Sunspots imaged by amateur astronomer Alan Friedman on July 10. Image credit: Alan Friedman/NASA Goddard Space Flight Center

Jun 28, 2015 Life on Mars? How about the Sun and Moon too!

Sen—With so many exciting space missions currently helping us learn more about the family of worlds around us, I took a trip two hundred years back in time this week for a different perspective on the Solar System.

I made my voyage, not via the TARDIS, but thanks to a book simply called The Solar System by a Scottish clergyman, the Rev. Thomas Dick, LL.D., that was published in the early 19th Century by The Religious Tract Society in London. It is a small book, but much bigger on the inside, albeit including some ideas that seem very strange today.

Of course there is no mention of Pluto—or of Neptune come to that—since the book was published long before either world was discovered. But Dick does tell us that there are ten planets other than Earth: the five visible to the naked eye—Mercury, Venus, Mars, Jupiter and Saturn—plus five telescopic worlds—Vesta, Juno, Ceres, Pallas and Uranus.

Evidently, at that time, our Solar System was being seen in a different way from today. There was no notion of an asteroid belt. Instead, those bodies that had been detected by the “Celestial Police”, of whom I have written before, were being thought of as planets to rank alongside those bright enough to have been known since antiquity.

The last of Dick’s “planets”, Vesta, was discovered in 1807 after which there was a long gap before renewed searching found a flurry more from 1845 onwards. Of course, as more and more of these new worlds were discovered, it was realised that new classification was needed. Other than Uranus, these other new bodies were not proper planets at all.

You can see that there is a remarkable parallel here with the discovery of more and more icy objects in the outer Solar System that led to Pluto’s relegation from the premier league of planets to a new classification. Whatever one thinks about that decision, we have here a clear demonstration of how our understanding of the Solar System is continually changing, due as much to changes in perception as to individual discoveries.

Going back to the book, I was fascinated to read about the Sun. Scientists of the day had a pretty good idea of its immense size and great distance—we learn that “a cannon ball, at its utmost speed of about five hundred miles an hour” would take 21 years and 245 days to reach it. Or to express that using more advanced technology of the day, a “steam carriage” setting out at the rate of 480 miles every day, “would require five hundred and forty-seven years before it could traverse the space which intervenes between us and that distant luminary.”

So far, so reasonable. But then we are introduced to sunspots, those apparently dark blotches that are frequently visible on the Sun to astronomers using proper protective filters. Today we know that these only appear dark due to contrast and would be seen to glow brightly if the rest of the Sun were not there. We know too that they are cooler regions on the Sun’s visible surface, or photosphere, due to concentrations in the magnetic field.

But in Dick’s day, these sunspots represented holes in a brilliant luminous atmosphere that surrounded a dark and solid body within. In other words they were windows onto this mysterious dark world. The clergyman was not making stuff up either, he was reporting the view of the highly respected astronomer William Herschel, who had not only discovered Uranus, but opened the door to infrared astronomy by noting that his thermometer recorded a temperature rise beyond the red end of the visible spectrum.

He was quite wrong about the Sun though, because he even imagined it might be inhabited. As Dick writes: “the internal part of the Sun may be considered as an immense solid ball, not altogether unlike the Earth and the other planets; and there is no great improbability in supposing that it is fitted for being the habitation of sensitive and intelligent beings, with constitutions adapted to the situation; and that it may constitute the most glorious habitation connected with the Solar System.”

Such a notion seems ludicrous nowadays, when we have a much better understanding of the nature of the Sun as a ball of gases that “burns” by nuclear fusion that converts hydrogen into helium. But our knowledge comes thanks to the studies of others over many years, and other mysteries remain to be solved. It is only now, for example, that we are learning why the Sun’s outer atmosphere, or corona, is nearly 200 times hotter than the surface, the cause being magnetically-driven jets of plasma from within the Sun delivering the energy needed.

It wasn’t only the Sun that Herschel considered must be populated by aliens. He compared the lunar surface to a countryside landscape. The Solar System tells me: “There can be little doubt that the Moon, like the Earth on which we dwell, is a world replenished with inhabitants . . . it is highly improbable that the Creator would leave a globe containing a surface of fifteen millions of square miles altogether destitute of sensitive and intellectual beings, especially when we behold its surface diversified and adorned with such a vast assemblage of picturesque and sublime scenery.”

Though those early 19th Century ideas are a long way from the reality of the “magnificent desolation” that Apollo astronauts found on the Moon, you can see the supposed logic of Herschel, which was basically to say, what is the point of a world being created if not to be lived on! So needless to say, Venus was thought to be home to “millions of sentient and intelligent beings—perhaps far superior in dignity to man”, while observations of Mars showed “strong presumptive proof that this planet is an inhabited world, and destined to afford existence and happiness to numerous orders of beings.”

Similar ideas were expressed a lot earlier by an Italian friar called Giordano Bruno when he travelled Europe in the 16th Century. He was ahead of his time, as he not only embraced Copernicus’s heliocentric model of the Universe, but also believed that planets surrounded all stars and were probably inhabited. Again, he had no special advance knowledge of exoplanets, he was simply applying what he saw as common sense. Such outspokenness cost him dear because he was burned at the stake, after several years in prison, for refusing to recant his views.