Exoplanets and the Search for Earth 2.0
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You look up at the stars. They stand 245 light-years away from the puffy clouds and blue skies of Earth. The Sun is merely a yellow dot. Most prominent in the sky is a big, puffy planet on the horizon. You take out your phone, snap a picture, and post it online; the location is tagged as “the moon of Kepler 16b.” Kepler 16b is a real planet that orbits a binary star system, much like the fictional planet Tatooine from “Star Wars.”
Kepler 16b is not the only planet that we have recently learned about. In fact, more than 4,000 planets outside our solar system have been discovered since 1992. They range from as close as 4.25 light-years to as far as 13 thousand light-years away. These planets are called exoplanets, and astronomers have been discovering a few each month. Most stars have, on average, at least one exoplanet. Of course, some star systems, such as HD 10180, host up to nine. Exoplanets are vastly different from each other. While one exoplanet is covered in diamonds (55 Cancri E), another is darker than coal (TrES-2b).
The different types of exoplanets include Neptune-like planets, gas giants, super-Earths, and terrestrial planets. Neptune-like exoplanets, as the name suggests, are similar to Uranus and Neptune, containing hydrogen, helium, water, ammonia, and methane in their atmospheres. The immense surface pressure of these planets makes them unlikely to possess life.
Meanwhile, gas giants can take on one of two different forms: cold gas giants, like Jupiter and Saturn, or hot gas giants. While both are mostly composed of helium and hydrogen, hot gas giants are extremely warm due to the proximity to their star, unlike the ones in our solar system. The astonishing gas giant J1407b, dubbed “Super Saturn,” is renowned for its ring system that’s 200 times more massive than Saturn’s. But don’t be fooled by the spectacle; with toxic gases and the absence of any solid surfaces, there’s no way life as we know it would exist on any gas giant.
On the other hand, super-Earths are terrestrial exoplanets that are larger than Earth but smaller than Neptune. There is evidence suggesting that Kepler-22b, a super-Earth, may be covered entirely by water. Terrestrial exoplanets, however, are rocky and of an equal or lesser size to Earth. Both super-Earths and terrestrial planets are at the focus of studies because they have similar compositions to Earth. The seven planets in the TRAPPIST-1 star system are believed to be rocky, making them prime subjects of investigation.
Each of these planets orbits a star (or maybe even two). Just as there are many different forms of exoplanets, there are various star types. Our Sun is a yellow dwarf star. From what we know, yellow dwarfs make up just 10 percent of all stars.
Conversely, the most prevalent and longest-living star is the red dwarf. Though not visible from Earth by eye, our neighboring star, Proxima Centauri, is a red dwarf. Other star types include brown dwarfs, orange dwarfs, blue giants, red giants, and white dwarfs. White and blue stars are the hottest, while red and brown stars are the coolest. However, research suggests that the most ideal hosts for life are the orange dwarfs, thanks to their low UV radiation and high stability. Giant stars have short life spans, making them highly unstable.
So how is it that scientists have found and approximately mapped so many distant planetary systems? For stars visible from Earth, a shift in the star’s brightness indicates an orbiting planet. Scientists can measure the amount of change to determine the size of the planet through the transit method. Unfortunately, this method only works for planets whose orbit paths are between the star and Earth. Another technique called the radial velocity method relies on the gravitational tug planets have on their star. Both the planet and star orbit a common center of mass called the barycenter. This pull makes the star appear “wobbly.” Because it’s difficult to see, astronomers utilize the Doppler Effect by examining the change in wavelengths. The same effect is observed when an ambulance passes an observer. The siren has a higher pitch when passing the observer as opposed to when it’s traveling away since the sound has a smaller wavelength. Likewise, when a star moves closer, the wavelengths compress and appear blue. When the wave stretches, it appears red. Naturally, the planets discovered using the radial velocity method are usually gas giants with great gravitational force.
Many other large exoplanets are found by direct imaging. Devices like the coronagraph and starshade are used to block light from the star, making it easier to image the planet(s). There are also more methods used to discover exoplanets, like microlensing, astrometry, and pulsar timing. Telescopes and spacecraft that have helped make these findings a reality include the Hubble Space Telescope, Chandra X-ray Observatory, and Kepler Space Telescope.
Nonetheless, what’s the use of discovering these diverse exoplanets if we can’t travel that far? For one, scientists hope to use their discoveries about young and old celestial objects to understand how life on Earth developed and what the future may hold.
Ultimately, the prime goal of NASA’s Exoplanet Program is to search for life on other planets. By analyzing light waves in a process called transmission spectroscopy, astronomers can determine the components of an exoplanet’s atmosphere and the planet’s mass. These characteristics are vital in determining whether an exoplanet is habitable or not. It’s also essential in the search for “Earth 2.0,” if we ever plan to move to another planet. Looking for an Earth 2.0 also means that there’s a greater chance of finding life. Imagine if these foreign life forms were just as developed as we are. The best candidates for life as we know it would lie in a star’s habitable zone, also known as the Goldilocks zone, where water can exist as a liquid. For hot stars like blue giants, the habitable zone would be far from the star; for cooler stars, like red dwarfs, it would lie much closer. So far, astronomers consider the TRAPPIST-1 star system the most likely to host life, having a total of three Earth-sized exoplanets in its habitable zone. Even our adjacent star system (Proxima Centauri) hosts a planet (Proxima Centauri B) in its habitable zone. Located four light-years from Earth, it will likely be the first exoplanet humans visit. But even traveling at the speed of light, it would take us a lengthy four years to complete the journey. Besides, it may be decades until we engineer technology to make that possible.
Simply determining which exoplanet has the potential to become Earth 2.0 will be extremely challenging due to the lack of knowledge about the discovered exoplanets. On top of that, finding an exoplanet with the same conditions as Earth is highly improbable. Earth 2.0 will need to have a nitrogen-oxygen atmosphere, have a Sun-like star, and be Earth-sized. Despite that, scientists have found planets better than Earth in terms of habitability. The stars they orbit are more stable, the planets contain more water (a key ingredient for life), and the planets are slightly warmer (Earth’s warm and moist climates comprise the most biodiversity).
The hope is that, in the future, a stronger familiarity with other worlds will improve our understanding of ours too. We may even uncover some aliens along the way.