51 Pegged — Re-Discovering the first exoplanet with Systemic Live

This post is the first in a series of Systemic Live tutorials. You can see all Systemic Live tutorials in this link.

In this post, I will show how to analyze the radial velocity dataset of the the planetary system that started it all, the original gangsta, 51 Peg. I will use the new web application Systemic Live, a simplified version of Systemic that runs in your browser.

Artist's concept of a "hot Jupiter", Credit: NASA/JPL-Caltech
Artist’s concept of a “hot Jupiter”, Credit: NASA/JPL-Caltech

51 Peg was announced in 1995 by a Swiss team led by Michel Mayor and Didier Queloz; it was later confirmed by an american team led by Geoff Marcy and Paul Butler at the Lick Observatory.  It was the very first exoplanet found to orbit a Sun-like star. Mayor and Queloz’s discovery of the hot Jupiter orbiting 51 Peg was truly a watershed event: their Nature paper has racked up 1225 ADS citations! (These are citations from other astronomical papers.)

We will analyze this data, and follow the same procedure used to unearth the evidence for the first planet orbiting a Sun-like star.


Launch Systemic Live. Upon launch, you will see a window similar to the one below.[ref]If Systemic suggests your browser might be slow, we recommend to use the Google Chrome browser for maximum performance.[/ref] Click on the blue question mark icons to get help on the various panels in the application.

You can either do the rest of this tutorial by following the instructions, or clicking on Big Blue buttons like these to show the step in Systemic.

Systemic Live upon first launch.
Systemic Live upon first launch.

The SYSTEM drop down lets you choose which dataset to analyze. The dataset name is the name of the star that was observed to produce the radial velocity data (for example, 14Her.sys is the dataset for 14 Herculis). You can find more information about the star by scrolling to the ABOUT THIS STAR section.

Click on the SYSTEM drop down, type “51peg” to find the dataset for 51 Peg. Choose “51peg.sys”. The data will be loaded, like in the screenshot below. The RADIAL VELOCITY plot shows the radial velocity data: each point is a single measurement. Time is on the x-axis, measured as a Julian Date (a  way to indicate time favored by astronomers). Radial velocity measurements, in meters per second, are on the y-axis. See in Systemic

The 51 Peg dataset loaded in Systemic Live. The observations were made from two observatories: the Swiss Observatoire de Haute-Provence (red) and the Lick Observatory (blue). Move your mouse to see the date associated with each measurement.

One of the datasets was published by the California-Carnegie Planet Search Team (red points), the other by the Geneva Extrasolar Planet Survey (blue points). The Swiss data set gives a long baseline of coverage, whereas the California-Carnegie dataset contains intensive observations taken mostly over the course of a single observing season in 1996.  You can move your mouse over the points to see human-readable dates instead of julian dates.

Scroll down to see the POWER SPECTRUM plot.

The Lomb-Scargle power spectrum shows the most prominent periodicities in the data.
The Lomb-Scargle power spectrum shows the most prominent periodicities in the data.

The POWER SPECTRUM plot shows which periodicities are present in the data. A prominent periodicity in this plot looks like a tall “peak”; a strong periodicity might be indicative of the presence of a planet orbiting a star at that period.

The peak at 4.23 days

In the case of 51 Peg, the power spectrum periodogram has an impressive tower of power at 4.231 days. This dataset contains a whopping-strong sinusoidal signal at that period! You can see a table of periodicities right under the plot. You can also “zoom in” and look at a more fine periodogram by changing the period interval. (Insert, for instance, 4 to 5 days as the interval and press Set; press Reset to return to the default interval). See in Systemic

Power spectrum between 4 and 5 days.
Power spectrum between 4 and 5 days.

Mousing over the power spectrum plot will show the “power” at a given period (the strength of the signal at that period) and also an estimate of the so-called “False Alarm Probability”, the probability that the signal might have arisen by chance (e.g. by an unlucky sequence of noise mimicking a sinusoid). In the case of the peak at 4.2306 days, the False Alarm Probability is astronomically low (10-168, an infinitesimally small probability). The periodicity is definitely there!

To work up the 51 Peg “b” planet, click on “Add planet”. This button will activate a table of orbital parameters: Period (the orbital period of the planet, in days), Mass (the mass of the planet, in Jupiter masses), Mean Anomaly (the phase of the planet at the time of the first measurement, in degrees), Eccentricity (the shape of the orbit) and Longitude of Periastron (the orientation of the orbit, in degrees).  Type 4.2306 (the period of the strong peak) in the Period box. You should see something like the plot below. See in Systemic

The Radial Velocity plot after the planet is added. The period of the signal is too small to be plotted! YELLOW BOX IS NOT HAPPY
The Radial Velocity plot after the planet is added. The period of the signal is too small to be plotted! YELLOW BOX IS NOT HAPPY

Systemic plots the radial velocity curve due to the presence of planet(s) as a thick black curve; the better the curve matches the points, the better the model (also called a fit). However, in the case of 51 Peg b, the plot is distorted. The reason is that the observations cover more than 9 years, while the curve has a period of only 4 days: the sinusoid has too many peaks and troughs to plot! A reproachful yellow alert informs you of this limitation.

To get a better plot, switch to the PHASED RADIAL VELOCITY plot. This switches the top plot to a new view. In this new view, the radial velocity points are “folded” to the period of the planet: the data points are shifted to cover the entire period of the planet.

The phased radial velocity plot. The sinusoidal signal is now a lot clearer.
The phased radial velocity plot. The sinusoidal signal is now a lot clearer.

Much better!

Finding the planet parameters

You can now see the full sinusoidal signal caused by the presence of the planet (the thick black line). The sinusoidal shape of the data is also evident. To match the black line (the model) with the points (the data), you would only need to shift it and increase its amplitude. This is done by varying the Mean Anomaly and Mass parameters. To automatically snap to the best values, use the checkboxes next to it to select them and click the Optimize button. The Optimize button automatically cycles to values to find the “best-fit”, the parameters of the model that best match the observations.

The fit is now quite good! See in Systemic

The black line (the model) is a better match for the points (the observations).
The black line (the model) is a better match for the points (the observations).

The improvement of the fit is measured by the Chi-square value (found under the STATISTICS table). A good fit has a value of Chi-square close to 1. The value of Chi-square for this model is 2.12 – pretty good!

We can do even better.  Check all the remaining parameters: the two offsetsPeriodEccentricity and Longitude of periastron. Then, click Optimize. The procedure will give a small improvement in Chi-square (from 2.12 to 2.01).

The final fit parameters for the planet give a period of 4.2308 days, a mass of about 0.5 Jupiter masses, and an eccentricity of 0.014, fully consistent with the original paper! This is what its orbit looks like: See in Systemic

Orbital plot of 51 Peg b. The planet is at only 0.05 AU from the star! HOT HOT HOT!
Orbital plot of 51 Peg b. The planet is at only 0.05 AU from the star! HOT HOT HOT!

Return to the POWER SPECTRUM plot one last time. The 4.23 days peak has been eliminated by the addition of the planet: the only strong period left is at about 359 days. See in Systemic

A residual peak at 359 days.
A residual peak at 359 days.

This residual peak is strong, though not quite as tall as the original one. Is it evidence for a second (“c”) planet? Not quite. The period of this peak is very close to the Earth’s orbital period (about 365.25 days). Turns out that certain periodicities, connected with the Earth’s and the Moon’s orbital period (among others), can show up in the data as artificial peaks. These spurious periodicities are related to gaps in the time coverage of the data and not due to the presence of exoplanets! From oklo.org’s post:

Aliases are a problem in Doppler surveys because observations are most efficiently done when the star is crossing the meridian, leading to a natural spacing of one sidereal day (23h 56m) between data points. Further periodicities in data-taking arise because RV survey time is usually granted during “bright” time when the Moon is up, and as a consequence of the yearly observing season for non-circumpolar stars. Aliases are minimized when observations are taken randomly, but the nuts and bolts of the celestial cycles impose regularity on the timestamps.

Saving and sharing a fit

You can save your fit by copying the current address (the URL) in your browser, or copying the content of the SHARING panel. It will look something like this:

You can copy and paste this address to your notes in order to save your work, or send it to other people to share your work.

Saving and printing plots

You can save or print each chart by clicking on the icon on the top-right corner:

Click the button to print or save a plot.
Click the button to print or save a plot.

What’s next?

Now that you have found your first planet with Systemic Live, take it for another spin with the star HD31253. HD31253 is another fish-in-a-barrel dataset for Systemic. It hosts a single planet — try to figure out its period and mass without looking them up!

Look out for the next installment, Trois Neptunes, in a future post. You can see all Systemic Live tutorials in this category.

[This post is an adaptation of the post “51 Pegged?” that originally ran on oklo.org on April 7th, 2006.]

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