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Planets outside our solar system: interesting graphics


-- ET

Response to Graphic of the day: Planets outside our solar system, Sky & Telescope, March 2002, p.22

For the 77 extrasolar planets (blue dots) discovered so far, this scatterplot shows their masses (lower limit) and orbital radius, for whatever that is worth. Note the log-log scaling for the scatterplot. There's not much structure to the relationship perhaps because we have a rather narrow band of information right now--but much more is to come.

Particularly interesting are the current detection limits shown on the plot, which indicates that we are close to being able to detect a Jupiter-equivalent (~1000 times the volume of Earth) around other stars.

The accompanying article in Sky & Telescope, written by Joshua Winn, reports on the work of a team led by David Charbonneau (Caltech) and Timothy Brown (National Center for Atmospheric Research) using the Hubble Space Telescope. They detected an atmosphere on an exoplanet orbiting a star 150 light-years away! This remains a local view since the volume of the universe is estimated at 350 million light years on a side. Further research is needed, as we say in the academic world.

The amazing work recently in astronomy has produced extraordinary images. A good place to start is the magazine of amateur astronomy, Sky & Telescope, which routinely displays spectacular photographs and well-drawn diagrams and maps. This work is high-resolution and much would not survive the low-resolution of computer screen displays (it is similar to trying to show a good printed map on the screen). Thus the Sky & Telescope website is thin; instead look at the beautiful printed magazine for good information displays of astronomical news.

-- ET

I'm no astronomer, but I do know that one potential significance of orbital radius comes from the fact that astronomers (at least used to) believe that there is a "range" of distance from a star at which life tends to be sustainable, and water tends to be liquid, under normal atmospheric presures. (This belief, however, predates the discovery of sulfur-based life in the volcanic ocean vents in the Earth's oceans.) Such a range is roughly equivalent to the orbit of the Earth around the Sun, or 1 A.U. Of course, in measuring the tendency toward liquid water at 1 A.U., n=1. This may be a fatal statistical flaw in this theory.

Still, it could be somewhat helpful to know how far the known extra-solar planets vary from an orbital radius of 1 A.U.; it may tell us how many of the planets may be able to support life in some iteration similar to what we see and know here on Earth.

Size is also important in this regard as well: the larger a planet is, the less likely it is to develop life in some form we're likely to recognize. The bottom line? By graphing extra-solar planets along axes of size and orbital radius, we get a rough idea of how many (or in this case, how few) of the extra-solar planets may sustain life in a form that may be similar to (or at least recognizable by) life here on Earth.

-- Dennis Kim-Prieto (email)

Following the introduction of the topic "Cleaning up Excel's poshlust", I thought you might be interested in the dot-dash plot of extrasolar planets I made in the last month based on the list at It is, of course, inspired by the dot-dash plots in The Visual Display of Quantitative Information. I constructed it in part as an exercise in pushing the limits of Excel's ability to mimic a more powerful graphing package. It's a 1770 wide by 2656 high GIF, but only 61K file size because of all the repetitious elements.

There are now around 200 planets discovered, many more than were known in 2002. The technical challenges involved in making such sensitive doppler measurements, when the star itself is spinning and boiling at hundreds or thousands of metres per second, are formidable.

Some things to note on the chart:

K is the conventional symbol for the doppler velocity which the detection technique is capable of measuring, that makes these discoveries possible. More recent discoveries in 2005/6 at K < 9m/s were only short period orbits. Larger period orbit detections in the 4-9m/s velocity range are to come as time goes on. The latest update shows planets now detected at K less than 4m/s!

Jupiter was within detection range at K=4m/s, but not Saturn, But from the latest detections it looks like Saturn is now detectable. Uranus mass planets have been detected, but not at Uranus distance.

Most planet orbits are highly eccentric, but planetary orbits are more circular if they orbit within 0.1 AU of the star.

The medians are 1.5Mjup and 0.78AU. 25% of the planets are greater than 1.76 AU, and 25% are less than 0.62 Mjup. So far only one planet is both, so just under 50% are neither.

The furthest planet is 7.73AU, between Jupiter and Saturn. The smallest planet is 0.02Mjup. The planet closest to its star (0.02 AU) is also the planet with the highest M sin i (18.40 Mjup)!

-- Derek Cotter (email)

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