We need to stop saying ‘Earth-like’ until we actually discover an exoplanet (or exomoon) that’s worthy of the title. While it’s exciting to hear the news of another discovery (and important to keep the public interested in the topic), it sets a poor precedence for how close we really are to discovering a planet that’s anywhere near ‘Earth-like’ or ‘Habitable’.
Here are just some of the recent articles that have come out that suggest we’ve discovered a shiny new Earth-like planet ready for colonization:
- Economic Times: Two Earth-like planets discovered around Dwarf Star
- Popular Mechanics: Newly Discovered Earth-like Planet Is Speeding Toward Us
- Science Daily: Two new Earth-like planets discovered near Teegarden’s Star
- Time: Why This Earth-like Planet is a Big Step Forward in the Search for Life
- USA Today: ‘Super-Earthy?’ Potentially habitable planet found
If articles like the one’s above are to be written about these planets, and the authors insist on including the phrase ‘Earth-like’, they should also include one of these qualifiers:
- ‘The most Earth-like yet’
- Or focus more on a singular feature, such as ‘Earth-sized’
Scientists confirmed the first exoplanet discovery in 1992. Since then we have been increasingly detecting and confirming exoplanets (and exomoons in a couple of cases) and now have confirmed thousands of exoplanets around stars as distant as a few thousand light years. This is a tremendous achievement as it’s incredibly difficult to detect a planet, especially one that’s smaller like Earth that’s also orbiting a star similar to our own.
Even more recently we’ve been focused on our solar system’s outer gas giants – Jupiter and Saturn. With over a hundred moons combined, a few of them have been of intense interest for the discovery of life. In particular, Europa and Titan have been getting a lot of press lately. Even with their promise for possibly complex life of some kind, they are nothing like Earth, nor are anywhere near what we’d want or need to colonize. Earth-like is not just ‘it has liquids and some atmosphere stuff on a rocky ball’.
If we are to recognize an exoplanet or exomoon as Earth-like, we need to first confirm the following with as much confidence as possible. For each perimeter I’ve included my own personal confidence designations based on past scientific research. The number is how critical the requirement is compared to the others, and the second reference is how confident we are in being able to confirm the property with current technology. Keep in mind that while this is a list, many of these have a previous qualifier, so finding a life-filled world may require all parameters and thus all could be considered “1”.
- 1/Confident: Position in the star’s (or perhaps gas giant’s) habitable zone (Mars qualifies), which speaks to temperature, UV radiance, and establishes the possibility of liquid water and other viability constraints.
- 2/Confident: About a third the mass of Mars or up to about 4 mass of Earth (the extremes will be partially determined by the habitable zone placement, and possibly orbit around another giant planet like Jupiter).
- 3/Somewhat Confident: Atmosphere of half a Bar to several Bars more in thickness than Earth that has some minimal level of carbon dioxide of about 150ppm (yes, this is necessary), nitrogen, and some other trace gases (Oxygen can come later with life).
- 4/Somewhat Confident: Liquid water on surface covering at least 30% (this speaks to the general temperature of the planet, as well as its orbital stability and long-term climate cycles)
- 5/Somewhat Confident: A magnetic field (may not always be necessary (Venus), a magnetic field can help ensure the possibility of surface life and retention of surface water for billions of years).
- 6/Somewhat Confident: Rotation that’s not too slow (Venus) or too fast. Slower in general is better though… even slower by several hours than what Earth has currently.
- 7/Not Confident: Atmosphere or ground that doesn’t contain universally toxic chemicals (I place this near the end of importance as it’s theoretically possible life could still evolve in ‘hostile’ environments).
- 8/Not Confident: Plate tectonics for atmospheric and ground chemical/mineral recycling (though this may or may not be a necessary feature, but it’s a good one).
It’s interesting to note how we’re more confident in the most important qualifiers than the supposed least important. This is a great start in our ability to confirm an Earth-like world, though again confirmation may still require all of these parameters.
Our technology is at the cusp of being able to consistently and accurately detect most of these features. Even evidence of plate tectonic activity is possible with the right mix of atmospheric gases, as well as the ability to possibly detect continents against the backdrop of an ocean.
The most important ingredient above all others though is the detection of liquid water on the surface. If you have that, most if not all of the other features are sure bets. Without many of them, such as the right sized planet, distance from its parent star, and so on, liquid water on the surface just wouldn’t be possible.
I could go on (does it have an ozone layer, Halley cells, etc.), but this is what would properly qualify at the foundation of a discovered planet that’s habitable and anything worth recognizing as ‘Earth-like’.
Learn about the limits of exoplanet habitability in my recent article and book, ‘Introducing Habitable Exoplanets: Red Dwarf Systems Like Trappist-1’.