Image showing where transits of our Solar System planets can be observed. Each line represents where one of the planets could be seen to transit, with the blue line representing Earth; an observer located here could detect us.
Credit: 2MASS /A. Mellinger/R. Wells


There are thousands of known exoplanets. That raises the question, is Earth under the watchful eye of other starfolk?

A group of scientists from Queen’s University Belfast and the Max Planck Institute for Solar System Research in Germany have turned exoplanet-hunting on its head.

Their new study takes a look at how an alien observer might be able to detect Earth using methods now in use.

Transits of Earth

They find that at least nine exoplanets are ideally placed to observe transits of Earth, in the new research published in the journal Monthly Notices of the Royal Astronomical Society.

The team identified sixty-eight worlds where observers on those planets would see one or more of the planets in our Solar System transit the Sun.

Nine of these planets are ideally placed to observe transits of Earth. But none of the worlds are deemed to be habitable.

Undiscovered worlds

However, the research team estimated that there should be approximately ten (currently undiscovered) worlds that are favorably located to detect the Earth and are capable of sustaining life as we know it.

Still, to date, no habitable planets have been discovered from which a civilization could detect the Earth with our current level of technology.

Diagram of a planet (e.g. the Earth, blue) transiting in front of its host star (e.g. the Sun, yellow). Left: The lower black curve shows the brightness of the star noticeably dimming over the transit event, when the planet is blocking some of the light from the star. Right: How the transit zone of a Solar System planet is projected out from the Sun. The observer on the green exoplanet is situated in the transit zone and can therefore see transits of the Earth.
Credit: R. Wells

Best viewing

The team identified parts of the distant sky from where various planets in our Solar System could be seen to pass in front of the Sun – so-called ‘transit zones’ — concluding that the terrestrial planets (Mercury, Venus, Earth, and Mars) are actually much more likely to be spotted than the more distant ‘Jovian’ planets (Jupiter, Saturn, Uranus, and Neptune), despite their much larger size.

In a press statement, lead author Robert Wells, a PhD student at Queen’s University Belfast said: “Larger planets would naturally block out more light as they pass in front of their star. However the more important factor is actually how close the planet is to its parent star – since the terrestrial planets are much closer to the Sun than the gas giants, they’ll be more likely to be seen in transit.”

Future work

The team’s plans for future work include targeting transit zones to search for exoplanets, in the hopes of finding some which could be habitable.

The new work “Transit Visibility Zones of the Solar System Planets”, R. Wells, K. Poppenhaeger, C.A. Watson, R. Heller, is published in the Monthly Notices of the Royal Astronomical Society.

A copy of the paper is available from:

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