Two nearby rocky worlds could offer alien astronomers views of planets appearing to reverse course

Aliens have long been a staple of science fiction. Yet the possibility of intelligent life beyond Earth is more than a matter of imagination—it is also a subject of serious scientific inquiry. Professor Woong-bae Zee of Sejong University’s School of Liberal Studies has recently published an astronomy paper that places the word “alien” front and center.

The study approaches the possibility of extraterrestrial civilizations from an entirely different perspective. The findings have been published in Monthly Notices of the Royal Astronomical Society.

From habitability to scientific development

Research on extraterrestrial life has traditionally focused on the environmental conditions of individual exoplanets. Scientists consider factors such as planetary temperature and the potential presence of liquid water to assess whether life could arise and survive. In astronomy, this is known as habitability.

However, a planet capable of supporting life does not necessarily give rise to a scientifically and technologically advanced civilization. The conditions that allow life to exist and those that enable its inhabitants to understand the universe and advance science are distinct questions.

For the search for extraterrestrial intelligence (SETI) to achieve its ultimate goal of detecting technological traces of alien civilizations, it is therefore necessary to look beyond whether a planet can support life and consider whether science and technology could develop there.

In this paper, Zee takes the idea of habitability a step further by proposing a new concept: “developmentability.” The concept asks how an exoplanet’s astronomical environment might influence the way intelligent life arising there understands the universe and develops science.

The study gives this perspective a concrete foundation by connecting the potential for scientific progress in extraterrestrial civilizations to specific astronomical observing conditions.

Retrograde motion as a scientific clue

The first phenomenon examined was the retrograde motion of neighboring planets.

In Earth’s night sky, planets such as Mars and Jupiter occasionally appear to reverse their usual direction of motion for a time. Astronomers call this “apparent retrograde motion.”

This phenomenon played an important role in the development of humanity’s understanding of the universe. Astronomers who believed Earth was at the center of the universe had to construct elaborate orbital models to explain the planets’ complex retrograde motions. When Earth and the other planets are understood to orbit the sun, however, retrograde motion can be explained as a natural consequence of planets moving at different orbital speeds.

Planetary retrograde motion thus provided an important observational clue that helped humanity move beyond an Earth-centered view of the universe and develop the heliocentric model.

Testing the view from alien worlds

But could intelligent beings on exoplanets also observe the retrograde motion of neighboring planets in their own night skies?

Over the past two decades or so, astronomers have discovered numerous exoplanets, including many systems in which several planets orbit the same star. By placing a hypothetical observer on each planet, researchers can calculate how neighboring planets would appear to move across that world’s sky.

Using orbital and physical data from the NASA Exoplanet Archive, Zee analyzed the apparent motions of neighboring planets as seen from these distant worlds.

The analysis went beyond simply determining whether retrograde motion occurs. It also examined whether the motion lasts long enough and spans a large enough angle to be noticeable, whether the neighboring planet is bright enough to be seen with the naked eye, and whether it appears sufficiently separated from its host star in the sky to be distinguished from the star’s glare.

Two nearby worlds with visible retrograde motion

The results identified two intriguing examples of potentially habitable rocky planets from which a neighboring planet’s retrograde motion is predicted to be clearly visible to the naked eye.

The first involves an observer on GJ 1002 b watching the neighboring planet GJ 1002 c, in a system orbiting the star GJ 1002 approximately 16 light-years from Earth. The second involves an observer on Teegarden’s Star c watching Teegarden’s Star d, in a system approximately 12.5 light-years away.

Both planetary systems are relatively close to Earth. The two planets chosen as observing locations are of particular interest because they may have conditions suitable for life.

Observational clues, not guarantees of civilization

Of course, the study does not establish that life or civilizations actually exist on these planets. Nor does the ability to observe a neighboring planet’s retrograde motion guarantee that a scientific civilization will develop.

Nevertheless, the study raises an intriguing question: Just as observations of planetary motion helped humanity develop its understanding of the universe, could astronomical phenomena in alien skies influence the scientific progress of civilizations that might arise there?

Perhaps hypothetical astronomers on GJ 1002 b or Teegarden’s Star c might observe a neighboring planet’s retrograde motion and come to realize that their own world is not the center of the universe. Could those worlds one day produce their own “alien Copernicus” and “alien Galileo”?

The study offers a new perspective on the search for extraterrestrial life by broadening the question from “Where can life exist?” to “What environments could support the emergence of civilizations capable of understanding the universe through science?”

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