# Multi planet transits

### #1

Posted 18 May 2022 - 07:57 PM

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### #2

Posted 18 May 2022 - 08:23 PM

Exoplanet transits are discovered by photometry, not imaging, but I think what you're asking is would it be possible to detect a two-planet transit and distinguish it from a single planet transit.

I recently attended this event,

https://calendar.usm...er#.YoWaXajMI2w

Dr Seagar showed some examples of real data showing the dip in a star's light curve as a potential exoplanet transited, including ambiguous examples (where slight variability in the star's brightness are an issue). In principle a two planet transit would produce a different signature than a single planet, but it might be difficult to recognize with real-world data.

Maybe Dr Seagar or another researcher will see your question, but here's a NASA resource.

https://exoplanets.n...ltiple-planets/

Jim

**Edited by JamesMStephens, 18 May 2022 - 10:06 PM.**

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### #3

Posted 18 May 2022 - 08:56 PM

Yes, I meant via photometry when I said imaging. I have detected many single planetary transits such as WASP80b I did many years ago as detailed below. But was interested in if any multiple planet transits were within range of amateur or semi professional equipment as your NASA link suggests. I imagine such a transit would have an appearance of a "dip within a dip" as one planet passed causing a primary dip in brightness, then a second dip within the first as a second planet passed before the first one egresses.

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### #4

Posted 18 May 2022 - 10:08 PM

StarmanDan--how did you get that data? Very nice.

Jim

### #5

Posted 18 May 2022 - 10:29 PM

If such an event would occur, it would be relatively easy to observe it, and deconvolution of the two light curves would be mathematically straightforward. The problem, though, is not an observational one, bur rather an issue of probability. To be in a position where and when you could observe such events is extremely improbable. That is why you are having trouble finding any data on it.

To use a nearby example, both Mercury and Venus transit the sun, fairly regularly, enough so that I have observed each of them. An article in S&T projected the next simultaneous transit of the two would be in the year 69,000. Since this event hasn't occurred in the historical era, a conservative estimate of the probability of it happening in any given year is at least on in 70,000. Further, the time during which Mercury is crossing the Sun's disk is approximately 2 hours during the 8765 hours in a year, or 1 part in 4380. Thus, if you observed the Sun at random for one hour, your chances of seeing a dual transit would be about 1 in 306 million, not very good odds. And that is the "best case" scenario, since your observing platform, the Earth, does lie within the very small zone where such observations are possible. This would not be the case with a another star, so the odds would be much lower in general than the crude estimate I provided.

So, in summary, the observation itself would be easy if you just happened to be in the right spot looking at the right star and exactly the right moment. Unfortunately, there is a vanishingly small chance of that happening.

I hope that explanation helps.

Brett

### #6

Posted 19 May 2022 - 01:15 AM

If such an event would occur, it would be relatively easy to observe it, and deconvolution of the two light curves would be mathematically straightforward. The problem, though, is not an observational one, bur rather an issue of probability. To be in a position where and when you could observe such events is extremely improbable. That is why you are having trouble finding any data on it.

To use a nearby example, both Mercury and Venus transit the sun, fairly regularly, enough so that I have observed each of them. An article in S&T projected the next simultaneous transit of the two would be in the year 69,000. Since this event hasn't occurred in the historical era, a conservative estimate of the probability of it happening in any given year is at least on in 70,000. Further, the time during which Mercury is crossing the Sun's disk is approximately 2 hours during the 8765 hours in a year, or 1 part in 4380. Thus, if you observed the Sun at random for one hour, your chances of seeing a dual transit would be about 1 in 306 million, not very good odds. And that is the "best case" scenario, since your observing platform, the Earth, does lie within the very small zone where such observations are possible. This would not be the case with a another star, so the odds would be much lower in general than the crude estimate I provided.

So, in summary, the observation itself would be easy if you just happened to be in the right spot looking at the right star and exactly the right moment. Unfortunately, there is a vanishingly small chance of that happening.

I hope that explanation helps.

Brett

Venus transits the Sun four times every 243 years, https://en.m.wikiped...ransit_of_Venus. From the Wikipedia article:

”Transits of Venus are among the rarest of predictable astronomical phenomena. They occur in a pattern that generally repeats every 243 years, with pairs of transits eight years apart separated by long gaps of 121.5 years and 105.5 years.”

So, those are pretty rare. Mercury happens a bit more frequently at once every 7 years or so. Here is another older article from NASA on Mercury and Venus transits, https://www.nasa.gov...anets-transit.

What something like the TRAPPIST-1 system has going for it is a large number of planets (7 observed) and the fact that they are very close to their host stars. The planets orbit their host star every 1.5 to 18 days, so the chances of more than one transiting the host star at once is greater. I don’t know how likely it would be, but I’m sure that that analysis has been done and is likely just a Google search away

Anil

### #7

Posted 19 May 2022 - 07:52 AM

Hi

If any of you are interested in pursuing exoplanet transit imaging or observing or imaging various types of variable stars, the AAVSO would be delighted to have you become part of their global scientific amateur community. There are a lot of resources to get you started and Mentors can be made available to help you learn some of the more technical aspects involving variable star and exoplanet transit imaging and analysis. Here’s a link: https://www.aavso.or...erving-sections

Gary

**Edited by GaryShaw, 19 May 2022 - 07:52 AM.**

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### #8

Posted 19 May 2022 - 08:12 PM

The problem, though, is not an observational one, bur rather an issue of probability. This would not be the case with a another star, so the odds would be much lower in general than the crude estimate I provided.

So, in summary, the observation itself would be easy if you just happened to be in the right spot looking at the right star and exactly the right moment. Unfortunately, there is a vanishingly small chance of that happening.

I hope that explanation helps.

Brett

Yes, I realize the probability may be small for planets such as Mercury and Venus which have considerably longer orbital periods compared to most exoplanets observed to date. However, I was hoping that many of the shorter exoplanet periods with orbits on the order of days and weeks might provide higher probability.

StarmanDan--how did you get that data? Very nice.

Jim

This observation was done using my club's 24 inch f9 RC and a Princton Instruments Pyxis camera.

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### #9

Posted 02 July 2022 - 01:38 PM

Worked on this topic some time ago. Apparently, there's only Kepler-9 and TRAPPIST-1 that are detectable by amateurs, from exoplanets discovered so far. The first one consists of two planets with ~0.008 mag transits each, but the target is almost 14 mag in V. There's a chance maybe once per few years, so it's not common. For TRAPPIST-1, you may need a 12" scope or larger with a camera that works well in infrared. A single transit may be lost in scatter, but a double transit (a few chances per year) that is deeper than 0.01 mag might be marginally detectable. Of course talking there about chances per observing site, from a space observatory there's several dozens of doubles per year.

There should be more such objects for observing soon. I'm following TESS and I've seen a few double transits observed from ground. These are almost certainly multi-planet systems than eclipsing trinaries (EA+EA), but I don't have the data about how many, how often or how bright/deep events are. Should be a few more per year, as TESS catches easier transits than Kepler does.

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