More than a dozen Earth-like planets have been discovered around nearby stars based on observations of changes in the brightness of their sun as they pass across its disk (transit events). If an Earth-like planet has a satellite like our Moon, its trajectory is slightly perturbed. Is this disturbance enough to reliably register a corresponding change in the brightness of their sun, indicating the presence of a “copy of the Earth + Moon”?
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$\begingroup$ Minor comment: "more than 5000" is a subset of "more than a dozen," so your first sentence is technically correct — though it gives a misleading impression of the size of the dataset that's available to study. $\endgroup$– rob ♦Commented Mar 14 at 15:56
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2$\begingroup$ Oh, my number is total planets, rather than Earth-like planets. So perhaps my annoying non-correction is actually incorrect. $\endgroup$– rob ♦Commented Mar 14 at 15:58
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2 Answers
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Hunting for exomoons is an active area of research. There is a list of exomoon candidates here, although none have been positively confirmed so far. Only one candidate exoplanet on that list, Kepler-409b, is close to the Earth in size, and its exomoon candidate is now deemed to be unlikely.
So it looks like the answer is that an Earth/Moon size exoplanet and exomoon could be detected in principle, but it would be very difficult and/or improbable with current technology.
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The transit method doesn't really give timing information apart from the times of the beginning and end of the transit. Here's a time-lapse of Mercury transiting our sun:
(source; credited there to NASA Goddard Spaceflight Center)
For distant stars, we only see the star's luminosity drop slightly when the transit begins, and rise again when the transit ends:
(source)
The irregularity you are describing would be very difficult to disentangle from the effect of "limb darkening" which prevents the bottom of the light curve from being flat. The difficulty is more obvious for noisier data:
(source)
One might ask whether there are transit light curves where the sensitivity is good enough to detect sequential transits of a planet and a comparably-size moon. I don't know the literature on this subject.
