By Matt Williams

In the past few decades, there has been an explosion in the number of extra-solar planets that have been discovered. As of October 1st, 2021, a total of 4.525 exoplanets have been confirmed in 2,808 systems, with 627 systems having more than one planet. In addition to expanding our knowledge of the Universe, the purpose of this search has been to find evidence of life beyond our Solar System.

Artist’s impression of how an an Earth-like exoplanet might look. - Image Credit: ESO.

In the course of looking for habitable planets, astronomers have used Earth as a guiding example. But would we recognize a truly “Earth-like” planet if we saw one? This question was addressed in a recent paper by two professors, one of whom is an exoplanet-hunter and the other, an Earth science and astrobiology expert. Together, they consider what advances (past and future) will be key to the search for Earth 2.0.

The paper, titled “Earth as an Exoplanet“, recently appeared online. The study was conducted by Tyler D. Robinson, a NASA Postdoctoral Fellow and an assistant professor from Northern Arizona University, and Christopher T. Reinhard – an assistant professor from the Georgia Institute of Technology’s School of of Earth and Atmospheric Studies.

For the sake of their study, Robinson and Reinhard focus on how the hunt for habitable and inhabited planets beyond our Solar System commonly focuses on Earth analogs. This is to be expected, since Earth is the only planet that we know of that can support life. As Professor Robinson told Universe Today via email:

“Earth is – currently! – our only example of a habitable and an inhabited world. Thus, when someone asks, “What will a habitable exoplanet look like?” or “What will a life-bearing exoplanet look like?”, our best option is to point to Earth and say, “Maybe it will look a lot like this.” While many studies have hypothesized other habitable planets (e.g., water-covered super-Earths), our leading example of a fully-functioning habitable planet will always be Earth.”

Thanks to advances in technology and detection methods, astronomers have detected multiple Earth-like planets in our galaxy. - Image Credit: NASA/JPL

The authors therefore consider how observations made by spacecraft of the Solar System have led to the development of approaches for detecting signatures of habitability and life on other worlds. These include the Pioneer 10 and 11 missions and Voyager 1 and 2 spacecraft, which conducted flybys of many Solar System bodies during the 1970s.

These missions, which conducted studies on the planets and moons of the Solar System using photometry and spectroscopy allowed scientists to learn a great deal about these bodies’ atmospheric chemistry and composition, as well as meteorlogical patterns and chemistry. Subsequent missions have added to this by revealing key details about the surface details and geological evolution of the Solar planets and moons.

In addition, the Galileo probe conducted flybys of Earth in December of 1990 and 1992, which provided planetary scientists with the first opportunity to analyze our planet using the same tools and techniques that had previously been applied throughout the Solar System. It was also the Voyager 1 probe that took a distant image of Earth, which Carl Sagan referred to as the “Pale Blue Dot” photo.

The “pale blue dot” of Earth captured by Voyager 1 spacecraft on Feb 14th, 1990. - Image Credit: NASA/JPL

However, they also note that Earth’s atmosphere and surface environment has evolved considerably over the past 4.5 billion years ago. In fact, according to various atmospheric and geological models, Earth has resembled many environments in the past that would be considered quite “alien” by today’s standards. These include Earth’s many ice ages and the earliest epochs, when Earth’s primordial atmosphere was the product of volcanic outgassing.

As Professor Robinson explained, this presents some complications when it comes to finding other examples of “Pale Blue Dots”:

“The key complication is being careful to not fall into the trap of thinking that Earth has always appeared the way it does today. So, our planet actually presents a huge array of options for what a habitable and/or inhabited planet might look like.”

In other words, our hunt for Earth analogs could reveal a plethora of worlds which are “Earth-like”, in the sense that they resemble a previous (or future) geological period of Earth. These include “Snowball Earth’s”, which would be covered by glacial sheets (but could still be life-bearing), or even what Earth looked like during the Hadean or Archean Eons, when oxygenic photosynthesis had not yet taken place.

Ice ages are characterized by a drop in average global temperatures, resulting in the expansion of ice sheets globally. - Image Credit: NASA

This would also have implications when it comes to what kinds of life would be able to exist there. For instance, if the planet is still young and its atmosphere was still in its primordial state, life could be strictly in microbial form. However, if the planet was billions of years old and in an interglacial period, more complex life forms may have evolved and be roaming the Earth.

Robinson and Reinhard go on to consider what future developments will aid in the spotting of “Pale Blue Dots”. These include next-generation telescopes like the James Webb Space Telescope (JWST) – scheduled for deployment in 2020 – and the Wide-Field Infrared Survey Telescope (WFIRST), which is currently under development. Other technologies include concepts like Starshade, which is intended to eliminate the glare of stars so that exoplanets can be directly imaged.

“Spotting true Pale Blue Dots – water-covered terrestrial worlds in the habitable zone of Sun-like stars – will require advancements in our ability to “directly image” exoplanets,” said Robinson. “Here, you use either optics inside the telescope or a futuristic-sounding “starshade” flying beyond the telescope to cancel out the light of a bright star thereby enabling you to see a faint planet orbiting that star. A number of different research groups, including some at NASA centers, are working to perfect these technologies.”

Once astronomers are able to image rocky exoplanets directly, they will at last be able to study their atmospheres in detail and place more accurate constraints on their potential habitability. Beyond that, there may come a day when we will be able to image the surfaces of these planets, either through extremely sensitive telescopes or spacecraft missions (such as Project Starshot).

Whether or not we find another “Pale Blue Dot” remains to be seen. But in the coming years, we may finally get a good idea of just how common (or rare) our world truly is.

Source: Universe Today - Further Reading: arXiv

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