30 years of Discovering Exoplanets
An accretion disk around a star, the material from which will go into forming exoplanets that orbit around it. Credit: NASA/JPL-Caltech

30 years of Discovering Exoplanets

20/01/2022Written by Dhara Patel

From one ‘Poltergeist’ to plenty more: Celebrating 30 years since the discovery of the first exoplanet.

Book online now and upgrade to a free annual pass

Book
mascot Telescope Right
Artist's impression of the pulsar Lich and its surrounding exoplanets. Credit: NASA/JPL-Caltech

In 1992, two radio astronomers detected the first ever extrasolar planet (or exoplanet for short). The search for such planets orbiting a star outside our Solar System was considered fringe science even 50 years ago, but in the 30 years since the first definitive detection, exoplanet research has become more mainstream with the hunt for other Earth-like planets and possible alien life.

The First Exoplanet Discovery

The First Exoplanet Discovery
1917 spectral evidence – two gaps in the thin line indicate the presence of calcium in the star which is unexpected for the particular star in question, a white dwarf. Credit: Carnegie Institution for Science
The First Exoplanet Discovery
Radiation beams emitted from a pulsar. Credit: B. Saxton, NRAO/AUI/NSF

Unbeknownst at the time, the first pieces of evidence for the existence of a planet orbiting a star outside our Solar System were recorded in 1917. The split-out light (spectrum) from a star hinted at the remains of an exoplanet that once existed around it.

Despite that, it took until 1992 for the first exoplanets to be discovered. They were aptly named “Poltergeist” a supernatural being and “Phobetor” a personification of dreaming, as it was an unlikely and startling find!

These two exoplanets were detected orbiting around a ‘dead’ star known as a neutron star or pulsar – the remnant of a massive star that has gone supernova. Neutron stars spin and emit beams of radiation (radio waves) and if Earth lies in the path of the beam, the star will appear to pulse as it spins, just like a lighthouse. These pulses are one of the best timekeepers going as they happen at very regular intervals.

But two radio astronomers Aleksander Wolszczan and Dale Frail noticed that this was not the case for a pulsar named Lich (meaning an undead wizard in Greek folktale). Every so often the pulse would be off beat and each time by the same amount – the result of exoplanets orbiting around it. Although it was the first definitive detection of an exoplanet (two in this case), it wasn’t until 1995 that the first exoplanet orbiting around a Sun-like star was detected.

Exoplanet Detection Methods

Exoplanet Detection Methods
Exoplanet detection methods. Credit: NASA/N. Batalha (NASA Ames)

The pulsar timing method used to detect the first exoplanets isn’t actually a popular method at all. Several techniques have been used to find worlds outside our Solar System, but some of the most common ones are:

Transit Method – when a planet crosses the face of a distant star from our viewpoint on Earth, the starlight received appears to dip. If a regular dip is seen in the detected starlight, it can indicate the presence of an exoplanet orbiting around the star.

Radial Velocity Method – contrary to what you might think, not only does the Earth orbit the Sun, but the Sun also does a tiny orbit too – both actually orbit around their common centre of mass, which means the Sun has a little wobble. Similarly, if an exoplanet is orbiting a distant star, the star will have a wobble, and this can be detected by examining the star’s light. As the star move slightly away in our line of sight, its light becomes slightly stretched out and redder and as it moves slightly towards us, the light becomes slightly compressed and bluer. This periodic movement and shifting of its light as the star wobbles back and forth, can reveal the existence of an exoplanet.

Instead of monitoring the towards and away motion, the Astrometry method measures the position of the star on the sky as it moves side to side due to it wobbling or orbiting around the centre of mass of the planetary system it lies within.

Gravitational Lensing – anything with mass has gravity so pulls things towards it. The more massive an object is, the greater its gravitational pull. So, a massive object like a galaxy can cause the light of a distant star hidden behind it to be bent around it due to its immense gravitational pull, and this is known as gravitational lensing. The star therefore appears brighter, and if an exoplanet is orbiting the star too, it has a tiny gravitational lensing effect causing an extra deviation or blip in the brightness.

Direct Imaging – because stars are so bright, an exoplanet can easily be outshined by its star, especially if it lies close to it, and that makes directly imaging an exoplanet very difficult. Rather than using visible light, astronomers often look for the light reflected off a planet in the infrared instead. At infrared wavelengths of light, it becomes a little easier to pick out exoplanets orbiting around their stars.

However, with all the methods above, the differences or deviations that scientists are trying to identify are so incredibly small and therefore difficult to detect – exoplanet hunting is an extremely sensitive science!

How Many Exoplanets Have Been Found?

How Many Exoplanets Have Been Found?
Kepler Space Telescope. Credit: NASA
How Many Exoplanets Have Been Found?
Artist's impression of a potentially habitable exoplanet. Credit: NASA
How Many Exoplanets Have Been Found?
Artist's impression of a super-Earth orbiting a small star. Credit: ESO/L. Calcada

As of January 2022, there are over 4,800 confirmed exoplanets, with their existence having been confirmed using at least two different methods. On top of that, NASA has over 8,400 other candidate exoplanets awaiting confirmation.

The hunt for exoplanets started off rather slowly. Between 1992 and 2000, just over 30 exoplanets were discovered. And by 2009, that number only increased to roughly 400. But in 2009, NASA’s Kepler Space Telescope launched. It was designed specifically to detect exoplanets in one small patch of the sky using the transit method and data collected in its 9 years of operation has confirmed the existence of around 2,400 of the exoplanets that we know of today.

Only a small amount of the exoplanets that have been detected are Earth-like though – by that we mean they are rocky planets somewhere between half and one and a half times the size of the Earth, which lie in the habitable zone of their star. In this goldilocks region they are at the right orbiting distance for liquid water to potentially exist. We know that Earth-like life needs liquid water, so it’s a good indicator of habitability.

There are many other types of exoplanets we’ve discovered which include Hot Jupiters (large gas planets close to their stars), Mini-Neptunes (planets resembling Neptune with thick hydrogen-helium atmospheres but less massive), and Super-Earths (planets that are twice to ten times the mass of Earth).

You can keep track of how many exoplanets have been discovered using NASA’s exoplanet exploration website.

What We Know About Exoplanets

What We Know About Exoplanets
The habitable zone of the Sun. Credit: ESO.M. Kornmesser
What We Know About Exoplanets
Studying a star’s light as it passes through an exoplanet’s atmosphere can help determine the molecules present in the atmosphere. Credit: NASA/ESA/Z. Levy (STScI)
What We Know About Exoplanets
Size comparison of the Hubble Space Telescope and the James Webb Space Telescope. Credit: NASA

Different exoplanet detection methods combined with known properties about the star they orbit can help determine things like the mass of an exoplanet, its orbiting distance, its size and density. These can give us clues about the characteristics of an exoplanet including its temperature and habitability. But a planet’s atmospheric composition can affect that. For example, Venus and Mars both lie along the inner and outer edges of our Sun’s goldilocks zone respectively. But Venus has a thick atmosphere rich in carbon dioxide which has turned it into an inferno world with surface temperatures of 460 degrees Celsius, whereas Mars was stripped of its atmosphere and has been unable to retain heat leaving it at average temperatures of -60 degrees Celsius – neither of which would allow water to exist as a liquid.

When passing in front of its star, an exoplanet’s atmosphere will absorb a very small amount of the star’s light. Exactly what light it absorbs depends on the chemical makeup of its atmosphere. So, starlight that has travelled through an exoplanet’s atmosphere can provide a ‘fingerprint’ or ‘barcode’. By comparing the starlight that has passed through the exoplanet’s atmosphere with the starlight that hasn’t, it’s possible to work out the composition of an exoplanet’s atmosphere.

The Earth’s atmosphere contains many of the same signatures we’re looking for in exoplanet atmospheres like water (H2O), carbon monoxide (CO), carbon dioxide (CO2) and Methane (CH4), so it has the potential to be mixed up in the results. This means that exoplanet atmospheres are best studied by space telescopes. But even telescopes like Hubble are pushed to their limits since it only has the tiny fraction of starlight that passes through an exoplanet’s atmosphere to work with. But since telescopes like the newly launched James Webb Space Telescope will have a larger mirror with greater light collecting power and better sensitivity, it’ll be able to make more headway with characterising exoplanet atmospheres and to help identify habitable, Earth-like exoplanet candidates.

Conclusion

Conclusion
Image Credit: ESA

We’ve only been searching for exoplanets with intent for 30 years and have found thousands within our own galaxy already. And with an estimated 200 billion stars in our galaxy, there’s likely to be billions of exoplanets to discover in the Milky Way itself, let alone the rest of the Universe. With roughly 25% of stars in our galaxy being Sun-like stars and around one in five of them likely to have an Earth-like exoplanet in the habitable zone, there could be at least 10 billion waiting to be discovered. And who knows, we might even find life on them!

About the author: Dhara Patel is a Space Expert at the National Space Centre.