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A Chronological Look at a Transiting Earth

Call it the Earth Transit Zone, that region of space from which putative astronomers on an exoplanet could see the Earth transit the Sun. Lisa Kaltenegger (Cornell University) is director of the Carl Sagan Institute and the author of a 2020 paper with Joshua Pepper (LeHigh University) that examined the stars within the ETZ (see Seeing Earth as a Transiting World).

While Kaltengger and Pepper identified 1004 main sequence stars within 100 parsecs that would see Earth as a transiting planet, Kaltenegger reminds us that stars are ever in motion. Given the abundant resources available in the European Space Agency’s Gaia eDR3 catalog, why not work out positions and stellar motions to examine the question over time? After all, there are SETI implications here. We study planetary atmospheres using data taken during transits. Are we, in turn, the subject of such study from astronomers elsewhere in the cosmos?

Thus Kaltenegger’s new paper in Nature, written with Jackie Faherty (American Museum of Natural History), which identifies 2,034 nearby star systems (within the same 100 parsecs, or 326 light years) that either could have seen the Earth transiting by observing our Sun within the last 5,000 years or will be able to within the same span of time going forward. Kaltenegger takes note of the dynamic nature of the dataset:

“From the exoplanets’ point-of-view, we are the aliens, we wanted to know which stars have the right vantage point to see Earth, as it blocks the Sun’s light. And because stars move in our dynamic cosmos, this vantage point is gained and lost.”

Image: With the plane of the Milky Way galaxy seen stretching from the top to the bottom of the image, this artistic view of the Earth and Sun from thousands of miles above our planet, shows that stars (with exoplanets in their own system) can enter and exit a position to see Earth transiting the Sun. Credit: Kaltenegger & Faherty/Cornell University.

Looking at the results more closely, we find 117 stars over the 10,000 year window that are within 100 light years of our Solar System, while 75 of these stars have been in the Earth Transit Zone since the advent of commercial radio roughly 100 years ago. From the paper:

Among those sources, 29 were in the ETZ in the past, 42 will enter it in the future, and 46 have been in the ETZ for some time. These 46 objects (2 F, 3 G, 2 K and 34 M stars and 5 WDs [white dwarfs]) would be able to see Earth transit the Sun while also being able to detect radio waves emitted from Earth, which would have reached those stars by now… Seven of the 2,034 stars are known exoplanet host stars… Four of the planet hosts are located within 30 pc of the Sun.

It’s intriguing to look at some well known systems in this context. Trappist-1, for example, with its seven transiting worlds, will not enter the Earth Transit Zone for 1,642 years, but once within it, will have the ability to see a transit for 2,371 years.

In fact, even the closest stars tend to spend a millennium or more in the ETZ once there, plenty of time for extraterrestrial astronomers, if such exist, to take note of biological and/or technological activity on our world. Ross 128 is another interesting system. Here we have the second-closest known possibly temperate exoplanet after Proxima b, orbiting a red dwarf in Virgo about 11 light years out. Denizens of this world had a 2,158 year window they entered about 3,057 years ago but moved out of 900 years ago.

As I’m curious about unusual venues for potential life, I found this interesting:

109 of the objects in our catalogue are WDs, dead stellar remnants. Whereas most searches for life on other planets concentrate on main sequence stars, the recent discovery of a giant planet around a WD opened the intriguing possibility that we might also find rocky planets orbiting evolved stars. Characterizing rocky planets in the HZ of a WD would answer intriguing questions on lifespans of biota or a second ‘genesis’ after a star’s death.

It should come as no surprise that Kaltenegger and Faherty find M dwarfs dominating the spectral types, given their wide distribution n the galaxy; 1,520 of these stars are M-dwarfs. They also find 194 G-class stars like the Sun and a range of other stellar types, of which 102 are K-class stars like Alpha Centauri B. At the present time, 1,402 stars within the 100 parsec bubble can see Earth as a transiting world. Early observations of stars in the ETZ have begun via Breakthrough Listen as well as the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China.

While technological activity may be difficult to observe, the authors point out that Earth’s biosphere has been at work on its atmosphere for billions of years, meaning observations of Earth transits would have identified it as a living world ever since the Great Oxidation Event. A transiting Earth’s biosignature should be hard to miss.

The paper is Kaltenegger & Faherty, “Past, present and future stars that can see Earth as a transiting exoplanet,” Nature 594 (23 June 2021), 505-507 (abstract).


Comments on this entry are closed.

  • ljk June 24, 2021, 11:42

    Round and round she goes, where she will stop nobody knows – until we get way better data!


  • henry cordova June 24, 2021, 12:41

    For those of you who have access, the Millennium Star Atlas, (R. Sinnott and M. Perryman, 1997, Sky Publishing Corp and the European Space Agency), has all 10,000+ stars within 200 light years of Earth and with Apparent Magnitude brighter than m = 11.0 plotted and marked with their distance. The MSA is, unfortunately, out of print, but copies are still available on the used market and in major libraries. This atlas was compiled with Hipparchos satellite data and is of high accuracy and precision.

    Also plotted is the Ecliptic, which will help identify those stars potentially in the Sun’s “transit zone”. Of course, many faint stars below the atlas’ limiting magnitude (many red and brown dwarfs) are left out, (those stars intrinsically fainter than an Absolute Magnitude M = 7).

    Another useful feature of this atlas is the marking of all stars (brighter than m = 11.0) with proper motions exceeding 0.2 arc-seconds per year regardless of distance. These stars are marked with a vector indicating the amount and direction of the proper motion. This will allow a calculation of when that star will be close enough to the ecliptic for an observer there to observe planetary transits around Sol.

  • Alex Tolley June 24, 2021, 13:59

    This is a perfect example of assuming ETI using the same technology as we do for characterizing exoplanets i.e. the transit method. This despite other methods being developed like direct imaging. If an ETI has been watching Earth for a millennium or so and is within 100 pc, then I would have thought that they would be using direct imaging with huge apertures like the suggested FOCAL mission and even sent an STL probe to lurk nearby to monitor us.

    IOW, the suggestion that these few planets would be suitable targets for investigating simply because they happen to be in a location that allows them to see Earth transits of our sun strikes me as rather presumptuous at best, not far off from numerology at worst. The planets around those stars needn’t even also be transiting so we have no means to observe them using the same technology.

    Seth Shostak:

    It really helps in the hunting if you know where the prey is located,…

    This is like saying that you will sit in the jungle looking in one direction for an animal that you hope will expose itself, rather than all around you.

    At 100 pc, a probe sent out at just 0.1 c would be sending data back to the ETI scientists in well under 4000 years. If they can manage near c flight velocities, then they would have data well within 500 years. (Hmm, about the same time Clarke’s monoliths could contact their masters to possibly destroy the Earth with our sun going nova).

    In another century, I would be surprised if we don’t have the technology to characterize exoplanets including direct imaging of the surface, and with that, a fleet of sublight probes using advanced propulsion to visit even those farthest stars within a few millennia at most. Those target planets will not be constrained by transit orbits, but will simply include them as part of a larger set. In a few centuries, we might even be buzzing their aircraft in “tictac” craft and only showing up as fuzzy images on their detection instruments while moving in ways that seem to defy physics.

    Note that 3 of the stars highlighted won’t even be able to observe our transits for hundreds, if not thousands of years in the future. Our rapidly degrading radio signals might well have washed over them before then, but that would apply to all stars within that volume. Why even bother to be concerned about their ability to view our transits so far in the future?

    I really do not see the logic of targeting stars where ETI might have seen us, rather than planets that have the highest probability of harboring life, intelligent or otherwise.

    Lisa Kaltenegger’s prior Arxiv paper can be read here – Which Stars can see Earth as a Transiting Exoplanet?

  • Mike Serfas June 24, 2021, 16:35

    I don’t have an intuitive idea about this: if we had 100 telescopes in different orbits throughout the outer Solar system, could we see *every* planet by transits? I suppose 30 x 2 AU/63000 AU gives roughly 1/1000 radians = 3 arc minutes, but I don’t know how far apart whatever stars usable for measuring transits of exoplanets are from each other. Nor am I so sure the “search pattern” would be workable.

    Then there is the matter of how many stars a telescope can examine at the same time. Exoplanet hunters improve this constantly, but when we speak of alien civilizations I wonder stranger things. Could a swarm of a billion little bits of straight wire, whose potential is carefully measured and timestamped to the attosecond (every attosecond) do massive parallel processing to construct an ultra high resolution image of every star in the sky at once?

    My gut feeling is that relying on planets transiting their own star is very primitive. I know that so far papers measuring microlensing or transits in other contexts can only report irreproducible data/one off events, but surely we can progress further from there?

  • Michael Fidler June 24, 2021, 22:21

    Something I realized the other day is that when looking at transiting planets in the habitable zone around M dwarfs;

    1. The planets around these type of stars are going to be tidally locked and have a large zone near the terminator that may be habitable no matter what the star sends out in radiation.

    2. The spectroscopic atmospheric data from planets around red dwarfs when transiting will be just that habitable zone area around the day/night terminator of the tidally locked planet. This is very likely to be the first and largest amount of atmospheric data gathered that may show indicators of life!

    Hopefully the JWST will give us that information for Trappist 1 and other nearby transiting planets around M dwarfs. We have a similar condition when the moon eclipses the sun in a total solar eclipse, we always see the same terminator of the moon because it is tidally locked to the earth. But in our case we see the much more active region of the lower suns atmosphere! ;-)

    • Robin Datta June 25, 2021, 7:21

      As the moon recedes over millions of years, it will increasingly resemble a transiting object rather than offering such an exact occultation.

  • Kamal Ali June 26, 2021, 5:00

    But probably in a few decades, won’t direct imaging cause photometric transits to become a also-ran method? Civs that are hundreds or thousands of years ahead of us would probably have huge telescopes in space so that they could get much better info on a planet than by using transits.

  • Jack Woods September 7, 2021, 19:30

    This is just one more study that the Gaia mission enables. I feel that Gaia is the most under-rated mission ever. Many lay people haven’t even heard of it and others have just recently found out. Maybe it is just because measuring stars (distance, velocity, spectrum) is not as “sexy” as some other missions, but when you understand the accuracy being achieved, it should be! Accurate distance is key to accurate luminosity. Velocity values allow us to wind the clock forward or backward and see where a star used to be and where it will be (keeping in mind that they move while the light is traveling the immense distances). These things have a ripple effect that reaches as far as better estimating the age of the universe. Hats off to Gaia!!

    • Paul Gilster September 8, 2021, 6:05

      Hats off indeed. Gaia is an extraordinary mission.