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Most Earth-Like Worlds Are Unborn

Credit: ESA/NASA

An artist's impression of innumerable Earth-like planets that have yet to be born over the next trillion years in the evolving universe. Credit: ESA/NASA

Astronomers have peered behind the Milky Way and determined that 92% of habitable planets have not yet been born.

An assessment of data collected by NASA’s Hubble Space Telescope and the Kepler space observatory has determined that only 8% of the potentially habitable planets that will ever form in the universe existed when our solar system was born 4.6 billion years ago.

“Our main motivation was understanding the Earth’s place in the context of the rest of the universe,” said study author Peter Behroozi of the Space Telescope Science Institute in Baltimore. “Compared to all the planets that will ever form in the universe, the Earth is actually quite early.”

Looking far back in time, Hubble has given astronomers a “family album” of galaxy observations that chronicle the universe’s star formation history as galaxies grew. The data show that the universe was making stars at a fast rate 10 billion years ago, but the fraction of the universe’s hydrogen and helium gas that was involved was very low.

Today, star birth is happening at a much slower rate than long ago, but there is so much leftover gas available that the universe will keep cooking up stars and planets for a very long time to come.

Kepler’s planet survey indicates that Earth-sized planets in a star’s habitable zone, the perfect distance that could allow water to pool on the surface, are everywhere in our galaxy. Scientists predict that there should be one billion Earth-sized worlds in the Milky Way galaxy at present, a good portion of which are presumed to be rocky. That estimate skyrockets when you include the other 100 billion galaxies in the observable universe.

The last star isn’t expected to burn out until 100 trillion years from now, leaving plenty of opportunities for untold more Earth-size planets to arise in the habitable zone.

An Ominous View behind the Milky Way

Hundreds of hidden nearby galaxies have been studied for the first time, shedding light on a mysterious gravitational anomaly dubbed the Great Attractor. Despite being just 250 million light years from Earth, the new galaxies had been hidden from view until now by our own galaxy, the Milky Way

Using CSIRO’s Parkes radio telescope equipped with an innovative receiver, an international team of scientists was able to see through the stars and dust of the Milky Way into a previously unexplored region of space.

The discovery may help to explain the “Great Attractor”, which appears to be drawing the Milky Way and hundreds of thousands of other galaxies towards it with a gravitational force equivalent to a million billion Suns.

The research, published in the Astronomical Journal (, found 883 galaxies, one-third of which had never been seen before.

Prof Lister Staveley-Smith of The University of Western Australia node of the International Centre for Radio Astronomy Research said that scientists have been trying to get to the bottom of this mysterious Great Attractor since major deviations from universal expansion were first discovered in the 1970s and 1980s.

“We don’t actually understand what’s causing this gravitational acceleration on the Milky Way or where it’s coming from,” Staveley-Smith said. “We know there are a few very large collections of galaxies we call clusters or super-clusters there, and our whole Milky Way is moving towards them at more than two million kilometres per hour.”

Astronomers have been trying to map the galaxy distribution hidden behind the Milky Way for decades in an attempt to explain its movement. They’ve used a range of techniques, but only radio observations have really succeeded in allowing them to see through the thickest foreground layer of dust and stars.

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at