How do we search for planets outside our solar system? What have we found so far? What do we know about the other planets in the universe?
Searching for exoplanets: radial velocity
We refer to planets orbiting stars outside our solar system as exoplanets, and the first confirmation of another planet orbiting a sun-like star was in 1995. 51 Pegasi b, officially named Dimidium, was discovered by the radial velocity method.
The radial velocity method works due to gravity. Stars exert a strong gravitational pull on their orbiting planets, while the effect of the orbiting planet’s gravitational force on the star is lesser.
This pull is enough to make the star wobble or dance very slightly. This tiny wobble stretches the wavelength of the light emitted by the stars very slightly because of the Doppler effect. We can use this tiny shift to discover planets around other stars.
We have so far discovered 936 planets using this method. Massive planets close to their stars are easiest to find using this method because of their gravitational pull – this describes many of the earliest planets found.
Searching for exoplanets: transit method
When an exoplanet passes in front of a distant star, the star briefly gets very slightly dimmer. It’s like the solar eclipse, which results from the moon passing between the Earth and the sun – but a much more subtle change. The planet’s journey between its star and an observer – someone standing on the Earth, for example – is known as a transit. Making certain observations about the transit can tell us more about the planet.
The dip in a star’s brightness that indicates the passage of a planet is known as a light curve. The larger the planet, the deeper the light curve as it blocks more light from its star. Longer transit events show planets a long way from their stars, with long orbits that take time to pass in front of the star.
Light travels through its atmosphere as a planet passes in front of this star. By analyzing this light, we can learn more about the chemical composition of this atmosphere.
The transit method is very useful indeed when searching for exoplanets. So far, we have found 3879 planets using this method.
Other ways of searching: gravitational lensing and astrometry
We have found a lot of planets using the radial velocity and transit methods. But astronomers are inventive and have other tools they can apply in this search.
Massive bodies bend space-time. And when a star moves in front of a more distant star, this bending focuses the latter’s light, making it brighten briefly. Detection of planets in this way is called gravitational lensing.
This effect is enhanced when the first star has a planet in tow, making the light from the more distant star brighter still. We can detect exoplanets thanks to this effect, but unfortunately, we can’t predict when it will happen. We just need to watch carefully. So far, 132 planets have been discovered this way, so it’s worth watching.
The tiny wobbles of stars we detect with the radial velocity method can also be seen in very tiny movements relative to other stars. However, these movements are so tiny they’re extremely difficult to track, requiring extremely precise optics. The method of tracking these tiny movements, called astrometry, has only been used to find 1 planet so far.
The problem with taking direct images and what we can do about it
Taking direct images of exoplanets is very challenging – sometimes impossible.
The main issue is one of brightness. Planets are small and dim, and the light from their nearby stars tends to drown them out. The chances of seeing an exoplanet directly are higher if it’s orbiting a long way from its star, or if it is particularly massive.
To combat the dazzling glare of a star, astronomers can use coronagraphs, devices which block light inside telescopes. Blotting out some of the light from stars that may have planets orbiting allows us to search for the planets themselves. So far, coronagraphs have been used in ground-based observatories but NASA’s planned Nancy Grace Roman Space Telescope will have a coronagraph on board.
Alternatively, star shades have been proposed for use with space telescopes. These could be deployed on spacecraft, which would position themselves exactly right to block the light from a particular star, allowing us to see what dim bodies might be nearby.
Kepler mission
The Kepler Space Telescope was designed with one purpose in mind: to look for Earth-sized planets in the habitable zones of other stars. The habitable zone is the region of space around a star where liquid water could exist on a planet’s surface. These are regions where temperatures could be just right – not too hot and not too cold, hence its nickname of ‘The Goldilocks Zone’.
The telescope was launched in 2009 and used the transit method to look for planets, first as the Kepler mission and later as the K2 mission.
In its 9 years of operation, Kepler discovered more than 2600 planets. It changed how we view the universe, revealing that small planets are more common than we thought and that there may be more planets in the universe than stars. It discovered that planets come in many sizes. The most common planets discovered were between the size of Earth and Neptune. This size of planet doesn’t exist in our own solar system, but there are many out there in the universe.
Characterizing exoplanets
Discovering that planets exist is one thing. But how do we learn more about them?
The mass of a planet can be measured by calculating how much of a gravitational tug the planet is exerting on its star. But while this is just about possible for more massive planets – the size of Jupiter, for example. For smaller Earth-sized planets, this is extremely difficult.
However, if more than one planet orbits a star, we can use them to make inferences about each other. The planets exert a pull on each other. So when we observe them transiting their stars, we can see them slowing down or speeding up very slightly due to their neighboring planets’ influence. From these slight variations, we can infer the mass of the planets.
Different elements and compounds absorb different wavelengths of light. The light that comes through the atmospheres of exoplanets can reveal more about what makes up those atmospheres. The Hubble Space Telescope has used this technique, known as transit spectroscopy, to discover Helium and water vapor in the atmospheres of exoplanets. The James Webb Space Telescope is likely to tell us more.
Gas giants
Gas giants are massive planets without solid surfaces, largely made up of helium and hydrogen – just like our Jupiter and Saturn.
Although Jupiter and Saturn are in the outer regions of our solar system, some of the earliest exoplanets we discovered are so-called ‘Hot Jupiters’. These are huge gas giants orbiting close to their stars and so reach extremely hot temperatures.
They exert a big gravitational pull and cause a pronounced wobble in the star because they are enormous in size, and are so close to their stars. These pronounced wobbles made them relatively easy to find when we started looking for exoplanets.
Evidence from the Spitzer Space Telescope suggests that gas giants form relatively quickly when solar systems are young – often in the first 10 million years of the life of a sun-like star.
Do planets migrate?
Hot Jupiters present us with a puzzle. What are these massive planets doing so close to their stars? Our gas giants are much further out in our solar system, away from the intense radiation of the sun.
The planets could have formed close to their stars, but the intense heat from a young star would be likely to vaporize the gasses as they formed. Young stars are also explosive and turbulent – likely to disrupt the formation of massive planets.
Further out from the star and past the snow line, it would be cool enough for a solid icy core to form and for gas to surround these cores.
It is possible that the planets then migrated inwards, closer to their stars. This could have happened early in the solar system – as the planet was pulled closer by the star’s gravitational field. An alternative theory suggests this could happen at a later point in the solar system’s history, as other planets influenced their giant neighbor. We’re still not sure about how planetary migration works.
Neptunian planets
Around 4 times the size of Earth, Neptunian planets are comparable to Uranus and Neptune in our solar system. Mini-Neptunes have also been discovered, somewhere between the size of Earth and Neptune – we don’t have any comparable planets in our solar system.
It has proven difficult to learn much about how Neptunian planets are made up. They often have thick clouds that block much light from coming through, making it difficult to analyze their atmospheres. Typically, they appear to have atmospheres with lots of hydrogen and helium. We don’t know exactly what their cores are made of, but they are likely to be heavy and rocky.
So far, we have confirmed the discovery of 1774 of these planets. One of these discoveries was HAT-P-11b, discovered in 2017. Thanks to its unusually clear skies, astronomers were able to identify water vapor in the atmosphere of this planet.
Super-Earths
Super-Earths differ greatly from any of the planets in our solar system. They’re not necessarily anything like Earth, despite the name. The term Super-Earth is just a reference to their size, bigger than Earth but smaller than Neptune. They are lighter than the ice giants of Neptune and Uranus, but at least twice as massive as Earth.
We have confirmed the discovery of 1577 Super-Earths as of 2022 – some of the most common planets we’ve found in our galaxy. Because we have nothing to compare them to in our own solar system, we don’t know much about them so far.
They could have a wide range of different compositions. Watery worlds, icy worlds, rocky worlds, worlds made of dense gas, or some combination of these – all are possible. At the top end of the scale for Super-Earths, we reach Mini-Neptunes, often grouped with Neptunian exoplanets.
Terrestrial planets
The familiar terrestrial planets of our solar system are Mercury, Venus, Earth, and Mars. Also known as rocky worlds, they are mostly made from some combination of rock, silicate, water, and carbon. Exoplanets from half the size of the Earth to twice as big as the Earth are considered terrestrial.
If more than twice the size of the Earth, a planet is considered a Super-Earth, though it may still be rocky. Planets between 1.5 and 2 times the size of the Earth are rare. The Fulton gap, as this gap in planet sizes is known, might be due to the way planets form. Studying it may help us learn more about this process.
We have confirmed the discovery of 188 terrestrial exoplanets as of 2022, but it’s estimated that between 2 and 12% of the stars in the sky could have rocky planets in their habitable zones. The TRAPPIST-1 system, discovered in 2017, contains the most Earth-sized worlds we’ve seen in the habitable zone of a star. This system has 7 rocky worlds – any of which could potentially have liquid water and the exciting possibility of life.
