{"id":54156,"date":"2024-01-19T08:39:28","date_gmt":"2024-01-19T08:39:28","guid":{"rendered":"http:\/\/90da871c-e59e-4aa3-9e22-a9124f24e58b"},"modified":"2024-01-19T09:32:36","modified_gmt":"2024-01-19T09:32:36","slug":"what-is-an-exoplanet","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/what-is-an-exoplanet\/","title":{"rendered":"What is an exoplanet?"},"content":{"rendered":"

Discover the science behind planets orbiting distant stars beyond the Solar System with our guide to exoplanets, and how astronomers detect them. <\/p>

By Paul Cockburn\n <\/p>

Published: Friday, 19 January 2024 at 08:39 AM<\/p>

\n\n

Exoplanets are planets orbiting distant stars beyond our Solar System, and since the first confirmed discoveries in the 1990s, astronomers have found all manner of weird and wonderful worlds.<\/p>

The list of exoplanets keeps on growing and astronomers calculate that for every star we see in the night sky, there is on average at least one exoplanet in orbit around it.<\/p>

This suggests that the possibility of discovering life beyond Earth could be greater than we ever imagined.<\/p>

Could we ever find life beyond Earth?<\/a> What make a planet habitable?<\/a><\/em><\/strong><\/p>

An illustration depicting the variety of exoplanets discovered so far. Credit: NASA\/JPL-Caltech\/Lizbeth B. De La Torre<\/figcaption><\/figure>

The concept of exoplanets is hardly new: as far back as 1584, Italian philosopher Giordano Bruno suggested space was filled by “an infinity of worlds of the same kind as our own”.<\/p>

Over the past century, numerous authors, TV directors and film producers set their stories on planets beyond our own Solar System.<\/p>

But in the summer of 1991, it seemed that exoplanets were finally about to switch from science fiction to science fact.<\/p>

\"An
An artist’s impression of Earth-like exoplanets in orbit around a star. Credit: NASA\/JPL-Caltech<\/figcaption><\/figure>

How the first exoplanet was discovered<\/strong><\/h2>

Observations carried out at Jodrell Bank Observatory in Cheshire had apparently detected a planet orbiting a pulsar<\/a>, the dense remnant of an exploded star.<\/p>

Pulsars are so called because, thanks to their rapid rotation, the beams of light they emit sweep across Earth at regular intervals.<\/p>

From our perspective, pulsars appear to flicker like a lighthouse beam, and the rate at which the pulses are seen from Earth implies the pulsar\u2019s spin rate.<\/p>

\"Artist's
Artist’s impression of a pulsar. Credit: NASA<\/figcaption><\/figure>

Unfortunately, the team at Jodrell Bank soon discovered a systematic error in the software used to analyse the pulsar data.<\/p>

Once corrected, the detectable \u2018drag\u2019 on the pulsar\u2019s spin, thought to have been due to an unseen planet\u2019s mass, vanished.<\/p>

Yet immediately after Jodrell Bank\u2019s Andrew Lyne had officially retracted his team\u2019s findings at the January 1992 meeting of the American Astronomical Society, held in Atlanta, he was followed on stage by Aleksander Wolszczan, principal author of a paper that detailed his own detection of at least two planets around another pulsar.<\/p>

This was to be the first confirmed discovery of an exoplanet.<\/p>

\"Astronomers
Astronomers Dale Frail (left) and Aleksander Wolszczan (right), discoverers of the first exoplanet.<\/figcaption><\/figure>

Wolszczan was particularly interested in \u2018millisecond pulsars\u2019, which spin hundreds of times a second.<\/p>

He had found it difficult to devise a sufficiently accurate mathematical model to explain how one particular millisecond pulsar, designated PSR B1257 12, was behaving.<\/p>

“Every time I came up with a model and then took some more data, the model failed to predict the pulse arrival times,” he says.<\/p>

\"An
An artist\u2019s impression of a habitable exoplanet orbiting a pulsar. Image Credit: Institute of Astronomy, University of CambridgeInstitute of Astronomy, University of Cambridge<\/figcaption><\/figure>

Following a concentrated observation period of almost a month, Wolszczan began to see a regular glitch in the rate that the pulses reached Earth.<\/p>

Many pulsars have companion dwarf stars providing material and energy, but Wolszczan realised that the observed peculiarities in these pulses were best explained by the existence of “two planetary mass objects” around the star.<\/p>

Unlike the Jodrell Bank team, Wolszczan knew that his result wasn\u2019t simply down to \u201csomething in the data analysis software\u201d, as he had previously observed another binary pulsar that had not shown the same irregularities.<\/p>

Together with fellow astronomer Dale Frail, Wolszczan\u2019s findings were independently confirmed several months later.<\/p>

Then in 1995 an exoplanet was detected for the first time around a Sun-like star \u2013 51 Pegasi \u2013 by Michel Mayor and Didier Queloz<\/a>. This \u2018hot Jupiter\u2019 \u2013 a massive gas giant in close orbit around its parent star \u2013 also came as a complete surprise.<\/p>

\"An
An artist’s impression of a hot Jupiter;: a gas giant similar to Jupiter but orbiting much closer to its host star. Credit: NASA\/Ames\/JPL-Caltech<\/figcaption><\/figure>

What exoplanets reveal about our own Solar System<\/strong><\/h2>

By finding different types of exoplanets, astronomers can learn much more about our own Solar System.<\/p>

We know that \u2018hot Jupiters\u2019, for example, can\u2019t have formed so close to their stars<\/a>, so they must have formed farther out and moved in.<\/p>

This suggests planets can move from their original formation sites and there’s the suggestion that in the early days of our own Solar System, there was a lot of movement: Jupiter and Saturn moving in, Neptune and Uranus perhaps being pushed out and even swapping places.<\/p>

\"Might
Might the planets of our own Solar System have moved around and swapped positions in the early days? Credit: MARK GARLICK\/SCIENCE PHOTO LIBRARY<\/figcaption><\/figure>

It\u2019s even proposed that this gas giant ballet is likely to have caused the Late Heavy Bombardment, a period (about half a million years after the formation of the Solar System<\/a>) during which huge amounts of material were fired towards the inner planets.<\/p>

There are also numerous types of exoplanet, like large ‘super Earths’, that don’t exist in our Solar System. In fact, the more we learn about other star systems, the more unusual our own Solar System appears.<\/p>

Find out more about exoplanetary extremities in our guide to the weirdest exoplanets<\/a>.<\/strong><\/em><\/p>

\"Artist's
Artist’s concept of a Jupiter-mass exoplanet orbiting the nearby star Epsilon Eridani. Credit: NASA, ESA, and G. Bacon (STScI)<\/figcaption><\/figure>

How many exoplanets are there?<\/strong><\/h2>

Over 5,000 exoplanets<\/a> have been discovered and confirmed to date, but the number keeps rising.<\/p>

The scale of this ever-growing list means astronomers can now replace some of the terms in the Drake Equation<\/a>, which famously attempts to calculate the number of active, communicative extraterrestrial civilisations in the Universe.<\/p>

If an Earth-like planet in an Earth-like orbit is the most likely place to find complex extraterrestrial life, we can be quite confident that those kinds of environments do actually exist in quite large numbers throughout the Galaxy<\/a>.<\/p>

But is that enough for life to exist? Will we ever find life beyond Earth<\/a>?<\/p>

\"A
A direct image of 2 exoplanets orbiting a Sun-like star. The planets are TYC 8998-760-1 b and c, and can be seen middle and lower right. Credit: ESO\/Bohn et al.<\/figcaption><\/figure>

Exoplanet extremes<\/strong><\/h2>

What are the closest, largest, weirdest exoplanets yet discovered?<\/p>

Closest exoplanet: Proxima b<\/strong><\/b><\/h3>
\"Closest
Credit: ESO<\/figcaption><\/figure>

In August 2016 astronomers announced that our nearest stellar neighbour, the red dwarf Proxima Centauri<\/a> (just 4.25 lightyears distant) is circled every 11 days by a possibly rocky world capable of possessing liquid water, slightly bigger than Earth.<\/p>

Farthest exoplanet: SWEEPS-11<\/strong><\/h3>
\"Farthest
Credit: NASA<\/figcaption><\/figure>

Detected by the Hubble Space Telescope<\/a> along with SWEEPS-4 in 2006, this giant gas planet has a radius 1.13 times that of Jupiter and is 27,710 lightyears away, making it one of the farthest exoplanets discovered.<\/p>

Smallest exoplanet: Kepler 37 b<\/strong><\/h3>
\"Smallest
Credit: NASA Ames\/JPL-Caltech\/T. Pyle<\/figcaption><\/figure>

Kepler 37 b is the smallest known exoplanet that orbits a Sun-like star. It orbits a main sequence star in Lyra, though the estimated mass of PSR B1257 12 b (aka Draugr) could trump it.<\/p>

Widest exoplanet: HAT-P-6b<\/strong><\/h3>
\"Widest
Credit: Livia Pietrow.<\/figcaption><\/figure>

HAT-P-67 b is a gas giant exoplanet whose mass is 0.34 Jupiters, but its diameter is over 2 times that of Jupiter. It takes nearly 5 Earth days to orbit its star, but it is incredibly close to the star: just 0.06 AU (where 1 AU is the average distance between Earth and the Sun).<\/p>

How astronomers find exoplanets<\/strong><\/h2>

There are a few techniques available to astronomers looking for planets orbiting distant stars:<\/p>

Radial velocity<\/strong><\/h3>
\"Widest
Radial velocity looks for a shift in the spectrum of light as a star wobbles due to the gravitational pull of an exoplanet in orbit around it. Credit: ESA<\/figcaption><\/figure>

The radial velocity method of exoplanet detection involves observing the host star being orbited by the exoplanet for signs of wobbling. This wobble is caused by the presence of a planet gravitationally tugging on the star, causing the star’s position to change ever so slightly.<\/p>

Astronomers use a spectrometer to measure how much the star’s lightwaves are shifted due to the wobble, as observed from Earth.<\/p>

Transit method<\/strong><\/h3>
\"Transit
Transit photometry reveals exoplanets by observing periodic dimming of the star’s light.<\/figcaption><\/figure>

You may have heard of astronomers on Earth observing a transit of Mercury<\/a> or transit of Venus<\/a>, whereby it’s possible to see the silhouette of either of the two inner planets passing across the face of the Sun. The transit method of exoplanet detection is much the same. It involves studying a ‘light curve’ of the host star, and dips in light indicate the potential presence of an orbiting exoplanet.<\/p>

Transits can be used to detect exoplanets, but can also be used to learn about the exoplanet’s size and how long it takes to orbit the star.<\/p>

Studying starlight that has passed through the exoplanet’s atmosphere on its way to Earth can reveal what sort of world it might be, for example what sort of elements are present in its atmosphere.<\/p>

Direct imaging<\/strong><\/h3>
\"Animation
Animation of 4 exoplanets in orbit around star HR 8799. Credit: Jason Wang (Caltech)\/Christian Marois (NRC Herzberg)<\/figcaption><\/figure>

It is possible to directly capture images of exoplanets orbiting a star, which is done by detecting light reflected off the exoplanet’s atmosphere in infrared.<\/p>

One of the issues with this technique is that the stars are much brighter than the exoplanets, and can completely drown them out.<\/p>

Astronomers get round this by direct imaging exoplanets that are particularly massive and have wide orbits far from their star, or by blocking out light from the host star in order to better see the exoplanets in orbit.<\/p>

This latter method was used in a study that directly imaged 4 exoplanets in orbit around young star HR 8799 (see animation above), and was announced in 2019.<\/p>

For more on the direct imaging method, listen to our interview with astronomer Beth Biller below:<\/p>