5 billion years ago
, so scientists use other clues in the search for early signs of life (SN: 2/8/14, p.
8 billion years ago
, in the form of single-celled creatures.
Furthermore, large collisions as late as about four billion years ago
may have repeatedly boiled away existing oceans into steamy atmospheres.
Wolf has been using the new climate model -- which is based on the Community Earth System Model maintained by the National Center for Atmospheric Research in Boulder -- in part to solve the "faint young sun paradox" that occurred several billion years ago
when the sun's output was only 70 to 80 percent of that today but when geologic evidence shows the climate was as warm or warmer than now.
7 billion years ago
until about 7 billion years ago
, when growth resumed.
Scientists theorize that more than 5 billion years ago
, a spinning cloud of gas and dust began to form the planets As the particles collided, they grew into larger and larger globules The inner planets--Mercury, Venus Earth, and Mars--formed closest to the sun There, temperatures were so hot that the gases "boiled off" and escaped into space So these planets are made of dense, rocky materials The outer planets, called "gas giants," formed farthest away from the sun, where gases could remain stable Pluto--an object in the Kuiper belt--is different from the inner and the outer planets
85 billion years ago
causing a crater 250 kilometres in diametre.
7 billion years ago
, all matter was smaller than the nucleus of an atom.
It will be sent into orbit far from Earth, and should be able to detect and analyze the faint, warm light produced when the first stars and galaxies formed a few hundred million years after the Big Bang, the theoretical beginning of the universe, some 14 billion years ago
These are the conditions at the beginning of the universe some ten billion years ago
, at the "big bang," as described by Nobel Prize-winning physicist Steven Weinberg in this classic.
It is believed that our planet is 4 million years old and that living organisms appeared 3 billion years ago
For example, we know now that 15 billion years ago
, the universe flared into being.