<p>They are <em>probably</em> tidally locked, given some assumptions about age of the planetary system. </p>

<p>There's a <a href="https://en.wikipedia.org/wiki/Tidal_locking#Moons" rel="noreferrer">formula</a> to calculate how long it'll take for an orbiting object to tidally lock, and one of the outcomes of that formula is that large objects in close orbits don't take very long to tidally lock. </p>

<p>Since many of the exoplanets we've found are large objects in close orbits of old stars, tidal locking is a likely outcome. </p>

CC BY-SA 4.0
2019-02-05T19:16:48.577Z
2019-02-05T19:16:48.577Z
Hobbes
<p>Often in discussing exoplanets we hear that they are close to the parent star (usually a red dwarf) but then there's a caveat that the planet is tidally locked to that star and that that fact severely decreases prospects for that planet to harbor life.</p>

<p>But how do we know that an exoplanet is tidally locked to a star? I doubt that we can directly observe the rotation of that planet. Does that mean that we conclude that it's tidally locked because that's what our theoretical models tell us, and not that we observe it directly?</p>

CC BY-SA 4.0
2019-02-04T02:22:20.413Z
We don't know. We compare age of a system (if known) to the tidal spin-down time (which is technically also unknown, but people make assumptions. See https://astronomy.stackexchange.com/questions/20199/requirements-for-a-satellite-planet-to-be-tidally-locked-to-a-planet-star
AtmosphericPrisonEscape
<orbital-mechanics><exoplanet><rotation>
How do we know that an exoplanet is tidally locked to the parent star?
stackzebra
# How do we know that an exoplanet is tidally locked to the parent star?

(Asked by: username_0 on 2019-02-01)

Often in discussing exoplanets we hear that they are close to the parent star (usually a red dwarf) but then there's a caveat that the planet is tidally locked to that star and that that fact severely decreases prospects for that planet to harbor life.

But how do we know that an exoplanet is tidally locked to a star? I doubt that we can directly observe the rotation of that planet. Does that mean that we conclude that it's tidally locked because that's what our theoretical models tell us, and not that we observe it directly?

**Tags:** orbital-mechanics, exoplanet, rotation
> **Comments:**
> **username_1**: We don't know. We compare age of a system (if known) to the tidal spin-down time (which is technically also unknown, but people make assumptions. See https://astronomy.stackexchange.com/questions/20199/requirements-for-a-satellite-planet-to-be-tidally-locked-to-a-planet-star  

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## 1 answers

### Answer by username_2 on 2019-02-05. Score: 7 | Accepted answer

They are *probably* tidally locked, given some assumptions about age of the planetary system.

There's a [formula](https://en.wikipedia.org/wiki/Tidal_locking#Moons) to calculate how long it'll take for an orbiting object to tidally lock, and one of the outcomes of that formula is that large objects in close orbits don't take very long to tidally lock.

Since many of the exoplanets we've found are large objects in close orbits of old stars, tidal locking is a likely outcome.


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