The European Southern Observatory provided this artist’s impression of the surface of Proxima b. “The double star Alpha Centauri AB also appears in the image to the upper-right of Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface.”
Proxima Centauri b has continued to excite over the weekend. The MacLean’s article “Why everyone is excited about an exoplanet named Proxima b”, by Amanda Shendrake, points out the apparent import for a layman’s audience.
Just because an exoplanet is in a star’s habitable zone, however, does not mean it can host life. It simply means that if the exoplanet featured a similar atmosphere and surface pressure as the Earth, the planet could sustain liquid water. Unfortunately, we know nothing about Proxima b’s atmosphere.
For an exoplanet to be potentially habitable, scientists consider more than just whether or not it can host water. The star around which the exoplanet orbits needs to be of a particular type—one that burns long enough to allow life a chance to evolve, and emits appropriate amounts of ultraviolet radiation.
Additionally, the exoplanet must have significant similarities to Earth. The Earth Similarity Index (ESI) is a way to measure this likeness, and it places objects on a scale of zero to one, where one is Earth itself. The closer to 1.0, the more similar the exoplanet is to our home. The measurement takes into account radius, density, escape velocity, and surface temperature.
No other planets in our solar system are Earth-like; however, in addition to being the nearest of the 44 potentially habitable exoplanets we’ve found, Proxima b also has the highest ESI.
(Nice infographics, too.)
At Scientific American, Elizabeth Tasker’s blog post “Yes, We’ve Discovered a Planet Orbiting the Nearest Star but…” notes the many, many caveats around identifying Proxima Centauri b as Earth-like. Sarah Scoles’ Wired article “Y’all Need to Chill About Proxima Centauri b” explores the same territory, and makes an argument as to the underlying motivation for this identification of worlds as potentially Earth-like.
[I]t’s not an Earth twin, no matter what the headlines say, and neither are any other planets scientists have found. Hot Jupiters may be cool; planets that rain glass may be a hit at parties; “Super-Earth” may be fun to say. And getting the full census of exoplanets is valuable. But most scientists, Messeri has found, really just want to find another Earth. Research priorities reflect that. The Kepler Space Telescope, which has discovered more planets than any other enterprise, was “specifically designed to survey a portion of our region of the Milky Way galaxy to discover dozens of Earth-size planets in or near the habitable zone,” according to NASA.
The search for an “Earth Twin” is the quest for a platonic ideal, says Messeri. “It allows us to imagine Earth at its best, Earth as we want to imagine it, Earth that isn’t hampered by climate change or war or disease,” she says.
In other words, Earth as we’ve never known it and never will. If scientists found a twin planet, Pure Earth would become, in our minds, a real place that still exists, somewhere out there.
But we haven’t discovered that place. And we might not ever. On a quest for the perfect partner, you usually find someone who’s pretty cool but yells at you when they’re hungry, or hates your mom. After you sign on to the perfect job, you discover that you’re supposed to wash everyone’s coffee mugs. In that way, finding Proxima Centauri b while searching for Pure Earth is just like every human quest for perfection.
“What we’ve actually found is something more real,” says Messeri, “and less ideal.”
Joseph Dussault at the Christian Science Monitor notes in “Do we need to change the way we talk about potentially habitable planets?” that the language used to describe these worlds may need to be altered, for lay audiences at least. Universe Today’s Matt Williams notes that Earth-like means something that is not a duplicate of Tahiti.
[F]inding a planet that is greater in size and mass than Earth, but significantly less than that of a gas giant, does not mean it is terrestrial. In fact, some scientists have recommended that the term “mini-Neptune” be used to describe planets that are more massive than Earth, but not necessarily composed of silicate minerals and metals.
And estimates of size and mass are not exactly metrics for determining whether or not a planet is “habitable”. This term is especially sticky when it comes to exoplanets. When scientists attach this word to extra-solar planets like Proxima b, Gliese 667 Cc, Kepler-452b, they are generally referring to the fact that the planet exists within its parent star’s “habitable zone” (aka. Goldilocks zone).
This term describes the region around a star where a planet will experience average surface temperatures that allow for liquid water to exist on its surface. For those planets that orbit too close to their star, they will experience intense heat that transforms surface waster into hydrogen and oxygen – the former escaping into space, the latter combining with carbon to form CO².
This is what scientists believe happened to Venus, where thick clouds of CO² and water vapor triggered a runaway greenhouse effect. This turned Venus from a world that once had oceans into the hellish environment we know today, where temperatures are hot enough to melt lead, atmospheric density if off the charts, and sulfuric acid rains from its thick clouds.
For planets that orbit beyond a star’s habitable zone, water ice will become frozen solid, and the only liquid water will likely be found in underground reservoirs (this is the case on Mars). As such, finding planets that are just right in terms of average surface temperature is intrinsic to the “low-hanging fruit” approach of searching for life in our Universe.
Even so, the optimism expressed by Bloomberg View’s Faye Flam in “What the New Planet Says About Life in the Universe” is something I would like to cling to. Proxima b’s ideal, or at least worlds like this ideal, might be perfectly suited for life.
Studying this planet could reveal something important about the timeline of life in the universe, and whether we earthlings are early to the party. That’s because stars like Proxima Centauri are the future of the universe. Called red dwarfs, these make up the majority of stars in the galaxy, and they live about 1,000 times as long as our sun.
In a paper made public last month, Harvard astronomer Avi Loeb looked at the cosmic implications of life surrounding red dwarfs. Loeb calculated that if life is just as likely to form around these stars as sun-like ones, then the vast majority of life in the universe has yet to be born, and we earthlings are not just early, he said, but “premature.”
That’s because scientists believe eventually, all the raw materials for star formation will be used up and all the stars will die, leaving a dark universe of dust and black holes. For most of the trillions of years stretching between now and cosmic darkness, red dwarfs will be the only game in town.
The papers at arXiV at least allow for hope. Why not encourage it?