Are we being watched? Tens of other worlds could spot the Earth

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

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, in a study that
instead looks at how an alien observer might be able to
detect Earth using our own methods. They find that at least
nine exoplanets are ideally placed to observe transits of
Earth, in a new work published in the journal Monthly
Notices of the Royal Astronomical Society
.

Thanks to facilities and missions such as SuperWASP and Kepler,
we have now discovered thousands of planets orbiting stars
other than our sun, worlds known as ‘exoplanets’. The vast
majority of these are found when the planets cross in front of
their host stars in what are known as ‘transits’, which allow
astronomers to see light from the host star dim slightly at
regular intervals every time the planet passes between us and
the distant star.

In the new study, the authors reverse this concept and ask,
“How would an alien observer see the solar system?” They
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 ‘ 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.

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

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

To look for worlds where civilisations would have the best
chance of spotting our solar system, the astronomers looked for
parts of the sky from which more than one planet could be seen
crossing the face of the sun. They found that three planets at
most could be observed from anywhere outside of the solar
system, and that not all combinations of three planets are
possible.

Katja Poppenhaeger, a co-author of the study, adds, “We
estimate that a randomly positioned observer would have roughly
a 1 in 40 chance of observing at least one planet. The
probability of detecting at least two planets would be about
ten times lower, and to detect three would be a further ten
times smaller than this.”

Of the thousands of known exoplanets, the team identified
sixty-eight worlds where observers 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,
although none of the worlds are deemed to be habitable.

In addition, the team estimate that there should be
approximately ten (currently undiscovered) worlds which are
favourably located to detect the Earth and are capable of
sustaining life as we know it. To date however, no have been discovered from which
a civilisation could detect the Earth with our current level of
technology.

The ongoing K2 mission of NASA’s Kepler spacecraft is to
continue to hunt for exoplanets in different regions of the sky
for a few months at a time. These regions are centred close to
the plane of Earth’s orbit, which means that there are many
target located in the transit zones of the solar
. The team’s plans for
future work include targeting these transit zones to search for
exoplanets, hopefully finding some which could be habitable.

Explore further:
Finding
a ‘lost’ planet, about the size of Neptune

More information: R. Wells et al. Transit Visibility
Zones of the Solar System Planets, Monthly Notices of the
Royal Astronomical Society
(2017). DOI:
10.1093/mnras/stx2077

Journal reference:
Monthly Notices of the Royal Astronomical Society

Provided by: Royal
Astronomical Society