# Exoplanets and sunspots **URL:** \ **Category:** Factual Questions\ **Created:** [May 18, 2013, 1:36pm UTC](https://boards.straightdope.com/t/exoplanets-and-sunspots/658676 "2013-05-18T13:36:13Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Quartz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/quartz/32/267_2.png) [@Quartz](https://boards.straightdope.com/u/Quartz)\ **Post date:** [May 18, 2013, 1:36pm UTC](https://boards.straightdope.com/t/exoplanets-and-sunspots/658676/1 "2013-05-18T13:36:13Z")
One of the methods of detecting an exoplanet is checking for dips in the brightness of the host star. But sunspots also cause a dip, so how do scientists differentiate between the two? How do they differentiate between a ‘hot Jupiter’ closely orbiting and a sunspot? ---
**Author:** ![SenorBeef](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senorbeef/32/2220_2.png) [@SenorBeef](https://boards.straightdope.com/u/SenorBeef)\ **Post date:** [May 18, 2013, 1:41pm UTC](https://boards.straightdope.com/t/exoplanets-and-sunspots/658676/2 "2013-05-18T13:41:34Z")
The dip in light from a transiting planet would happen on a regular orbital period. If you saw that dip exactly every 174.31 days or whatever, it’s a pretty good indication that it’s a planet. I think they wait for 3 orbits to declare an exoplanet discovered. The odds of transient events creating such events randomly are rather small. ---
**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:** [May 18, 2013, 1:54pm UTC](https://boards.straightdope.com/t/exoplanets-and-sunspots/658676/3 "2013-05-18T13:54:12Z")
It would take a very extreme starspot to produce as much of a dip in luminosity as a planet, and any star that’s volatile enough to produce such extreme starspots would show its volatility clearly enough to eliminate it from consideration. There’s also the matter of the time profile. For a typical star, a spot will remain visible for days or weeks, but a transit will be over in hours. The change in luminosity due to a spot, meanwhile, will vary over the entire time its visible, while a transit will leave the luminosity constant for most of the duration of the event, with the drop at the beginning and rise at the end following a very particular pattern. ---
**Author:** ![Quartz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/quartz/32/267_2.png) [@Quartz](https://boards.straightdope.com/u/Quartz)\ **Post date:** [May 18, 2013, 2:17pm UTC](https://boards.straightdope.com/t/exoplanets-and-sunspots/658676/4 "2013-05-18T14:17:01Z")
> [@SenorBeef](#): > > The dip in light from a transiting planet would happen on a regular orbital period. Yes, just like the rotational period of the star… > [@Chronos](#): > > There’s also the matter of the time profile. For a typical star, a spot will remain visible for days or weeks, but a transit will be over in hours. Doesn’t that depend upon how fast the star rotates and the presumed location of the presumed planet? The sun has a rotational period of 25 days or so at its equator; the Earth has a rotational period of 1 day. ---
**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:** [May 18, 2013, 3:33pm UTC](https://boards.straightdope.com/t/exoplanets-and-sunspots/658676/5 "2013-05-18T15:33:04Z")
The Earth’s rotational period is irrelevant; it’s the orbital period (1 year) that matters here. And any Earthlike planet is certainly going to have a period somewhere around that. The rotational periods of stars can vary somewhat, but so far as we know not enough to overlap with that. ---
**Author:** ![GIGObuster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gigobuster/32/421_2.png) [@GIGObuster](https://boards.straightdope.com/u/GIGObuster)\ **Post date:** [May 18, 2013, 3:44pm UTC](https://boards.straightdope.com/t/exoplanets-and-sunspots/658676/6 "2013-05-18T15:44:10Z")
Sadly, the latest from the Kepler mission, that was the best in finding exoplanets, is that its planet hunting days are over. > **[NASA's Planet-Hunting Kepler Telescope Will Have Long Legacy, Despite Big Glitch](https://www.space.com/21210-nasa-kepler-exoplanet-telescope-legacy.html)** > > Whether or not NASA's Kepler spacecraft can bounce back from the malfunction that has stalled its search for alien planets, the mission's place in history is assured. > [@](#): > > Kepler has spotted more than 2,700 potential exoplanets to date, with many more waiting to be plucked from the mission’s huge dataset. Its discoveries have opened the eyes of scientists and the public alike, revealing that the Milky Way galaxy abounds with an incredible diversity of alien worlds. > > “Before we flew Kepler, we didn’t know that Earth-sized planets in habitable zones were common throughout our galaxy,” Hertz added. “We didn’t know that virtually every star in the sky had planets around them. Now we know that.” > > An uncertain future > > The Kepler spacecraft launched in March 2009, kicking off a 3.5-year prime mission to determine how common Earth-like planets are throughout the galaxy. > > The observatory spots alien worlds by detecting the tiny brightness dips caused when they pass in front of their parent stars from the instrument’s perspective. To stay locked onto its 150,000-plus target stars, Kepler needs three functioning reaction wheels, gyroscope-like devices that allow the spacecraft to maintain its position in space. > > Kepler has four such wheels. But one, known as No. 2, failed in July 2012. And No. 4 has now failed as well, NASA officials announced Wednesday. > > Mission engineers will try to bring the two failed wheels back into service over the coming weeks. If they cannot recover at least one wheel, Kepler’s planet-hunting days are almost certainly over, though the observatory may get a new mission that doesn’t require incredibly precise pointing.