolve, starting with arthropods, followed by fish 530 million years ago (Ma), land plants 475Ma and forests 385Ma. Mammals didn’t evolve until 200Ma and our own species, Homo sapiens, only 200,000 years ago. So humans have been around for a mere 0.004% of the Earth’s history.

Jump to: Geological timeline | Geological time periods | Big Five mass extinction events |
Mass extinction theories | Ancient Earth habitats

Geological timeline

During its dramatic 4.5 billion year history, Earth has gone through a series of major geological and biological changes. The timescale below highlights a number of notable prehistoric events and the geological periods in which they occurred. As things didn’t get interesting from a biological perspective until around 570 million years ago, we’ve included a couple of zoomed in timelines to show the detail of more recent evolutionary history. Show text only timeline

4.6 billion years ago The origin of the Earth
3.8 billion years ago First life arises
2.1 billion years ago Eukaryotes evolved
1.1 billion years ago First sexually reproducing organisms
570 million years ago First arthropods evolve
530 million years ago The first fish
475 million years ago First land plants
385 million years ago First forests
370 million years ago The first amphibians
320 million years ago The earliest reptiles
225 million years ago The dinosaurs evolve
200 million years ago The mammals evolve
150 million years ago First birds
130 million years ago Flowering plants evolve
100 million years ago The first bees evolve
65 million years ago Dinosaurs and ammonites become extinct
14 million years ago The first great apes appear
2.5 million years ago Genus Homo evolves
200 thousand years ago Our species, Homo sapiens evolves
10 thousand years ago End of the last Ice Age

Geological time periods

Geologists have organised the history of the Earth into a timescale on which large chunks of time are called periods and smaller ones called epochs. Each period is separated by a major geological or palaeontological event, such as the mass extinction of the dinosaurs which occurred at the boundary between the Cretaceous period and the Paleocene epoch.

Big Five mass extinction events

Although the Cretaceous-Tertiary (or K-T) extinction event is the most well-known because it wiped out the dinosaurs, a series of other mass extinction events has occurred throughout the history of the Earth, some even more devastating than K-T. Mass extinctions are periods in Earth’s history when abnormally large numbers of species die out simultaneously or within a limited time frame. The most severe occurred at the end of the Permian period when 96% of all species perished. This along with K-T are two of the Big Five mass extinctions, each of which wiped out at least half of all species. Many smaller scale mass extinctions have occurred, indeed the disappearance of many animals and plants at the hands of man in prehistoric, historic and modern times will eventually show up in the fossil record as mass extinctions. Discover more about Earth’s major extinction events below.

Mass extinction theories

Asteroid impacts, climate change, volcanoes – there have been many theories about the causes of mass extinctions. In some cases, such as the Cretaceous mass extinction event, more than one such factor was involved in the global catastrophe.

Ancient Earth habitats

If you were able to travel back far in time, you’d find Earth to be a very different place – at times a giant hot molten ball of rock, at others a frozen planet completely covered in snow and ice. During its long history, Earth has been covered by habitats and experienced climates that no longer exist. Discover more about these and about the dramatic story of ancient Earth.

Artist's conception of Kepler 22-b

Painting of Milky Way galaxy used as backgroun...

Image via Wikipedia

The planet, shown here in an artist’s conception, circles its host star in 290 days

Astronomers have confirmed the existence of an Earth-like planet in the “habitable zone” around a star not unlike our own.

The planet, Kepler 22-b, lies about 600 light-years away and is about 2.4 times the size of Earth, and has a temperature of about 22C.

It is the closest confirmed planet yet to one like ours – an “Earth 2.0”.

However, the team does not yet know if Kepler 22-b is made mostly of rock, gas or liquid.

During the conference at which the result was announced, the Kepler team also said that it had spotted some 1,094 new candidate planets – nearly doubling the telescope’s haul of potential far-flung worlds.

Kepler 22-b was one of 54 exoplanet candidates in habitable zones reported by the Kepler team in February, and is just the first to be formally confirmed using other telescopes.

More of these “Earth 2.0” candidates are likely to be confirmed in the near future, though a redefinition of the habitable zone’s boundaries has brought that number down to 48. Ten of those are Earth-sized.

‘Superb opportunity’

The Kepler space telescope was designed to look at a fixed swathe of the night sky, staring intently at about 150,000 stars. The telescope is sensitive enough to see when a planet passes in front of its host star, dimming the star’s light by a minuscule amount.

Kepler identifies these slight changes in starlight as candidate planets, which are then confirmed by further observations by Kepler and other telescopes in orbit and on Earth.

Kepler Space Telescope

Infographic (BBC)
  • Stares fixedly at a patch corresponding to 1/400th of the sky
  • Looks at more than 155,000 stars
  • Has so far found 2,326 candidate planets
  • Among them are 207 Earth-sized planets, 10 of which are in the “habitable zone” where liquid water can exist

Kepler 22-b lies 15% closer to its sun than the Earth is to our Sun, and its year takes about 290 days. However, the planet’s host star puts out about 25% less light, keeping the planet at its balmy temperature that would support the existence of liquid water.

The Kepler team had to wait for three passes of the planet before upping its status from “candidate” to “confirmed”.

“Fortune smiled upon us with the detection of this planet,” said William Borucki, Kepler principal investigator at Nasa’s Ames Research Center.

“The first transit was captured just three days after we declared the spacecraft operationally ready. We witnessed the defining third transit over the 2010 holiday season.”

The results were announced at the Kepler telescope’s first science conference, alongside the staggering number of new candidate planets. The total number of candidates spotted by the telescope is now 2,326 – of which 207 are approximately Earth-sized.

In total, the results suggest that planets ranging from Earth-sized to about four times Earth’s size – so-called “super-Earths” – may be more common than previously thought.

As candidates for planets similar to Earth are confirmed, the Search for Extraterrestrial Intelligence (Seti) has a narrower focus for its ongoing hunt.

“This is a superb opportunity for Seti observations,” said Jill Tarter, the director of the Center for Seti Research at the Seti Institute.

“For the first time, we can point our telescopes at stars, and know that those stars actually host planetary systems – including at least one that begins to approximate an Earth analogue in the habitable zone around its host star.

Kepler 22-b infographic

Fifty new exoplanets discovered

By Paul Rincon Science editor, BBC News Website

Artist's impression of exoplanet
Liquid water is considered essential if a planet is to support life

Astronomers using a telescope in Chile have discovered 50 previously unknown exoplanets.

The bumper haul of new worlds includes 16 “super-Earths” – planets with a greater mass than our own, but below those of gas giants such as Jupiter.

One of these super-Earths orbits inside the habitable zone – the region around a star where conditions could be hospitable to life.

The planets were identified using the Harps instrument in La Silla in Chile.

The new findings are being presented at a meeting called Extreme Solar Systems in Wyoming, US, and will appear in the journal Astronomy and Astrophysics.

Lead author Dr Michel Mayor, from the University of Geneva in Switzerland, said the haul included “an exceptionally rich population of super-Earths and Neptune-type planets hosted by stars very similar to our Sun”.

He added: “The new results show that the pace of discovery is accelerating.”

Life markers

Of the new finds, a total of five planets have masses that are less than five times that of Earth.

“These planets will be among the best targets for future space telescopes to look for signs of life in the planet’s atmosphere by looking for chemical signatures such as evidence of oxygen,” said Francesco Pepe, from the Geneva Observatory, who contributed to the research.

HD 85512
The star HD 85512 lies some 35 light-years away and hosts a potentially habitable planet

One of the worlds, called HD 85512 b, is estimated to be only 3.6 times the mass of the Earth.

It is located at the edge of the habitable zone – the narrow strip around a star where liquid water can be present on the surface of a planet. Liquid water is considered essential for the existence of life.

Observations with Harps have also allowed astronomers to come up with an improved estimate of the likelihood that a star such as the Sun will host low-mass planets such as the Earth (as opposed to giants such as Jupiter).

They found that about 40% of such stars have at least one planet less massive than Saturn.

Harps (High Accuracy Radial Velocity Planet Searcher) is a precision instrument known as a spectrograph that is installed on the 3.6m telescope at Chile’s La Silla Observatory.

The instrument searches for planets using the radial velocity method. This looks for spectral signs that a star is wobbling due to gravitational tugs from an orbiting planet.