Looking for Habitable Planets
From discovering the first planets beyond our solar system (exoplanets) using ground-based telescopes, to future plans for analyzing the atmospheres of these bodies light years away, NASA and the European Space Agency (ESA) have guided our search for habitable worlds beyond Earth.
Writing in Hidden in the Heavens: How the Kepler Mission’s Quest for New Planets Changed How We View Our Own, physicist Jason Steffen describes the techniques that have contributed to our understanding of planets beyond our solar system. “Exoplanet discoveries, while concentrated in the last few decades,” he writes, “were enabled by several technological advances that occurred over the last few centuries.” Steffen is part of the science team that designed the Kepler telescope and continues the analysis of its data.
Since the 1800s scientists have used the spectroscope to determine the chemical composition of stars. Further understanding of the spectrum in more recent times has enabled astrophysicists to detect star motion, or “wobbles,” caused by the gravitational pull of unseen planets. Two Swiss researchers, while observing this phenomenon, discovered the first exoplanet orbiting a sunlike star; they received the 2019 Nobel Prize in physics for their work.
“We are discovering not merely new continents, like the explorers of old, but whole new worlds circling other stars.”
Detecting the dimming of starlight caused by these planets crossing in front of their host star was the next important step. “This transit measurement was a big deal,” Steffen notes. “It opened a completely new arena for investigation.” Digitizing and refining this system and building it into space telescopes has made it possible to do the seemingly impossible, writes Steffen, “like looking at Las Vegas from space and detecting a fly buzzing around a streetlight.”
It’s hard to imagine the incredible odds against our coming into just the right alignment with a star to observe an exoplanet crossing its face. Yet the Kepler telescope was able to accomplish this for star systems in its field of view 500 to 3,000 light years away.
“Exoplanet detection,” writes Nathalie Cabrol in The Secret Life of the Universe, “is a highly dynamic field that is likely to remain vibrant and exciting for many decades.” Cabrol is confident that other life is out there waiting to be found: “With a minimum of 300 million exoplanets located in the habitable zone of their parent stars in our galaxy alone, thinking that we are alone in this cosmic ocean is simply a statistical absurdity” (see “A Timeline of Firsts in Exoplanet Discovery,” below).
This NASA illustration depicts Kepler-186f, the first validated Earth-size planet to orbit a distant star within its habitable zone.
NASA Ames/JPL-Caltech/T. Pyle
“Ever since the discovery of the first exoplanet,” Steffen concludes, “and especially with the large number of planets that we’ve seen in the subsequent few decades, how we view the Earth, its sibling planets, and our shared history will never be the same.”
The Search for Extraterrestrial Intelligence, or SETI (since 1960), and Breakthrough Listen (since 2016) have swept the sky for evidence of communications from extraterrestrial intelligence (ETI). To date, no messages have been detected. The idea of physical planets existing beyond our solar system, on the other hand, has been debated for centuries. It was only 30 years ago that their presence was actually confirmed. Many rocky, Earthlike exoplanets have now been and are continuing to be discovered. Time will tell whether or not these exhibit conditions that are conducive to life or reveal evidence of life.
“While Kepler did not answer the ultimate question about our collective solitude in the universe, it clearly showed the wondrous variety of creation that is out there.”
What we do know from these ongoing discoveries is that our situation—a planet perfect for life, with an intelligent species creating a complex, technological civilization—is, to say the least, rare. In Rare Earth: Why Complex Life Is Uncommon in the Universe, Peter Ward and Donald Brownlee suggest just that: based on an examination of the factors that make Earth hospitable for animal life (factors that go far beyond rockiness, size, or distance from its sun), finding complex life elsewhere is extremely unlikely. They argue that this should give us pause to consider the powerful role we play in the world. “What if,” they ask, Earth is “utterly unique: the only planet with animals in this galaxy or even in the visible Universe, a bastion of animals amid a sea of microbe-infested worlds?”
How, then, should we see our role as intelligent inhabitants, able to understand cause and effect, and then to manage our relationship to life on Earth? In terms of the biosphere, there is no replacement for the things we destroy; tragically, it is the ultimate hubris to believe our actions are without consequences. “If that is the case,” Ward and Brownlee conclude, “how much greater the loss the Universe sustains for each species of animal or plant driven to extinction through the careless stewardship of Homo sapiens?”
It doesn’t really matter how many other Earthlike exoplanets exist up there. Our ongoing exploration and search for other habitable worlds should inspire in us a heightened awareness of our impact and responsibilities down here.
A Timeline of Firsts in Exoplanet Discovery
- 1992 – Astronomers announce their discovery of a rocky exoplanet, 1,170 light years away.
- 1995 – Swiss scientists find an exoplanet orbiting a sunlike star.
- 2003 – NASA launches the Spitzer Space Telescope for infrared detection of exoplanets.
- 2007 – The CoRoT (Convection, Rotation and planetary Transits) satellite detects exoplanets transiting their stars; ESO (European Southern Observatory) announces discovery of the Gliese 581 system, including a “Super-Earth” found in the habitable zone of its star.
- 2008 – A digital time capsule containing 501 messages is aimed toward Gliese 581c from a radio telescope in Ukraine. It will arrive there in 2029.
- 2009 – The Kepler Space Telescope begins scanning a small area of sky containing more than 150,000 stars, eventually detecting more than 2,700 confirmed exoplanets and almost 400 in the habitable zone of their stars.
- 2017 – Kepler scientists discover the Trappist-1 system, including seven Earthlike planets.
- 2018 – A consortium of US space exploration organizations launches the Transiting Exoplanet Survey Satellite (TESS) to continue the search for exoplanets.
- 2019 – The European Space Agency (ESA) launches the CHaracterizing ExOPlanet Satellite (CHEOPS) to gather further data from known exoplanets.
- 2021 –The James Webb Space Telescope (JWST) explores the deep history of the universe and helps characterize exoplanets.
- 2026 – ESA’s planned launch of PLAnetary Transits and Oscillations of stars (PLATO) will advance the discovery of exoplanets and the evolution of extrasolar systems.
- 2027 – The Nancy Roman Space Telescope will create direct images of Jupiter-size exoplanets.
- 2029 –The Atmospheric Remote-sensing Infrared Exoplanet Large-survey mission (ARIEL) will examine the atmospheric structure of exoplanets to uncover their history and evolution.
- 2040 – The Habitable Worlds Observatory (HabEx), presently in the design stages, will directly image Earthlike exoplanets.