Posted by Until 1992, nobody could prove that a single planet existed outside the Solar System. Today the count of confirmed planets in outer space has passed 6,000, and statistical surveys suggest that, on average, every star in the Milky Way has at least one. That is a few hundred billion planets in our galaxy alone, most of them orbiting faint red dwarfs no one will ever name. The question has changed from “are there other worlds?” to “what are they actually like?”, and the second question is a lot harder.
The first ones were the wrong kind
The earliest confirmed exoplanets were found in 1992 by Aleksander Wolszczan and Dale Frail, orbiting a pulsar, the spinning corpse of an exploded star. Not the sort of place anyone was looking for. The first planet around an ordinary Sun-like star came in 1995, when Michel Mayor and Didier Queloz at the University of Geneva announced 51 Pegasi b, a gas giant at least half the mass of Jupiter whipping around its star every 4.2 days. Nobody had expected a giant planet that close to anything. The discovery earned them a share of the 2019 Nobel Prize in Physics and introduced a new category, the hot Jupiter, that theory had not predicted.
That pattern has repeated ever since. Almost every new survey has turned up something the textbooks did not allow for.
Shadows and wobbles
Most planets in outer space are never seen directly. They are too faint and too close to stars billions of times brighter. Astronomers detect them through their effects instead.
The radial velocity method, used for 51 Pegasi b, watches the star sway back and forth as the planet tugs on it, shifting the star’s light slightly toward blue and then toward red. Modern spectrographs such as HARPS in Chile and ESPRESSO on the Very Large Telescope can detect stellar motions of around a metre per second or better, roughly walking pace. That gives a planet’s minimum mass.
The transit method watches for the tiny dip in brightness when a planet crosses the face of its star. NASA’s Kepler telescope stared at one patch of sky in Cygnus and Lyra from 2009 to 2013, monitoring roughly 150,000 stars, and its data have yielded thousands of confirmed planets. TESS, launched in 2018, does a similar job across almost the entire sky, focusing on brighter, closer stars that are easier to follow up. A transit gives the planet’s size. Combine size from a transit with mass from radial velocity and you get density, which tells you whether a world is mostly rock, mostly gas or something in between.
There are other tricks. Gravitational microlensing catches planets when a foreground star briefly magnifies a background one, and it is especially good at finding cold planets far from their stars. Direct imaging, which blocks out the starlight, photographed the four giant planets of HR 8799 in 2008. Astrometry, measuring a star’s tiny side-to-side motion on the sky, is getting a boost from ESA’s Gaia mission.
Worlds with no local equivalent
The single most common kind of planet Kepler found is one our Solar System does not have at all: something between Earth and Neptune in size. Astronomers split these into super-Earths, which are probably rocky, and mini-Neptunes, which likely wrap a core in a thick envelope of hydrogen, helium or water. In 2017 a careful look at Kepler’s planets revealed a gap in the size distribution around 1.5 to 2 times Earth’s radius. Few planets sit there. One explanation is that some smaller planets were born with gas envelopes and lost them to their stars’ radiation, leaving bare rocky cores behind.
Then there are the oddballs. Lava worlds so close to their stars that their dayside is likely molten. Planets on orbits more eccentric than any comet’s. Planets with no star at all, drifting through the galaxy alone, found through microlensing.
Each of these says something about how planetary systems form and rearrange themselves. Giant planets clearly migrate, sometimes a long way, and our own system’s tidy layout may be more unusual than it looks.
The habitable zone is a starting point, not a verdict
The habitable zone is the range of distances from a star where a planet with a suitable atmosphere could keep liquid water on its surface. It is a useful filter. It is easy to over-read.
Proxima Centauri b, announced in 2016, sits in the habitable zone of the nearest star to the Sun, 4.2 light-years away. That sounds promising until you remember that Proxima is a flaring red dwarf that blasts its planets with X-rays and ultraviolet light, and that a planet that close may be tidally locked, with one side in permanent day. Whether it has kept any atmosphere at all is unknown. Being in the zone tells you where to look. It does not tell you what you will find.
Estimates of how many Sun-like stars host a rocky planet in the habitable zone have ranged from under a tenth to around half, depending on how researchers handle the gaps in Kepler’s data. The honest summary is that such planets are common but not yet precisely counted.
Reading the air of a distant world
Detection is the easy part now. Characterisation is the frontier.
When a planet transits, a sliver of starlight passes through its atmosphere, and gases in that atmosphere absorb specific wavelengths. The James Webb Space Telescope, operating since 2022, has turned this into routine work for giant planets, detecting water, carbon dioxide, sulphur dioxide and other molecules. For small rocky planets the job is much harder. Webb’s measurements of the innermost TRAPPIST-1 planets suggest they lack thick atmospheres, which is disappointing but also informative, since it hints that rocky worlds around active red dwarfs may struggle to hang on to their air.
The most publicised case is K2-18b, a planet about eight times Earth’s mass. Webb found methane and carbon dioxide there, and one team has argued for traces of dimethyl sulphide, a gas produced on Earth mostly by marine life. Other researchers reanalysing the data have found the signal weak or not statistically convincing, and it is not even settled whether K2-18b has an ocean at all. It is a good illustration of how a single spectral bump becomes a headline long before it becomes a result.
Where this goes next
The next generation of instruments is designed to push from describing planets toward comparing them. NASA’s Nancy Grace Roman Space Telescope is set to run a large microlensing survey. ESA’s PLATO mission is built to find Earth-sized planets around Sun-like stars, the category Kepler struggled with. On the ground, the Extremely Large Telescope in Chile, with a 39-metre mirror, should be able to probe the atmospheres of some nearby rocky planets directly. Further off, NASA’s proposed Habitable Worlds Observatory aims to image Earth-like planets around nearby stars and look for oxygen, methane and water in the same atmosphere.
None of these will answer the big question on their own. They will narrow it.
There is something almost dizzying about the speed of all this. Within a single human lifetime, planets in outer space went from a reasonable guess to a catalogue too large for anyone to memorise. If you want to keep up with what the telescopes find next, and what it might mean for life elsewhere, SETIworld is a good place to keep reading.
