Sunday, August 21, 2016

Sol-like Solar Systems

Before we on Earth started discovering exo-planets, the idea was common that our solar system was typical, and other solar systems, once we discovered them, would look like ours. Specifically, all we knew about at that time were the eight planets and Pluto, plus some asteroids and comets. What we expected to see was a spread of planets similar to ours, with a couple of big gas giants out at six to ten AU, and some little ones inside their orbit and some medium sized ones outside it.

Then there started to be exo-planets discovered and, surprisingly, they were mostly of large planets orbiting inside the orbit of Mercury, maybe at only 0.1 or 0.2 AU. The assumption was turned on its head, and there was some talk about our solar system being the unique or unusual one. Is it? Suppose there were just hordes of sol-like solar systems out in the galaxy, even in the corner we inhabit. Are they visible or invisible?

Everyone knows that the wobble technique of finding exo-planets, where a star's velocity is spectroscopically measured to great accuracy, and if the variation corresponds to what the gravity of a planet would do, a candidate is declared. Wobble works best with larger planets and smaller stars, so the relative motion of the star is enhanced. It also works best with planets in-close to the star, as the frequency of motion is greater and easier to detect, and it is larger as well. If you look at the list of exo-planets which have been detected by wobble, there is no Jupiter-sized or less planets detected at Jupiter’s orbit radius, about 6 AU. There are some bigger ones out there, but no Jupiters. So Jupiter is invisible to current wobble capability. There are no planets of Earth mass or less at Earth’s radius or more. In other words, wobble can’t detect Earth or Jupiter. Too bad.

The other prominent technique is transit. There the total output power of the star is monitored, and when it diminished by a small but constant amount, from a planet passing between the star and the telescope, a possible candidate is found. The smaller the planet, the harder it is to see the signal in the noise. Also, the further out the planet is, the smaller the diminishment will be. Add to this the difficulty in finding single occurrences, as opposed to multiple passes. And on top of this, there is the third dimension. A planet which is in a larger orbit has a lesser chance of happening to be between the observed star and the telescope observing it, because the effect of orbital inclination is larger for larger orbital radii. Passage time is also shorter. Everything works against seeing planets at Jupiter’s radius and small ones at Earth’s radius. And yes, in the list of exo-planets discovered by transit, there is no Earth-size or smaller at Earth radius or larger, and there is no Jupiter-size or smaller at Jupiter radius or larger. Which means that Earth and Jupiter are invisible to this method at the current state of technology as well.

Another technique which is gaining prominence is direct observation. The planet’s own light, typically the infrared portion of it, is detected. If the orbital radius is huge, the problem of blocking by the light of the parent star isn’t so bad, but if it is not, but small like Earth’s or moderate like Jupiter’s, the light of the star must be obscured, unless it is a faint star. The first time this was accomplished was for planets around a white dwarf, which is not bright at all. So, for stars like our own, nothing the brightness of Jupiter or less at Jupiter’s radius or smaller, and nothing the brightness of Earth or less at Earth’s radius or smaller, have been detected. In other words, even with direct imaging, Earth and Jupiter are invisible.

There are no other techniques that have been perfected yet for finding Earth-like planets around Sol-like stars, or Jupiter-like planets around the same. If Jupiter is not detectable, neither would be Saturn, and Uranus and Neptune may also have the same problem. Our solar system is invisible to current technology. Perhaps for close-in, neighboring stars, there might be an exception, or perhaps with longer data collection times, this barrier might be crossed. Maybe the next generation of technology is just about to crack through these limits and see solar systems like ours. But until that happens, and until it becomes popular enough to detect our type of solar system in numbers corresponding to other types, our type of solar system is virtually invisible and the hastily-concocted comments about our solar system being the exception are simply untenable.

If you told me there were no ants on your picnic table, but I knew you left your glasses home and couldn’t see some no matter how hard you tried, I would wonder about your credibility in general. However, that doesn’t apply to astronomers who have just accomplished some absolutely astonishing technological developments, both in instruments and in data processing, and have started to see exo-planets after centuries of wondering about them. The credibility of the astronomy community has never been higher, and coverage of astronomical discoveries is much greater now that it has been in the distant past. Yes, the existence of science fiction since the time of HG Wells has certainly helped, but the drama of discovery is in itself something that interests large numbers of Earth residents. Yes, having astronauts and cosmonauts and taikonauts, plus a space station, plus for really old people the visits to the moon, add to the excitement that now envelops astronomy. All together, there is a great demand for news about new Earths and so on. Perhaps this excuses the failure of science commentators to mention that these unusual solar systems are the detectable ones, and ones like ours may be by far the most common, but they are invisible. As far as it goes for figuring out the prevalence of alien civilizations and their propensity for visiting us sometime soon, it is misleading. If there are special conditions that obtained here on Earth for originating life, it is simply not possible to extrapolate out to other solar systems in general until this gap in our observational technology is rectified. Kepler has seen a couple of thousand stars with planets out of the hundred thousand or so they observed, and these two thousand are pretty distinct from Sol-like. But there could be twenty thousand Sol-like systems, and they are just not yet seen. So, another barrier to figuring out if extraterrestrials are nearby exists, waiting to be ruptured and surpassed.

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