# Astronomy question **URL:** \ **Category:** Factual Questions\ **Created:** [July 26, 2015, 10:44pm UTC](https://boards.straightdope.com/t/astronomy-question/726270 "2015-07-26T22:44:02Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![Bijou\_Drains](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bijou_drains/32/2814_2.png) [@Bijou\_Drains](https://boards.straightdope.com/u/Bijou_Drains)\ **Post date:** [July 26, 2015, 10:44pm UTC](https://boards.straightdope.com/t/astronomy-question/726270/1 "2015-07-26T22:44:02Z")
Planets have been found around stars that are very far away. Based on the star dimming. That makes sense. But I wonder how they figure out the planet size ? That would seem to be very hard. Also the same problem would be true on the distance from the star to the planet. If the planet is 1400 light years away, the distance from star to planet is some very,very small percent of that . ---
**Author:** ![ITR\_champion](https://avatars.discourse-cdn.com/v4/letter/i/c67d28/32.png) [@ITR\_champion](https://boards.straightdope.com/u/ITR_champion)\ **Post date:** [July 26, 2015, 10:51pm UTC](https://boards.straightdope.com/t/astronomy-question/726270/2 "2015-07-26T22:51:10Z")
> [@Bijou\_Drains](#): > > Planets have been found around stars that are very far away. Based on the star dimming. That makes sense. > > But I wonder how they figure out the planet size ? That would seem to be very hard. Also the same problem would be true on the distance from the star to the planet. If the planet is 1400 light years away, the distance from star to planet is some very,very small percent of that . The first planets to be discovered outside the Solar System were detected because of the star’s ‘wobble’. In other words, the star gets pulled around, a very small amount, as the planet orbits around it. By observing the period of the wobble, scientists know the period of the planet’s orbit; from that, they can figure out the planet’s distance from the star. Based on the magnitude of the wobble, they can figure out the planet’s mass. ---
**Author:** ![Bijou\_Drains](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bijou_drains/32/2814_2.png) [@Bijou\_Drains](https://boards.straightdope.com/u/Bijou_Drains)\ **Post date:** [July 26, 2015, 11:00pm UTC](https://boards.straightdope.com/t/astronomy-question/726270/3 "2015-07-26T23:00:11Z")
and the wobble is measured by what technique? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [July 26, 2015, 11:11pm UTC](https://boards.straightdope.com/t/astronomy-question/726270/4 "2015-07-26T23:11:02Z")
A wobbleometer. (OK, seriously: [Kepler space telescope - Wikipedia](https://en.wikipedia.org/wiki/Kepler_(spacecraft)#Planet_finding_process)) ---
**Author:** ![wolfstu](https://avatars.discourse-cdn.com/v4/letter/w/d6d6ee/32.png) [@wolfstu](https://boards.straightdope.com/u/wolfstu)\ **Post date:** [July 26, 2015, 11:13pm UTC](https://boards.straightdope.com/t/astronomy-question/726270/5 "2015-07-26T23:13:09Z")
By the “Radial velocity method”, in which small doppler shifts in the star’s light spectrum indicate that it’s moving alternately towards or away from us in proportion to the magnitude of a planet’s tug on it. The planet’s pull on the star depends on its distance from the star, and its mass. > **[Doppler spectroscopy](https://en.wikipedia.org/wiki/Doppler_spectroscopy)** > > Doppler spectroscopy (also known as the radial-velocity method, or colloquially, the wobble method) is an indirect method for finding extrasolar planets and brown dwarfs from radial-velocity measurements via observation of Doppler shifts in the spectrum of the planet's parent star. > As of November 2022, about 19.5% of known extrasolar planets (1,018 of the total) have been discovered using Doppler spectroscopy. > Otto Struve proposed in 1952 the use of powerful spectrographs to detect distant plane... > **[5 Ways to Find a Planet | Explore – Exoplanet Exploration: Planets Beyond our...](https://exoplanets.nasa.gov/alien-worlds/ways-to-find-a-planet/)** > > NASA’s Exoplanet Exploration Program, the search for planets and life beyond our solar system. ---
**Author:** ![MEBuckner](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mebuckner/32/2896_2.png) [@MEBuckner](https://boards.straightdope.com/u/MEBuckner)\ **Post date:** [July 27, 2015, 12:14am UTC](https://boards.straightdope.com/t/astronomy-question/726270/6 "2015-07-27T00:14:45Z")
> [@Bijou\_Drains](#): > > But I wonder how they figure out the planet size ? They can actually [deduce the radius of the planet by measuring how much the parent star’s brightness decreases during the transit](https://www.paulanthonywilson.com/exoplanets/exoplanet-detection-techniques/the-exoplanet-transit-method/). ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [July 27, 2015, 7:12am UTC](https://boards.straightdope.com/t/astronomy-question/726270/7 "2015-07-27T07:12:23Z")
> [@ITR\_champion](#): > > The first planets to be discovered outside the Solar System were detected because of the star’s ‘wobble’. In other words, the star gets pulled around, a very small amount, as the planet orbits around it. By observing the period of the wobble, scientists know the period of the planet’s orbit; from that, they can figure out the planet’s distance from the star. Based on the magnitude of the wobble, they can figure out the planet’s mass. Wouldn’t that get immensely complicated at best and totally confounded at worst, if the star has multiple planets whizzing around it, which may all be on various sides of the star at any time? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 27, 2015, 12:09pm UTC](https://boards.straightdope.com/t/astronomy-question/726270/8 "2015-07-27T12:09:06Z")
As for the distance from the star: They don’t consider it a proper detection via the occultation method until they’ve observed it happen two or three times (which, yes, does take years). This means that they can determine the period, and given that and the star’s mass (which can be inferred from its spectral type), it’s easy to calculate the average orbital period. As a double-check, you can also infer the star’s size from its spectral type, and given that, the orbital distance and speed, and the inclination (which you can get from the shape of the dip in brightness), you can calculate how long the occultation should last. Compare that with the duration you actually measure. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 27, 2015, 12:10pm UTC](https://boards.straightdope.com/t/astronomy-question/726270/9 "2015-07-27T12:10:33Z")
> [@](#): > > Quoth **Senegoid** : > > Wouldn’t that get immensely complicated at best and totally confounded at worst, if the star has multiple planets whizzing around it, which may all be on various sides of the star at any time? Not really. A simple Fourier transform will separate out all of the information into neat little piles, one corresponding to each planet. Some of them will have a larger spike than others, and those will be the ones detected first, but with patience and good data, you can find the others. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [July 28, 2015, 4:59am UTC](https://boards.straightdope.com/t/astronomy-question/726270/10 "2015-07-28T04:59:41Z")
The very first extra-solar planets were actually discovered around a type of star no one expected to have planets: a [neutron star](https://en.wikipedia.org/wiki/PSR_1257%2B12). It was also a pulsar and the planets caused irregularities in the timing of its pulses. However, such pulsar planets are very rare and the next planets were discovered by the wobble method (which method is not what Kepler uses). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 28, 2015, 11:09am UTC](https://boards.straightdope.com/t/astronomy-question/726270/11 "2015-07-28T11:09:41Z")
Well, the pulsar planets were still detected by the wobble; it’s just that the wobble was detected in a different way than with most stars. ---
**Author:** ![Kinthalis](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kinthalis/32/16084_2.png) [@Kinthalis](https://boards.straightdope.com/u/Kinthalis)\ **Post date:** [July 28, 2015, 1:36pm UTC](https://boards.straightdope.com/t/astronomy-question/726270/12 "2015-07-28T13:36:12Z")
> [@ITR\_champion](#): > > The first planets to be discovered outside the Solar System were detected because of the star’s ‘wobble’. In other words, the star gets pulled around, a very small amount, as the planet orbits around it. By observing the period of the wobble, scientists know the period of the planet’s orbit; from that, they can figure out the planet’s distance from the star. Based on the magnitude of the wobble, they can figure out the planet’s mass. I never understood this… I can see clearly how this would work on a solar system with 1 planet, but I think most have more than 1. How could you determine anything about say, earth, by observing the wobble of our sun? There are a ton of more planets pulling and tugging at our star. I swear my browser only rendered the first post when I replied to this. Weird. So a Fourier transform. Amazing, that function is. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 28, 2015, 11:52pm UTC](https://boards.straightdope.com/t/astronomy-question/726270/13 "2015-07-28T23:52:45Z")
For what it’s worth, our instruments would not be able to detect Earth (or rather, a planet of the same size and distance) around any other star (except possibly a pulsar) via the wobble method. For that matter, we’d have an extremely hard time even finding Jupiter that way. Most of the planets found via wobble are “hot Jupiters”, gas giants closer to their star than Mercury is to ours. We can detect Earthlike planets via occultation, but only if we’re lucky (the plane of the orbit has to be nearly exactly edge-on to us). We make that luck by the simple method of checking a great many stars.