Did We Receive a Message from a Planet Orbiting the Nearest Star?

On December 18, 2020, Ian Sample of the Guardian published a report about a tantalizing radio signal at 982.002MHz that was detected within the Breakthrough Listen project by the Parkes telescope in Australia from the nearest star to the sun, Proxima Centauri. This infrared star hosts an Earth-size planet, Proxima b, in its habitable zone, where liquid water could allow the chemistry of life on the planet’s surface. There was no scientific paper accompanying the report, and therefore it’s too early to draw any inferences.

Astronomers must verify that the signal cannot originate from radio interference on Earth or some natural emission mechanism. Terrestrial interference should be different for telescopes at different locations on Earth. If the radio source repeats and resides on Proxima b, then it should show an 11-day modulation associated with the planet’s orbital (and spin) period. As soon as I saw the news report, I wrote to the publisher of my forthcoming book Extraterrestrial on the search for intelligent life: “We might have friends out there. Better than a five-star review is getting reassurance for the book’s content from an actual star on the sky.”

Following on this report, Jonathan O’Callaghan and Lee Billings of Scientific American published more details about the detected signal, labeled BLC1, an abbreviation for the first Breakthrough Listen Candidate event. Based on the information they provided, I was immediately able to conclude that the transmitter cannot be on the surface of Proxima b, or else its radio frequency would drift much more than observed based on its known acceleration around Proxima Centauri (which is directly measured using momentum conservation from the reflex motion of this star). Since the news came out of an unintended leak, and I am not a member of the discovery team, I was unaware of the BLC1 details before reading these excellent new reports.

But even without examining the event details, one might wonder whether it is plausible for a radio signal to originate from our nearest star system. In a new paper with my student Amir Siraj, we show that the likelihood of another civilization transmitting such radio waves is exceedingly low based on the Copernican principle. Terrestrial radio technology appeared only over the last century of the 4.5-billion-year history of the Earth. The Copernican principle asserts that humans on Earth are not privileged observers.

This principle agrees with everything we know about the universe. Unlike Aristotle’s cosmology, which placed the Earth at the center and was popular for a millennium, the current scientific perspective on the physical universe implies that Earth-size planets reside in the habitable zone of roughly half of all sunlike stars, that tens of billions of sunlike stars reside in the Milky Way galaxy alone, that tens of billions of Milky Way–like galaxies exist in the observable volume of the present-day universe, and that the universe has no center but is nearly uniform to within one part in a thousand on the largest scale. Therefore, it is reasonable to apply the same Copernican principle to the technological universe. Following this argument, the quantitative paper with Amir shows that the chance of a radio signal appearing now from our nearest star is miniscule. BLC1 most likely originated from a human-built radio emitting oscillator on Earth that contaminated the telescope side lobes with an intrinsic frequency drift.

There is one caveat to this conclusion, namely if intelligent life on Earth and its nearest star are correlated. Stars enter and leave the immediate neighborhood of the solar system because of their random motions. Interestingly, Proxima Centauri became our nearest star around the same time when Homo sapiens appeared on Earth. Is that a mere coincidence?

Either way, there are now more reasons to visit our neighboring planetary system. A probe sent at a fraction of the speed of light could get us the first photographs. The Breakthrough Starshot initiative aims to develop the technology that would allow us to launch such a probe using a powerful (100 gigawatt) laser pushing on a lightweight (gram-scale) light-sail on the length-scale of a human, to which a miniaturized camera and communication device are attached.

Since Proxima b is 20 times closer to its star than the Earth is from the sun, it is expected to be tidally locked, facing the star with a permanent dayside. My daughters suggested that the permanent sunset strip between the two sides should have the highest real estate value since it is ideal for vacations. If there is a civilization on Proxima b, it would likely cover the permanent dayside with photovoltaic cells and transfer electricity to heat and illuminate the nightside.

In a paper published with my former postdoc Manasvi Lingam, we showed that if such cells cover a substantial fraction of the planet’s landscape, the spectral edge in their reflectance could be identified by future telescopes. In another new paper I am currently writing with Stanford undergraduate student Elisa Tabor, we show that the James Webb Space Telescope could constrain the amount of artificial illumination on the nightside of Proxima b, especially if it is based on LED technology. This type of illumination might be particularly appealing to the infrared eyes of our hypothetical neighbors.

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