Is There a Planet Vulcan? The Story of the Planet That Never Was but the Asteroids That Could Be
Ever heard about the planet before Mercury? Didn’t think so. Once thought to exist based off a series of important calculations in the 19th century, the planet Vulcan (not the one from Star Trek, mind you) has been tossed into the trash bin of history after years of observations and revisions to gravity came to the forefront of science. However, the quest did spurn an idea for which no definite conclusion has been reached - yet. But, I have gotten ahead of myself so let’s start at the beginning.
How Math Led Us Astray
The first search for the planet Vulcan began in 1611 after Christoph Scheimer saw a dark spot on the surface of the Sun. Mercury was not around that position at the time, so what could it be? Scientists now suspect he saw a sunspot, but at the time, it was a big mystery. However, Mercury does occasionally transit in front of the Sun, and in the 1700’s scientists wanted to record them so that they could calculate solar system distances, with the Mercury-Sun distance as a reference, using trigonometry. However, predictions of the transits proved to be difficult with many scientists being off by as much as an hour! How could this happen? Slowly they began to realize that everything, and not just the Sun, pulls on Mercury courtesy of Newton’s gravity. With this in mind, long and tedious calculations were made to try to take these tugs into account, therefore getting an accurate Mercury orbit (Plait 35-6, Asimov).
By the 1840’s, Urbain Le Verrier, known for his discovery of Neptune, noticed that some irregularities still existed in the orbit of Mercury despite astronomers best efforts to reign it in. He found that something unaccounted for seemed to pull on it when Mercury was at perihelion, or its closest approach to the Sun. Plus, the orbit was still off by 1.28 seconds each year. Le Verrier, in a great twist of irony, preceeded Einstein's new thoughts on gravity when he postulated that maybe gravity needed some modification. He didn't pursue this avenue however because Neptune's discovery solidified gravity as a stable theory. But a readily testable possibility remained. Could a mystery planet exist? He called this postulated planet Vulcan after the god of the forge (for it would be a hot place, being in such close proximity to the Sun) and began an immediate search (Plait 35-6, Asimov, Weintraub 123, Levenson 65).
He got even more excited when astronomer Lescarbault, after hearing about the transit of Mercury in 1845, reported a small dot about a quarter the diameter of Mercury passing in front of the Sun on March 26, 1859, and it was not Mercury nor Venus. The object appeared at 3:59:46 pm local time and disappeared at 5:16:55 pm local time, giving a total transit of 1h, 17m, 9s. Le Verrier jumped on this information and after reviewing the data he found that if the object was similar in properties to Mercury, would be an average of 21 million miles from the Sun, would have a small diameter of 2600 kilometers and would have a year of 19.7 days, and if similar in make-up to Mercury would be about 1/17 Mercury’s mass. But Vulcan would also be at most about 8 degrees above/below the Sun, so viewing Vulcan could only happen at twilight. After visiting Lescarbault to verify that his viewing equipment wasn't at fault, Le Verrier began to use the Paris Observatory in tandem with his mathematical prowess to better solidify the range of the unknowns. It was during this that Le Verrier realized Vulcan was not massive enough to account for Mercury’s movement so he thought maybe more asteroids were present also. Regardless, it wasn't the object Le Verrier was looking for. He found how the perihelion of Mercury shifted by 565 arcseconds every 100 years, and so sought to see how much each major solar system body contributed to that. He found it all adds up to 526.7 arcseconds per 100 years, and published his results in Comptes Rendus on September 12, 1859. What was causing the remaining 38 or so arcseconds? He wasn't sure (Asimov, Weintraub 124, Levenson 65-77).
But the science community as a whole was so confident and thrilled in the work that it didn't matter if he solved the Vulcan situation; he was awarded the Gold Medal from the Royal Astronomical Society in 1876 for his Vulcan solution. Many expeditions went out and hunted for Vulcan but all they found were sunspots. The best chance for spotting an unknown object close to the sun would be an eclipse, and one occurred on July 29, 1878. Many astronomers around the world claimed to see two different objects at the event but they neither agree with each other nor with Le Verrier's work. As it turns out, they were stars mistaken for solar objects (Weintraub 125-7).
Telescopes by Le Verrier’s time had gotten much better but no signs of a planet were found despite Simon Newcomb’s finding that Mercury’s orbit was found to be off by 0.104 seconds of arc, implying that something should be there. However, those same calculations found that Le Verrier had some errors in his own work as well. But we cannot blame Le Verrier for any of his mistakes. He was working solely with Newtonian gravity. But we have Einstein’s relativity, and the mystery of the orbit was solved. As it turns out, Mercury is close enough to the Sun that it suffers frame dragging of the space-time fabric, a result of Einstein’s relativity, affecting its orbit when close to our star (Plait 36, Asimov, Weintraub 127).
But now the idea was planted in people’s minds. Could something be there? Or somethings? After all, Urbain said it was either a planet or some debris orbiting the Sun. Could there be tons of leftovers from the formation of the solar system between the Sun and Mercury, hidden from us by the intensity of the Sun? Other zones like between Mars and Jupiter and past Neptune are full of a group of objects, so why not this zone also? (Plait 35-6, Campbell 214)
To be clear, it is a very specific zone. If anything exists there, it cannot be too close to the Sun otherwise it would burn up but if it were too close to Mercury then that planet would capture it and the asteroids would collide with it. Some think the surface of Mercury already displays the evidence of this. Don’t forget the Yarkovsky effect, which deals with the heated versus cooled sides of an orbiting object exerting a net force away. Plus, erosion from the solar wind may have completely faded any material that was there, so models have to be tweaked constantly with new data to even show that Vulcanoids could have survived the 4.5 billion years post-solar system birth. But with these considerations in hand, a possible zone between 6.5-20 million miles from the Sun does exist. Altogether, it is a few quadrillion square miles to search (Plait 36, Campins 88-9, Stern 2).
Now, how big are Vulcanoids if they exist? Well they would have to be bigger than the average piece of space dust because the solar wind pushes that away from the Sun. In fact, something 100’s of meters would be affected by the solar wind. However, the Vulcanoids cannot be larger than 40 miles in diameter, for they would have been bright enough to be seen by now (Plait 36).
On top of those conditions, they would be spread out a max of 12 degrees of sky with the only chance of seeing them being at sunrise and sunset. One only has minutes a day to view under the best circumstances possible, and even then, you need software to remove the solar interference. On top of that, out atmosphere scatters light entering it, making it even more difficult to spot any Vulcanoids (36-7).
On The Hunt
The early hunt for Vulcanoids was first conducted with photographic plates during total solar eclipses when the Sun would be blotted out long enough for any nearby objects to be detected. Searches by Perrine in 1902, 1906, 1909; Campbell and Trumpler in 1923; and Courten in 1976 found nothing of large size but did not rule out asteroids as possibly being present (Campins 86-7).
From 1979 through 1981 astronomers at the Kitt Peak Observatory used the 1.3-meter telescope to look at a 9 to 12 degree stretch of sky from the Sun, approximately 6 square degrees in total. Based on the likely composition of Vulcanoids (mainly iron) and the brightness of the Sun at the orbital range of the Vulcanoids, the team was hunting for 5th magnitude objects which correspond to a minimum radius of 5 kilometers based on reflectivity models. Nothing was found but those in the study acknowledge the limited span of sky searched and felt nothing negated the possibility of Vulcanoids still (91).
But the new promise of infrared array detectors prompted a new search from Kitt Peak in 1989. Because of the heat-seeking nature of the technology, fainter objects would stand out better due to their heat near the Sun. Potentially, 6th magnitude objects could be seen. Alas, a downside of the detector was the long exposure rate of 15 minutes. Vulcanoids according to Kepler’s Laws of Planetary motion would move at about 5 arc minutes an hour and with the proximity of the field, being examined by the time the exposure was done anything could have moved out of frame and become diffused to the point of not being seen (91-2).
Alan Stern, the man behind the New Horizons mission, and Dan Durda have been looking for the objects for over 15 years now. They think that Vulcanoids are not only real but that we can actually image them directly without having a blob of light to study. To accommodate for the Earth’s atmosphere and the glare of the sun, they designed a special UV camera nicknamed VULCAM that can fly on an F-18 jet, which is capable of going over 50,000 feet. In 2002, they gave it a go but amazingly, the sun was still too bright to image anything around it, even when the attempt was made at twilight. So what about space cameras? Unfortunately, because sunrises and sunsets are the only way to see Vulcanoids combined with the fast rate, which objects orbit the Earth means that observing time is down to a few seconds. Beyond Earth, the Solar Dynamic Observatory, MESSENGER, and STEREO all looked but came up with nil (Plait 35, 37; Britt). So while the story seems to have its conclusion in hand, one never knows what could happen...
Asimov, Isaac. “The Planet That Wasn’t.” The Magazine of Fantasy and Science Fiction May 1975. Print.
Britt, Robert Roy. “Vulcanoid Search Reaches New Heights.” NBCNews.com. NBC Universal, 26 Jan. 2004. Web. 31 Aug. 2015.
Campbell, W.W. and R. Trumpler. “Search for Intramercurial Bodies.” Astronomical Society of the Pacific 1923: 214. Print.
Campins, H. et al. “Searching for Vulcanoids.” Astronomical Society of the Pacific 1996: 86-91. Print.
Levenson, Thomas. The Hunt for Vulcan. Pandin House: New York, 2015. Print. 65-77.
Plait, Phil. “Invisible Planetoids.” Discover Jul. /Aug. 2010: 35-7. Print.
Stern, Alan S. and Daniel D. Durda. “Collisional Evolution in the Vulcanoid Region: Implications for Present-Day Population Constraints.” arXiv:astro-Ph/9911249v1.
Weintraub, David A. Is Pluto a Planet? New Jersey: Princeton University Press, 2007: 123-7. Print.
Questions & Answers
© 2015 Leonard Kelley