• Question: Hi everyone, How do scientists find exoplanets and how are they photographed?

    Asked by anon-244767 on 30 Apr 2020.
    • Photo: Susan Cartwright

      Susan Cartwright answered on 30 Apr 2020:

      This is an excellent question, thank you!

      The first thing to say is that very few exoplanets can be photographed. The problem is that the star is very close to the planets and very much brighter, so in most cases it would be like trying to photograph a firefly next to a searchlight. The only exceptions are large planets a long way from their stars – and then only when those planets are rather young and therefore still hot from their formation, which means that they glow brightly in the infra-red waveband.

      Most exoplanets are discovered by one of two methodsL transits and radial velocity.

      If the exoplanetary system is viewed very nearly edge-on, then once per rotation period each planet will pass between the star and our solar system. This will dim the star’s light very slightly (Jupiter would dim the Sun’s light by 1%, the Earth by only 0.01%) which can be detected with very careful monitoring of the star. The Kepler spacecraft has been the most productive in terms of detecting transiting exoplanets. Because planets transit only once per orbit, this is most effective at detecting planets close to their stars.

      The other common method is radial velocity. Planets don’t really orbit their stars: in fact, both planet and star orbit their common centre of mass. The ratio of orbital speeds is the inverse of the ratio of masses: Jupiter’s mass is 1/1000 of the mass of the Sun, and Jupiter’s orbital speed is 13 km/s, so the Sun’s orbital speed around the centre of mass of the Sun-Jupiter system is 13 m/s. This is very small, but can be picked up as a Doppler shift of the star’s spectral lines (the relative change in wavelength is given by v/c, where v is the speed towards or away from us, so 13 m/s would produce a wavelength shift of 13/300000000 – which is tiny, but just about detectable by secial equipment. This method is most sensitive to massive planets close to their stars, because massive planets move the centre of mass further from the star, and close-in orbits have higher orbital speeds.

      There are other methods that are responsible for the detection of relatively small numbers of planets, but these are the main two.

    • Photo: Roan Haggar

      Roan Haggar answered on 30 Apr 2020:

      There are loads of different techniques used to find exoplanets. Mostly these are indirect ways of detecting exoplanets, and so they use measurements we can make of the motion of exoplanets, rather than actually photographing them.
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      One of the most common methods is using ‘transits’. This involves looking at stars, and measuring their brightness over time. If the star has a planet orbiting around it, the planet can pass between the star and us during its orbit. This blocks out some of the light from the star, and so measuring a drop in the star’s brightness can tell us that a planet is orbiting around this star. This was the main method used by the Kepler telescope, which was amazingly successful and detected several thousand exoplanets!
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      When a planet orbits around a star, the gravity of the planet causes the star to wobble a little bit. Measuring this wobble can also allow us to detect exoplanets — we can either actually see the star moving in the sky, or we can detect changes in the frequency of light from it, caused by the Doppler effect as it moves towards and away from us.
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      Only a handful of exoplanets have actually been photographed, as they’re very faint, and so are much easier to find using one of these other methods. These photos usually just look like tiny dots of light. The detailed pictures of exoplanets that you see on the news are usually just drawings of what we think these planets look like, rather than real photos — unfortunately the only planets that are close enough for us to get detailed photos of are the ones in our solar system. Hope this helps!

    • Photo: anon

      anon answered on 30 Apr 2020:

      There are several ways. NASA has an excellent animation showing them, look it up!

      The easiest one is when the planet happens to pass in front of its star (called “transit”), then we can see the dimming in the light from the star. But for this to happen, the planetary orbits must align more or less perfectly with our line of sight – so that we see the planetary system “edge on”.

      With recent advances in technology, we can also see when planets tug at the star – this causes tiny wiggles in the light from the star, it gets ever so slightly redder as the planets pull the star away from us and ever so slightly bluer as the planets pull the star towards us. But this doesn’t work if we see the planet’s orbital plane from the top – because the star will move sideways, not towards/away from us!
      In such a case, however, we can still detect the wobbling of the star by comparing its position with respect to the neighbouring stars.

      Direct imaging has become possible very recently. If you ever saw videos of eruptions from our own Sun – when they block the sun with a circular plate to see the much fainter eruptions – that is exactly how it works. We block the star with a disk matching its size, and then we can pick up the very, very faint light from the planet around it.

      Another funky method is to look for gravitational lensing – the light gets deflected by the planet’s mass, and focuses it slightly (just like a glass lens) as the planet passes between us and the star.

      Each of the methods has its own limits and advantages, but generally the bigger the planet is, the easier it is to detect. All of them also require several consecutive observations to confirm periodicity of the planet’s orbital motion. And then we have to distinguish them from binary star systems, which will have very similar “symptoms”, except that the smaller star will also emit light.

    • Photo: Ry Cutter

      Ry Cutter answered on 30 Apr 2020:

      All of the above answers are brilliant! I’d like to add some other ways we know there are exoplanets, but without actually seeing them 😀
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      When a star turns into a white dwarf at the end of its lifecycle, it’s atmosphere turns into a blank canvas of hydrogen (and sometimes Helium). When a planet (or planetesimal) gets too close to the white dwarf, the planet gets ripped up. The material of the planet then lands on the white dwarf, “polluting” the hydrogen atmosphere. We can tell by what elements we see in the white dwarf’s atmosphere what elements the planet was made of. We have found evidence of tectonic plates and water from objects in other solar systems this way!
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      We have also seen gaps in planet forming discs! This is a way to tell where baby planets are. I’m not an expert in this field, but check out ALMA protoplanetary discs, these are high resolution images of young stellar systems making baby planets.
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      Another cool thing we’ve started doing is looking at exoplanet atmospheres. We do this by watching the way different wavelengths of light get refracted as they pass through the planet. Astronomers have been able to confirm diamond rain on some distant planets because of this!
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      A really technical method called astrotomography can also be used to find planetesimals in accretion discs. This cool technique uses a statistical combination of lots of observations to see inside the disc without taking a direct picture! It has only recently been used to find objects around stars, but this technique confirmed the first planetesimal around a white dwarf!
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      Great Question,

      Ry

    • Photo: Marios Kalomenopoulos

      Marios Kalomenopoulos answered on 30 Apr 2020:

      Just wanted to suggest this very nice page, which describes the main techniques using illustrations: https://exoplanets.nasa.gov/alien-worlds/ways-to-find-a-planet/#

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