Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.

Detecting planets via transitss

You are familiar with the basic idea behind the transit method, right?

If not, use this tool to refresh your memory

The idea is pretty simple: if a star has a planetary companion, AND if a distant observer lies in the orbital plane of that planet, then ever now and then the planet will pass in front of the star. As the planet makes its transit, it will block some of the star's light, causing a brief, small dip in its brightness. Find a series of little dips, and *boom* there's the planet.

So, what's so hard about that?

   Q:  Really -- why is this so difficult?

Well, let's begin by seeing just how large these dips in brightness might be, and how long they last. Let's take as an example the solar system, and two of the most interesting of the planets:

    Object               Radius               Orbital speed 
                          (m)                     (m/s)

    Sun                   6.96 x 108               ------

    Earth                 6.37 x 106               29,800

    Jupiter               7.15 x 107               13,100


     Q:   What fraction of the Sun's disk will Earth block?
          How long will it take Earth to travel across the Sun?

     Q:   What fraction of the Sun's disk will Jupiter block?
          How long will it take Jupiter to travel across the Sun?

The answers

You should find results somewhat like this:

Yes, these dips in brightness are pretty darn small: Jupiter blocks about 1% of the Sun's disk, while the Earth blocks about 0.01%. For this reason, astronomers use the following terms when discussing transits:

So, if we want to find planets via the transit method, we need high precision photometry.

We also need patience. Consider a poor ground-based astronomer who is looking for new planets with this technique.

   Q:  How long does a transit from an Earth-like planet last?

   Q:  How often will a transit occur?

   Q:  What fraction of the time will one particular system 
          be in transit?
          (Compare this to the fraction of time one particular
           system shows radial velocity variations)

   Q:  How many transits must one observe in order to be sure
          that the dip is due to a planet, and not to some
          other effect, or due to noise?

The answers

Complications in transit detection

Well, okay, the signals are small -- maybe very small -- and they are rare. Fine. We have marvelous detectors, such as CCDs, and we have lots of time. Surely it can't be so hard to find these transits?

In one sense, no, it isn't. After all, the first transit ever observed was measured with the 4-inch STARE telescope:

Image courtesy of the STARE project

But, in practice, there are a number of complications which make the detection of transits a lot harder than one might think at first.

Putting it all together: high-precision photometry is hard!

If you aren't a regular observer, working frequently with the real data, you might over-estimate the quality of your measurements.

A number of years ago, while I was working on the SDSS, I checked the precision of photometric measurements of stars in the SDSS catalogs.

Has this inconvenient truth been inserted into the SDSS catalogs? Well, I just now (Mar 19, 2015) made a query using the SDSS website. Take a look at what I found:

Wierd things often appear at the bright end of stellar distribution. For example, when I examined some images taken with the HDI camera on the WIYN 0.9-m telescope, I found -- well, you take a look:

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Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.