There are billions of planets out there in our galaxy, the milky way, and beyond. These planets can be insanely different from ours. Imagine the year lasting 18 hours or an atmosphere so hot it could melt metal. As hostile as most of these planets are, astrophysics tells us some of these worlds may be habitable planets. Space agencies like NASA, the European Space Agency (ESA) are expanding their search for these worlds beyond our solar system. Any planet outside our solar system is called an exoplanet. So far, scientists have discovered over 4,000 diverse exoplanets. While most of these exoplanets are nothing like our Earth, a small handful are Earth-Like planets.
It’s incredible that humans have figured out how to find exoplanets at all. They are about the same size as the planets in our own solar system but are trillions of miles away! Have you ever looked at Venus, Mars, or Jupiter through a telescope? Even then they look tiny! The nearest exoplanet, Proxima B, is 4 light-years away. That’s 24 trillion miles. Mars, by comparison, is only about 35 million miles away. Astronomers confirmed the first exoplanet just 30 years ago in 1992.
While land-based observatories can find some exoplanets, space telescopes are much more effective. Dr. Nancy Grace Roman pioneered the practice of launching telescopes into space with the Hubble space telescope. Space telescopes like NASA’s Kepler space telescope have numerous advantages for finding exoplanets. They are ultra-stable, receive no light pollution from humans, and are above Earth’s pesky, hazy atmosphere and weather.
Finding an object the size of Earth more than 24 trillion miles away takes some sophisticated techniques. Two methods, the transit method, and the wobble method are currently the best ways to discover exoplanets.
Once an exoplanet candidate is discovered by a telescope it must be verified by two other telescopes. For example, NASA’s transiting exoplanet survey satellite (TESS) discovered over 2,500 exoplanet candidates but only 125 have been confirmed as exoplanets. This is why only about 4,000 exoplanets are confirmed. New telescopes like the James Webb space telescope, planned for launch in 2021, will help us discover and confirm more Earth-like planets.
The transit method of finding exoplanets is the most common way to discover exoplanets. This method has led to the discovery of more than half of all exoplanets. When an exoplanet passes between one of our telescopes and its solar system’s star, the planet slightly dims the light of that star when observed by the telescope.
Try it yourself. If you hold something in front of a lamp lightbulb, the light coming from your lamp becomes dimmer to your eyes. Since planets are so much smaller than stars this effect is much more subtle than your experiment. Our sensitive astronomical instruments, however, can pick up on those extremely subtle differences in light. The bigger a planet, the more light it blocks. This means bigger planets are easier to discover.
The transit method works best with red dwarf star systems. These systems are the most common in the universe. Red dwarfs are relatively cool, small stars. Since they are cooler and smaller than stars like our sun, they emit less light. This means that planets that pass in front a red dwarf block a larger percentage of the total light from that sun compared to planets that pass in front of a hotter sun like ours. This increased light blocking makes the detection of red dwarf system exoplanets easier.
The wobble method of discovering is a bit more difficult to imagine. It is the second most common method of exoplanet discovery, resulting in about 25% of all discoveries.
To get a grasp on this method, let’s start with our own moon. The tides on Earth are a result of the push and pull from the moon. The moon always pulls water towards it, so as it spins around the Earth the tides are higher on the side of the Earth closest to the moon and lower on the side of the Earth further away from the moon. This is an example of the moon exerting gravity on the Earth. Even though it is much, much smaller than the Earth, the moon still has a gravitational effect on our world.
Similarly, planets have an effect on the gravity of their parent stars. These planets obviously orbit their sun-like stars due to gravity, as our Earth does. But these planets also pull their stars towards themselves in, again, subtle ways. This pull from the planets produces a ‘wobble‘ effect in the rotation of the star.
Smaller planets, like Mercury, would be very difficult to detect using the wobble method. These small planets have so little mass compared to their stars that the wobble in the star is nearly undetectable. Larger, more massive planets, like Jupiter, have a stronger pull on the star, making the wobble easier to detect.
Similarly, if our moon was 10 times the size it currently is its gravitational presence on Earth would be stronger. Our tides would be much stronger than they currently are!
As demonstrated above, it is difficult to learn all that much about planets that are so far out there. And it is easier to learn about planets that are much bigger than Earth. Most of our knowledge of these planetary systems is inferred from astrophysics rather than directly observed. That being said, these inferences are incredibly valuable. They can clue us in to an exoplanet’s atmosphere, size, mass, and whether the planet may be habitable or not.
Just capturing the effects of an exoplanet by the wobble method or the transit method is difficult enough. To add to that difficultly, there are too few space telescopes to answer all the questions astronomers have. These telescopes’ schedules are jam-packed. Due to this lack of observation equipment, we don’t know as much about alien worlds as we could.
Given enough time on a telescope, we can sometimes learn interesting facts about exoplanets. In some situations, the transit method can also clue us into the atmosphere of exoplanets. When the light from a star is dimmed by a planet, not all light is dimmed equally. The planet, if solid, will block all wavelengths of light. If the planet has an atmosphere, the different gasses in that atmosphere will block different wavelengths of light to varying degrees. Transit spectroscopy measures the different wavelengths of light that pass through exoplanet atmospheres. Carbon dioxide, for example, blocks different wavelengths of light compared to methane or water. By using this technique we can learn about the composition of planets trillions of miles away.
This technique, unfortunately, takes eight or more transits to be accurate. That’s a lot of telescope time! Clouds and other atmospheric events on the exoplanet can also alter the result of transit spectroscopy. It’s not a perfect method, but it’s one of the only methods we have.
A newer method, the Webb method measures the difference in heat on an exoplanet between the side facing the sun and the dark side of the planet. That difference in heat can also tell us something about the atmosphere. If the planet has no atmosphere, the temperature difference between the light and dark sides will be very great. If there is an atmosphere, that difference will be less because the atmosphere regulates the heat on the planet as a whole. This method only takes two transits, making it much more efficient than the transit spectroscopy method. The new James Webb space telescope aims to dig deeper into this technique.
The ‘zone of habitability is defined as the orbital belt around a star where liquid water could be present on a planet. Astrobiologists (maybe the coolest job title ever) believe that liquid water is crucial for life to develop. While this may seem an Earth-centric view of life, water does have some very unique capabilities which render it particularly favorable for life. As astrobiologist Philip Ball put it, “1. Life most probably needs a solvent. 2. That solvent needs to perform an active, diverse, and flexible role. 3. Water is so far the only common liquid we know that is capable of this.”
In our solar system, the Earth and the moon are the only planets within the zone of habitability. Venus is too hot for liquid water and Mars is too cold.
Kepler-186f was the first discovered similar-sized planet to Earth within the habitable zone. This planet was discovered in just 2014, which gives you a sense of how new this field of study is. We’ve had the iPhone 6 longer than we’ve known of exoplanets in the habitable zone. In addition to being in the habitable zone, exoplanets must be small enough that they aren’t gaseous planets and large enough that they aren’t simply small space rocks in order to be truly habitable. Of 3300 exoplanets (a somewhat outdated number), only 53 are potentially habitable exoplanets.
The width of this ‘Goldilocks zone’ as well as the distance from the sun depends on the size and the heat output of the host star. If the host star is huge and fiery hot, the zone of habitability will be further away than it is in our solar system. If the star is a red dwarf, that zone will be closer and the band will be more narrow.
There are four broad categories of exoplanets, two of which could be Earth-like planets. The size and mass of the planet determine which of the four categories they are placed into. These two factors largely dictate the surface and general characteristics of the planet.
How can we determine mass? By the wobble method. How can we determine size? By how much light the planet blocks via the transit method. But, as mentioned before, both of the techniques are more difficult for smaller planets because they don’t make as much of a wobble or block as much light. If we garner both the size and mass of a planet, we can calculate the planet’s density. Knowing this density gives us big clues into the surface, atmosphere, and creation of the exoplanet.
Gas giants are not earth-like planets. These massive planets have enormous atmospheres of hydrogen and helium. Unlike oxygen and nitrogen in our own atmosphere, these gasses stay gasses until very cold temperatures (around -450F). If our planet was that cold we would have liquid oxygen and nitrogen and no atmosphere.
These planets, sometimes called Jovians since they are huge like Jupiter, somehow form during the early stages of star formation. During this phase, the star sheds tons of hydrogen and helium which gas giants can suck up with their gravity. These planets are more than 10 times the diameter of Earth.
Neptune-like planets are gaseous with a rocky core. They have 4 to 10 times the diameter of Earth. These planets tend to form on the very edges of solar systems and are extremely cold. Therefore, these are not earth-like planets. Astronomers don’t understand how these planets are formed.
Super-Earths are ‘super’ because they are two to ten times as heavy as our own Earth. These super-earths may have surfaces of lava, water, rock, or gasses. The TOI-270 system has two super-Earths that more closely resembles our Neptune than Earth. Some super-Earths are considered habitable, unlike Neptunians or Jovian gas giants. Most astronomers now think that there might be a super-earth on the very edges of our own solar system. Apparently, assuming a super-earth exists out there solves more astronomy problems than it creates. We could have a planet past Pluto!
As you may guess, terrestrial planets are our best bet for Earth-like planets outside our solar system. These more-or-less Earth-size planets can be up to two times the mass of Earth. Because they are small, these planets don’t attract huge, prohibitive atmospheres like the gas giants and Neptune-like planets.
Unfortunately, these little terrestrial planets are difficult to discover. As we explored before, the wobble and transit method of planet discovery works much better for large planets. Therefore, of the 4500+ confirmed exoplanets, only 165 are terrestrial. This may not be because they are rarer in the universe but simply because it is more difficult for us to ‘see’ them. In fact, astronomers think there may be millions of planets like Earth in the Milky Way alone.
The word ‘terrestrial’ might be a bit of a misnomer. Some of these planets may be rocky planets, but others may have totally liquid surfaces. Even some rocky terrestrial planets, like our own Mercury, may have no atmosphere whatsoever. These terrestrial planets are our best bet for intergalactic Earth-like planets.
In 2015 astronomers discovered the Trappist-1 solar system using the Belgian Trappist observatory. Subsequent observations made by the Kepler and Spitzer space telescopes added to our understanding of this fascinating system. This system has a very cool, small sun known as an ultra-cool red dwarf. Like a gnome with sunglasses? Nope.
Because this star is so cool, we were able to learn a lot about the planets orbiting it via the wobble and transit methods. So far, astronomers have discovered 7 terrestrial planets orbiting Trappist-1. These planets are extra close to their star, taking between 1.5 and 19 days to complete a revolution. Imagine an entire year being one and a half days. What would that even be like?
Even though these terrestrial planets are so close to their star, they are not insanely hot because the star itself only puts out 0.05% of the heat of our sun. Of the seven planets, five are within the habitable zone where liquid water is possible.
Exoplanet research is still in its infancy. Remember, the first exoplanet was discovered in just 1990. Human knowledge of space is sure to become much stronger in the coming decades as technologies improve and costs of operation decrease. New space telescopes will increase our understanding of these diverse, alien worlds. Maybe, just maybe, we will even find signs of life trillions of miles away.