…now what about ET?

Astronomers are searching the galaxy for exoplanets — planets orbiting other stars — hoping to find any that might support life. So far, they haven’t found alien life, but they have found over 4000 exoplanets, primarily via the transit method — detecting the tiny dimming of light from a star when one of its planets passes through our line of sight.

This image shows Mercury (small dot, upper right) transiting the Sun, as seen from Earth, with a sunspot in the lower left. Transits are very difficult to detect, but we have become good at it.

When a transiting exoplanet is found, the next step is analyzing its atmosphere for signs of life, the presence of molecules of organic origin, such as O₂. Since oxygen molecules are highly reactive, O₂ cannot be abundant without a continuous source, for which life is the most probable candidate. We are still learning how to analyze exoplanet atmospheres — no great discoveries yet.

Since we are searching for ET by looking for transits of distant stars, perhaps ET is searching for us by looking for Earth transiting our star.

Astrophysicists L. Kaltenegger of the Carl Sagan Institute of Cornell University and J. K. Faherty of the American Museum of Natural History in New York authored a paper identifying ETZ (Earth Transit Zone) stars, stars from which Earth was/is/will be viewable transiting our Sun, at any time from 3000 BC to 7000 AD.

People once thought stars were “fixed” in the sky. Actually, all stars are in constant motion, although it’s rarely detectable without sophisticated instruments. The ESA’s (European Space Agency) Gaia satellite is now measuring the positions and motions of billions of stars. We can now determine which stars were/are/will be in the ETZ, near enough to the plane of Earth’s orbit and close enough to detect an Earth transit.

The authors find 2034 ETZ stars within 326 light-years. Of these, 313 were once, 319 will be, and 1402 are in positions to see Earth transits. Of ETZ stars, 194 are G-type stars like our Sun, while most are much less hospitable, including 1520 M dwarfs, 109 white dwarfs, and 12 red giants.

Additionally, 46 of the ETZ stars are, and 42 will be, within 100 light-years of Earth, close enough to potentially detect our radio transmissions, which began about 100 years ago (S. G. Marconi, 1922). These 46 stars, which include 3 G-type stars, can now see Earth transits and detect our radio emissions.

Seven of the 2034 ETZ stars are known to host exoplanets, and four of those are within 100 light-years — more details in the footnote.

The authors note that Earth’s atmosphere has had abundant oxygen — a compelling sign of life — for a billion years. Extrapolating from their 10-millennia calculations, they estimate ETs in millions of star systems could have already detected an inviting Earth.

To paraphrase Nobel Laureate Enrico Fermi: “So, where are they?” — meaning, if there are so many ETs, why haven’t they visited or even called?

FOOTNOTE: More about the nearest ETZ stars.

Ross128 is about 11 light-years away, with one Earth-size exoplanet. It entered and exited the ETZ 3057 and 900 years ago, respectively.

Teegarden is 12 light-years away, with two exoplanets of about Earth’s mass. It will enter the ETZ in 2050 AD and exit 410 years later.

GJ9066 is 15 light-years away, with one exoplanet. It will enter the ETZ 846 years from now and exit 932 years later.

Trappist-1 is 41 light-years away, with 7 Earth-size exoplanets. It will enter the ETZ 1642 years from now and exit 2371 years later.

Best Wishes,

Robert

March 2022

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