For scientists and dreamers around the world, there has been a long-term quest to find a second Earth, a planet that could potentially support life as our Earth. With thousands of exoplanets and planets outside our solar system having been discovered in the last years, the possibility of finding "Earth 2.0" feels more distant than ever from simply being out there. How close are we, really? Let's dive into this fascinating world of exoplanets, explore some of the detection tools and techniques, and tease out that fascinating possibility of extraterrestrial life.
The extrasolar planets or exoplanets are planets outside our solar system, orbiting other stars other than the Sun. Since its first confirmed sighting in 1992, there have been discoveries of more than 5,000 exoplanets, thousands of which still await confirmation. They range in size from larger gas giants as big as Jupiter to small and rocky worlds akin to Earth.
Now, we discover exoplanets which have changed everything we thought we knew about the universe. We now know that systems of planets are really common and yet incredibly diverse. But the final aim is finding one where it lies inside its star's "habitable zone," an area where conditions might just be right for liquid water to exist, a key ingredient for life as we know it.
Habitable zones, also known as "Goldilocks zones," are ranges of distances from a star with temperatures that are neither too hot nor too cold to support the existence of liquid water on a planet's surface. Water is considered essential for life, as we know it, and this is why distance from the star matters. The planet would be too scorching hot if it sat too close to a star and would even evaporate the water. A planet that sits too far away would freeze your toes off, meaning that water would freeze into ice.
But it doesn't mean a planet in a habitable zone will be Earth-like. Many other conditions decide how life can be supported on the planet, like atmospheric conditions, composition, and stability. Venus and Mars both are within habitable zones of the Sun but none of them is alive. Venus is in a runaway greenhouse effect while on Mars, only a thin layer of atmosphere prevails, with no liquid water on the surface.
Probably one of the most wonderful milestones in exoplanet searches was the launching of NASA's Kepler Space Telescope in 2009. Its mission was to survey a small fraction of our galaxy to determine how many stars host Earth-sized planets in their habitable zones. In the nine years of its mission, Kepler discovered more than 2,600 confirmed exoplanets, including many that are Earth-sized and found in their stars' habitable zones.
Kepler used the "transit method" to detect the exoplanet. In this, the technique measures the star's light dimming caused by a planet passing across it, and the amount of dimming and the frequency of transits is a decision point for scientists in determining the size of the planet, orbit, and distance from its star.
Perhaps one of the most interesting discoveries Kepler has made is the so-called "Earth's cousin," which goes by the name Kepler-452b. It is an enormous 60 percent larger than our Earth, orbiting a star that resembles the Sun located in the habitable zone. We do not know if it has an atmosphere or liquid water, but it did mark a huge step for humanity in searching for Earth-like worlds.
While Kepler revolutionized the discovery of exoplanets, it only marked the beginning. New missions and technologies continue to advance the detection and understanding of distant worlds.
Launched in 2018, TESS is NASA's follow-up mission to Kepler. Kepler looked at a small corner of the sky, whereas TESS is an all-sky survey, imaging almost the whole sky to detect exoplanets orbiting around the brightest and closest stars. So far, TESS has detected thousands of exoplanet candidates, with some of those Earth-sized planets lying within the habitable zone.
The James Webb Space Telescope, a telescope launched late in 2021, has been the first and most powerful that has ever gone into space. Although its direct mission is in the study of the early universe, JWST is also intended to study the exoplanet in unprecedented levels of detail; it can break down the detailed composition of such a planet or its temperature in addition to examining the presence of potential biosignatures—those chemicals that possibly indicate life.
From the space-based telescopes, ground-based observatories are now their turn to greatly contribute to exoplanet research. In Chile, the ELT would increase resolution and sensitivity, as well as come closer to finding signs of life by studying exoplanet atmospheres.
The ultimate goal of exoplanet research is to answer one of the most profound questions for humanity: Are we alone in the universe? Although we have not yet found evidence of life beyond Earth, the discovery of potentially habitable exoplanets brings us closer to answering this question.
An obvious way scientists search for alien life is by studying the composition of a planet's atmosphere for biosignatures. That is to say, oxygen, methane, and ozone are so far considered to be potential biosignatures: they are produced by processes of life here on Earth, so their presence elsewhere in principle could be taken as an indication of life.
Detection of biosignatures is really difficult. Accurate measurements of the atmosphere of a planet require proximity, which cannot be done from such a huge distance. Also, biosignatures of certain types are created by natural, non-biological processes; thus, their alternative explanations have to be strictly excluded.
In addition to biosignatures, scientists are now looking for technosignatures, evidence of advanced technology, such as artificial lights, megastructures, and even radio signals. Technosignatures, however, remain undetected, though some projects like the Search for Extraterrestrial Intelligence (SETI) are actively scanning the skies for intelligent life.
Despite the tremendous progress that has been realized in exoplanetary research, much remains challenging. Identifying Earth-sized planets in the habitable zone continues to prove challenging, especially when orbiting bright stars, and especially if those planets have atmospheres that can be complexly characterized, ensuring them to be potentially habitable.
But the future of exoplanet research is bright. Soon, missions such as the European Space Agency's PLATO (PLAnetary Transits and Oscillations of Stars) and NASA's Nancy Grace Roman Space Telescope will greatly enhance our capacity to detect and study exoplanets. Such missions will further give us a better view of the diversity of planetary systems and bring us closer to finding Earth 2.0.
One of the most exciting and rapidly advancing fields in science is the search for habitable exoplanets. While we haven't found a true Earth 2.0, what we have found so far suggests that Earth-like planets are probably common in the universe. With new technologies and missions on the horizon, we are closer than ever to answering the age-old question: Are we alone?
The more we explore the universe, the possibility of finding that planet that would support life on it or discover signs of life elsewhere remains alluring. It doesn't matter whether we find Earth 2.0 in the next few years, but the exploration will certainly take us to unknown wonders and add to our understanding of the cosmos we live in.
This content was created by AI