Homeless planets ‘outnumber stars’

Orphan planets are roaming the universe and may be even more common than stars, a new NASA-funded study suggests. The solitary worlds, abandoned long ago by their parent suns, have until now been floating through the vast void of space unnoticed.

An impression of an orphan world drifting in space
An impression of an orphan world drifting in space (NASA)

But a small number have been detected by a powerful instrument on a large telescope in New Zealand that was trained towards the centre of our Milky Way galaxy.

It was able to spot subtle warping effects on the light of certain stars caused by the gravitational pull of invisible planets passing between us and them – a technique known as microlensing.

Up to ten so-called exoplanets have so far been detected in this way, all believed to be giants at least as big as our own mighty Jupiter. Astronomers believe these were ejected from alien solar systems in the early stages of their formation.

But according to theories of how this would happen, involving encounters with other stars or planets, a greater number of smaller worlds more the size of Earth would be kicked out into space. Their small size just makes them impossible to spot using microlensing.

Researchers conclude that the Jupiter-sized worlds alone are likely to be twice as common in the galaxy as the stars meaning there must be many hundreds of billions of them in the Milky Way.

But it is impossible to imagine any life as we know it inhabiting such planetary outcasts because they must lack the warmth and energy that a sun gives to allow life to flourish.

The NASA-funded study was led by Takahiro Sumi, of Japan’s Osaka University, and is revealed this week in the journal Nature. Observations were made from Mount John Observatory in New Zealand, an ideal site because the centre of our Milky Way rises high in the sky from the southern hemisphere.

The survey is called the Microlensing Observations in Astrophysics (MOA) and is named partly after an extinct giant flightless bird that once inhabited New Zealand. A 1.8-meter (5.9ft) telescope was used and contributions were also made using a similar technique called the Optical Gravitational Lensing Experiment (OGLE) with a 1.3-meter (4.2ft) in Chile.

David Bennett, co-author of the study from the University of Notre Dame, Indiana, said: “Our survey is like a population census. We sampled a portion of the galaxy, and based on these data, can estimate overall numbers in the galaxy.”

“If free-floating planets formed like stars, then we would have expected to see only one or two of them in our survey instead of 10. Our results suggest that planetary systems often become unstable, with planets being kicked out from their places of birth.”

Exoplanet expert Mario Perez, at NASA hq in Washington, said: “Although free-floating planets have been predicted, they finally have been detected. This has major implications for models of planetary formation and evolution.”

Reporter: Paul Sutherland

Paul Sutherland

Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.
Paul Sutherland

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Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.

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