A shadow may not be the only way to spot alien worlds NASA, ESA, L. Calçada

Echoes can reveal the unseen. Similar to how a killer whale can “see” through pitch-black water by bouncing high-frequency sound waves off objects, we could use light to discover exoplanets.

Whenever a star emits a bright flare of radiation, some of its light may reach Earth where astronomers will measure a burst of brightness – but the display isn’t over. Because the light emanates in all directions, it will also head towards any circling exoplanets. Once it reaches a planet, it will reflect off it and could potentially bounce towards Earth, producing a second – albeit fainter – burst, like an echo.

Though such light echoes have often been observed from supernovae, William Sparks of the Space Telescope Science Institute in Baltimore, Maryland, and his colleagues argue that such a signal could be used to detect otherwise invisible planets around flaring stars.


Tough to detect

At the moment, most planets are discovered when we see their shadows dance in front of their host star – a technique that greatly limits detections because it requires the planet to travel in the exact line of sight between Earth and its star. But you should be able to see light echoes from planets in any configuration, says Sparks. “If we can make it work, it opens up the discovery space enormously.”

Making it work will be a challenge. “The signal would be really hard to pick up – not impossible, but really hard,” says Edward Guinan at Villanova University in Pennsylvania.

While both Guinan and Sparks agree that the Hubble Space Telescope may be able to detect light echoes, Sparks hopes that future observatories with massive mirrors, such as the proposed 15.1-metre LUVOIR space telescope, will more easily spot such signals from hidden worlds.

Blazing worlds

Still, Guinan worries about a second issue, especially around dwarf stars. “When M-stars are young, they are on fire – they are flaring every hour or two,” he says. That could mean that any planets Sparks discovers will be uninhabitable.

But it is unclear how much damage flaring stars cause to their circling planets, says Sparks. And Guinan agrees that these planets can remain habitable under certain scenarios. The stars do calm down over time. Should planets migrate into the habitable zone at that point, they would be perfectly safe. It is also possible that meteorites could deliver water to a sterile planet and help replenish its atmosphere, creating a world where life can thrive.

“You could imagine ways in which the planet could protect life and ways in which life could protect itself,” says Sparks. “There are UV-resistant microbes down on Earth, for example.” For this reason, he is hopeful that the technique might one day bring more habitable worlds to light.

Reference: arxiv.org/abs/1801.01144

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