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Seeing shadows

· ​Mercury transits ·

Starting around 1:12 pm on Monday and continuing until sunset, the unusual sight of planet Mercury passing between us and the Sun can be seen (weather permitting!) by observers here in Rome and wherever the Sun is visible during this event. (The transit ends at 8:42 pm Rome time).

Of course, Mercury is so small and the Sun is so bright that to see the transit you’ll need a telescope outfitted with special filters. (Never look directly at the Sun, with or without a telescope, unless you have the proper protection for your eyes!) What you would see through such a telescope is the shadow of Mercury, a small black spot slowly moving across the Sun’s surface.

Such events are called “transits” by the astronomers. They’re important for a number of reasons. For example, by comparing the observations of a transit as seen from two different locations on Earth you can eventually work out how many kilometers away from Earth the transiting planet lies; that is how astronomers first measured the size of the solar system in the 18th century. Furthermore, Mercury transits give us a very precise way of measuring Mercury’s orbit. Such measurements first suggested something was slightly askew with Newton’s understanding of the Sun’s gravity, which was finally only explained by Einstein’s theory of General Relativity.

Mercury swings around the Sun every 88 days. But because its orbit is slightly tilted compared to Earth’s, most of the time it appears from our vantage point to pass above or below the Sun, rather than cutting across its disk. Thus, most Mercury orbits do not result in transits. The next transit after Monday can be seen from Earth in 2019, and then you’ll have to wait until 2032 to see it again. Venus also orbits between us and the Sun; it has transits as well. But even compared to Mercury transits, Venus transits are exceedingly rare. The last one was in 2012 and the next will not occur until 2117. The farther the sun lies behind the planet, the less chance there is that we will see the planet’s path cross the Sun’s disk.

There have been 55 Mercury transits since the telescope was invented, some 400 years ago. At that time, Kepler with his new laws of planetary orbits was able to predict when these transits would occur; and in 1631 Father Pierre Gassendi, the noted French philosopher and Catholic priest, was the first person to observe such a transit.

Why had no one noticed Mercury blotting out a part of the Sun before then? Unlike the eclipses caused by the Moon, which is so close to us that it can completely cover the Sun, these little Mercurian “eclipses” cover only the tiniest fraction of the Sun, reducing its brightness to us by only one part in 25,000.

Of course, transits would be easier to notice if Mercury were much larger, or the Sun much smaller. And we’d see transits more often if Mercury orbited closer to the Sun. That’s not the case in our solar system, but in fact we’ve been able to discover planets orbiting other stars in exactly that way. As expected, most of these planets are giants, the size of Neptune or larger; and most orbit very close to their star. These are the kinds of planets and orbits that are easiest to detect. So far, about 2000 such transiting planets have been observed.

To see smaller planets, closer to Earth’s size, you would have to look for transits of smaller, dimmer stars. It takes longer to get precise measurements from dimmer stars, so fewer stars can be checked for planets. And since as we’ve seen it’s rare for planets to be lined up exactly right for them to appear to transit their star, the planets would likely need to be very close to their star. Even then, one would have to be very lucky to actually catch such a transit.

But recently, the luck held for a team of astronomers at the University of Liege, Belgium.

They and their colleagues at the Geneva Observatory have built “TRAPPIST” — the Transiting Planets and Planetesimals Small Telescope — in the desert mountains of Chile to make just such measurements of small, dim stars. This past week, Michaël Gillon and his team at Liege and elsewhere reported discovering the three planets roughly the size of Earth around a star now designated TRAPPIST-1.

Even more exciting, all three planets orbit close enough to their cool, dim star to have surface temperatures that might make life possible there.

Discovering if they have life, of course, remains still in the realm of science fiction. But the time is not far off when we might be able to convert that fantasy into science fact. This star is only 40 light years away from us, close enough that perhaps in the next decade or two we’ll be able to see if any of those planets have atmospheres where we might look for gases produced by living creatures.

Watching the shadows of Venus and Mercury in past centuries helped us understand our location in the physical universe. From that, we know now that our star is not the only star and Earth is not the only world. Likewise, watching for the shadows of other planets around other stars will also shape the way we understand our place in the universe, especially if we do find that our Earth is not the only home for life.

In both cases, it is interesting that we are learning about these planets not by seeing them, but only their shadows. The philosopher scientists of the 17th century, like Fr Gassendi, would have recognized the echo of the story of Plato allegory of watching shadows in a cave. The more we learn, the more we are humbled to realize how little we yet know about God’s creation.

Guy Consolmagno




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Nov. 24, 2017