Stupid Question ™
Sept. 13, 2004
By John Ruch
Q: How many planets are there outside of our solar system?
—anonymous, from the Internet
A: This question came in before the Sept. 10 announcement of a possible composite photograph of an “extrasolar” planet, so—one more than before, I guess.
The current count is anywhere from 124 to 140, depending on who’s counting, how and with what degree of skepticism.
They are all immensely huge. The smallest (and most recently discovered) would be about the size of Neptune—which is about 60 times the size of Earth. Most are much larger than Jupiter, our solar system’s largest planet, which is about 1,300 times the size of Earth. Like both Neptune and Jupiter, the “known” extrasolar planets are likely big balls of gas.
You wouldn’t know it from the very definitive language in astronomy textbooks and NASA and university web sites, but there is certainly room to argue the reality of any of these observations—at least, if they’re to be called “planets.”
Leaving aside the question of the possible photograph (and another similar image announced in May), none of the extrasolar planet “discoveries” involves direct observation.
At best, this is an art of interpreting “wobbles” and (most frequently and credibly) velocity shifts (sometimes also known as “wobbles”) in the movements of stars, from which it is inferred that gravitational pull from an orbiting planet is the cause. The size of the planet can be guessed from the movement.
Another mode is presuming a regular change in the beat of a pulsar—a star that emits bursts of energy rather than just glowing steadily—is due to the gravitational suction of a nearby planet. And still another is that the slight dimming of a star is due to a planet coming between it and us (a frequent “confirmation” method to other forms of detection).
The latest method is optical interferometry, which in theory will allow planets to be literally, actually seen (via infrared telescopy, usually) by using multiple images taken from various angles that cancel out the otherwise overwhelming glare of the accompanying star. If planets are out there, anyhow. It can also include adding spectrography that can determine chemical elemental makeup based on the light the object emits. (This led to the claimed discovery of a sodium-containing “atmosphere” on one “planet” in 2001.)
The Sept. 10 image of a possible planet around the star identified as 2M1207 was produced by that method by the European Southern Observatory.
The velocity-shift detection method kicked off the whole discovery boom in the early 1990s, and since then there’s essentially been an extrasolar planet discovered every month.
It’s an exciting field of observation, but there are philosophical and practical reasons for skepticism.
We’re talking about very minute, indirect observations driven by a romantic quest to, as NASA’s web site baldly puts it, be able to say we “have found another Earth.” There is a strong tea-leaf aspect to this.
And what is often left out of discussions of this field is its history of wishful-thinking errors and suspicious data. For example, as NASA again puts it, “many” of the observed planets “are bizarre,” displaying highly eccentric orbits and a proximity to their stars that doesn’t square with accepted planetary-creation theory. Why such a high degree of “bizarre” behavior? How about observer error or wishful thinking? The individual explanations become very involved.
Even more curiously, it has been determined that our Sun just so happens to have no detectable wobble from all the planets around it—this being blamed on sunspots masking the electromagnetic detection of such a shimmy. Hmm.
It’s also worth noting there is no agreement on what a “planet” is. You and I think of a ball orbiting a star; but some observing astronomers accept a definition that includes a body that is not in (stellar) orbit. They may be “failed stars,” if you will.
The earliest claimed extrasolar planet detection, in the late 1960s, was almost certainly bunk. It involved a supposed wobble of a star. Just about nobody else could detect any wobble. Instrument error and wishful thinking were likely culprits. The wobble method is not favored today.
Extrasolar excitement renewed in 1991 with a pulsar observation that was later acknowledged to be a calculation error. This did not stop a 1992 observation that claimed to find three planets around another pulsar. The observation remains controversial. (It’s also worth noting that pulsars are infamous for bizarre changes and fluctuations, and nobody has satisfactorily explained their nature in the first place.)
Wisely moving to yet another detection method, astronomers really kicked things into gear in 1995, with a flood of velocity-shift observations. Many of these are considered quite convincing and will prove to be either a watershed or a scientific fad. Things have not let up since.
I am being hyper-skeptical here because somebody has to be. Astronomers certainly put individual observations to the fire in their journals, but the overall idea of the reality of these observations is rarely questioned openly and certainly not for public consumption; quite the opposite, in fact, as misleadingly positive statements are made by highly respected sources.
The good news is that interferometry has the potential to provide direct images of some types of extrasolar planets. That could really settle some matters.
On the other hand, it may only raise more questions. Take the Sept. 10 picture, for example. The “planet” is pretty convincing as an image, but since that image is two-dimensional, there’s no way to tell the two objects’ actual relationship to each other, let alone if one is orbiting the other. The “planet” could be light-years closer or farther away from us than the star is.
Bringing most or all of the observation methods together may paint a convincing “yea” or “nay” picture of extrasolar planets. Or maybe it will take interstellar space flight to determine the question for sure.