Could detected exoplanets actually just be a wandering planet passing in front of the star?

This could be possible for the few direct detections of objects near stars and also the few objects detected by gravitational micro-lensing observations.

However, the vast majority of objects have been detected either by transit observations (such as by the Kepler space telescope) or by radial velocity observations (the 'wobble' method using high precision spectroscopy) of stars.

Thus what we observe is either the star dipping in brightness periodically or the star's motion towards and away from us changing periodically. Such periodic observations wouldn't be caused by a planetary object having a single chance encounter with a star.

Periodic brightness changes could be caused by some form of stellar variability, which is partly why so many of Kepler's detections are only classed as planetary candidates at present, rather than confirmed planets.

Your presumption ('a wandering planet passing in front of the star, which is in fact not even close to the star') can be ruled out. If it is that close to a star that it can cast a shadow (on us on Earth, by which we are able to pick it up), then it can't escape its gravity & will be a planet of it. If it is too far from the star, it wouldn't be able to cast a shadow (shut of its light; the same thing though) then, as either of them would be out of focus (by the usual laws of Physics) and we wouldn't be seeing anything to give us a clue that there lurks a planet-like body between us (that can be near us even or midway between & has nothing to do with that star even etc etc) & that star. The whole game of our detecting a planet orbiting a star is off.

Darn. There are three methods to appreciate your question, and every one provides an additional reply. 1. When a planet from our sunlight approach passes in entrance of a far off star, the amount of starlight decreases as the edge of the planet (together with a possible surroundings with a non-zero thickness) passes in entrance of the celebrity. The starlight drops to zero because the planet in the end "eclipses" the superstar. 2. When that big name's possess planet passes in entrance of it (this would make the planet a "transiting exoplanet"), the quantity of the curb is proportional to the rectangular of "radius of the planet over the radius of the celebrity". However, the proper terminology for such an occasion includes the phrase "exoplanet" (to denote a planet that's not in our possess sunlight method). 3. For the duration of the search for MACHOs (significant Astrophysical Compact Halo Objects), some devices had been looking for rogue planets (free planets not in orbit round stars) by using looking for gravitational lensing as one of the rogue planets might pass in front of far-off stars. In such cases, the expand in located starlight could be proportional to the mass of the planet. --- headquartered on the choice of solutions, you look to be coping with transiting exoplanets.

No. These objects are not seen directly. Their presence is inferred by *preiodic* variations in the intensity of the stellar light (which is the transit), by periodic oscillations of red and blue shift, by periodic changes in the exact location of the parent star. In no case can a single perturbation be considered an exoplanet---how could it be? There would be no way of detecting the difference of a single transit from noise or observational error.

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But don't take my word for it--see for yourself. Go to the Planet Hunters website and you can learn exactly what is involved in identifying possible exoplanets based on light intensity, by hunting for them yourself.

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No, because astronomers wait until the planet passes in front of the star at regular, predictable intervals (meaning that the planet is orbiting the star) before they declare it as a planet orbiting that star.

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They don't just pick one transit. They watch and observe for periodic transits. If it's periodic it's not a wandering planet, it has to be orbiting the star.

I believe that the Kepler mission required three transits at the expected interval to confirm an exo-planet. Wandering planets wouldn't make three orbits, or they wouldn't be wondering.

>> Or when detecting these exoplanets do they watch it orbit more than once? Which would rule out a wandering planet...<<

Bingo.

Ya but if its a wandering planet it's still a planet