FAQ

Vulcanoids and the Vulcanoid Zone

I found a Vulcanoid. Will you pay me for my data?

Question 1.1: What is a Vulcanoid?

A Vulcanoid is an object in a long-term stable orbit around our Sun whose mean distance to the Sun is less than that of Mercury’s. Mercury’s mean distance from the Sun is about 0.387 a.u. (about 57.9 million km).

Any object whose orbit is completely inside Mercury’s orbit would be a Vulcanoid. However, because Mercury’s orbit is elliptical, it is possible for an object to cross Mercury’s orbit and still be a Vulcanoid. For example, the following diagram shows a theoretical Vulcanoid orbit that crosses Mercury’s orbit part of the time:

Question 1.2: How many Vulcanoid have been discovered?

As of 15 October 2005, there are no known Vulcanoids.

Question 1.3: What is the stable Vulcanoid zone?

The stable Vulcanoid zone is the zone where Vulcanoids can reside in stable orbits for several Gyr (109) of years. The stable Vulcanoid zone lines between 0.08 a.u. (about 11.97 million km) and 0.18 a.u. (about 26.93 million km) from the Sun:

This location of the stable Vulcanoid zone is based on a revised model by Neil Wyn W. Evans (University of Oxford) and Serge A. Tabachnik (Princeton University Observatory).

Question 1.4: I read that the Vulcanoid zone extends out to 0.21 a.u., why do you use 0.18 a.u. as the outside stable limit?

According to Dr. Durda (Southwest Research Institute), Neil Wyn W. Evans (University of Oxford) and Serge A. Tabachnik (Princeton University Observatory) revised their model a changed the outside edge of the Vulcanoid zone from 0.21 a.u. to 0.18 a.u. They found that while objects out to 0.21 a.u. might be stable on the order of a 100 MYr, they are not stable for several Gyr. Any long term stable Vulcanoids that have remained in their orbit since the formation of our solar system (age > 4.5 Gyr) must reside inside the 0.18 a.u. limit.

Question 1.5: Why are orbits that closer to the Sun than 0.08 unstable?

Neil Wyn W. Evans (University of Oxford) and Serge A. Tabachnik (Princeton University Observatory) revised states that objects closer than 0.08 a.u. would be perturbed by extreme solar heating and dynamical transport mechanisms and would either push it away or pull it in to the Sun.

Objected very close to the Sun would become soft or even melt. Tidal forces of the Sun would very likely destroy such objects. Very small objects would be ejected by the force of the solar radiation and wind. Larger objects that did not melt would be subjected to the Yarkovsky effect causing them to migrate.

Question 1.6: Some grazing comets come very close to the Sun. Why aren’t they Vulcanoids?

All known grazing comets that survive their very close perihelion passage to the Sun have an aphelion that is much farther away. Their mean distance to the Sun is greater than that of Mercury. Some comets that pass close to the Sun eject fragments that temporarily orbit. Those fragments soon vaporize and/or fall into the Sun and therefore are not permanent residents of the Vulcanoid zone.

Question 1.7: I heard that object XYZZY approached the Sun closer than Mercury. Is it a Vulcanoid?

Just because an object passes inside Mercury’s orbit does not make it a Vulcanoid. It needs a stable orbit with a mean distance that is less than Mercury’s 0.387 a.u. (about 57.9 million km) to be a Vulcanoid. See Question 1.1: “What is a Vulcanoid?” for more details.

Section 2: Modern Vulcanoid Theory

Question 2.1: How many Vulcanoids are there?

While there are no known Vulcanoids as of 15 October 2005, there are potentially many Vulcanoids with a diameter > 1km that may exist.

In 2000, a team led by Dr. Durda (Southwest Research Institute) used images from the Large-Angle Spectroscopic Coronagraph aboard the Solar and Heliospheric Observatory (SOHO) spacecraft to conduct the most extensive search to date. They searched for Vulcanoids as faint as magnitude +8.0. Even though Durda’s team failed to find any Vulcanoids, the existence of fainter ones is clearly a possibility. If the largest Vulcanoids are just under Durda’s search limits, then the Dohnanyi power-law implies there could be as many as 1,800 to 42,000 Vulcanoids larger than 1 km!

See http://www.boulder.swri.edu/preprints/preprint.cgi?submit=Display&id=184 for more information on Dr. Durda’s SOHO search.

Question 2.2: What does a Vulcanoid look like?

Because (as of 15 Oct 2005), no Vulcanoids have been discovered, we do not know what they look like. Vulcanoids, if they exist at all, are likely to be:

• Mercury-like in reflectivity: Mercury reflects only 10% to 12% of sunlight. Vulcanoids, because of the Sun’s intense radiation, are likely as dark if not darker than Mercury.

• Mercury-like in color: Mercury is often described as “reddish-brown” in color. Vulcanoids are likely to favor the red end of the visible spectrum.

• Smaller than 60km in diameter: Dr. Durda’s (Southwest Research Institute) SOHO Vulcanoid search team did not find and Vulcanoids brighter than +8.0 magnitude. If Vulcanoids are Mercury-like in color and reflectivity, then they must be less than 60km in diameter. Anything larger would have been bright enough to be seen by the SOHO Large-Angle Spectroscopic Coronagraph C3.

Here is an artist’s concept of what a Vulcanoid might look like:

The artwork above depicts the Sun in Hydrogen-Alpha light. In visible light, it depicts a Vulcanoid in the foreground and Mercury (as a star-like object) in the distant background.

Question 2.3: Why do you believe that Vulcanoids exist?

To paraphrase the Astronomer Carl Sagan: Vulcanoids will exist or not exist regardless of if we believe in them or not. A better question to ask might be Question 2.4: “What evidence points to the existence of Vulcanoids?”.

Question 2.4: What evidence points to the existence of Vulcanoids?

We do not know if Vulcanoids exist. However, the following is a partial list of evidence points toward the existence of Vulcanoids:

• The revised model by Neil Wyn W. Evans (University of Oxford) and Serge A. Tabachnik (Princeton University Observatory) suggests that there is a zone of stable orbits where Vulcanoids can reside for 4 or more Gyr.See Question 1.3: “What is the stable Vulcanoid zone?” for mode details.

• The Mariner 10 probe photograph of Mercury shows that it was impacted with numerous objects. Mercury would have collided with only a small fraction of objects in the inner solar system. Some of those inner solar system objects that did not impact Mercury may have made it into the stable Vulcanoid zone.

• The Earth has been impacted by fragments blasted off the Moon and Mars when asteroids impacted those objects. The Earth only collided with a small fraction of such fragments. A number of such fragments may have been able to migrate all the way into the Vulcanoid zone, including possible fragments from collisions with Earth, Venus, and Mercury.

• The leading modern theory of how the Moon formed suggests that a Mars-sized planetoid collected with the proto-Earth some 0.5 Gyr after the proto-Earth formed. The resulting collision destroyed the proto-Earth. A significant portion of the combined mass would up in the Earth. A significant portion of the mass of the Moon came from of the mantle of the Mars-side planetoid. It is very possible that some material, perhaps a significant amount, escaped the gravity well of the impact and would its way into the inner solar system to become Vulcanoids.

• During the early formation period of the solar system, Mercury accreted material near the orbit in which it formed. It is possible that material closer to the Sun (particularly material inside and near the stable Vulcanoid zone) would have escaped accretion into Mercury. Like the asteroid belt between Mars and the massive Jupiter, primordial material between Mercury and the Sun may have been unable to accrete into a planetoid of significant size.

• Our current methods of detection of extra-solar planets favor detection of massive planets close to their stars and therefore present us with a biased solar system sample. Still the existence of planets close to their central stars suggests that material close to our Sucn is a possibility and still reside within the stable Vulcanoid zone today.

 

Question 2.5: What evidence points to the non-existence of Vulcanoids?

Despite evidence that points to the existence of Vulcanoids (see Question 2.4: “What evidence points to the existence of Vulcanoids?”) it is possible that Vulcanoids do not exist. The following is a partial list of evident against Vulcanoids:

• As of 15 October 2005, no known Vulcanoids exist.

• Based on the SOHO search by Dr. Durda, it is very unlikely that any Vulcanoid larger than 60km in diameter exists. The upper size limit could be a small as 20 km if we use the more reflective limit of asteroid belt members. There are about 200 asteroids with diameters larger than 97 km but none of them has been found within or even near the Vulcanoid zone.

• Recent models of the history of our solar system suggest that planets migrate. It is therefore possible that Mercury formed in the Vulcanoid zone, accreted or ejected the material in that zone and them migrated outward to it s present orbit leaving the stable Vulcanoid bare.

• It is possible that the revised model by Neil Wyn W. Evans (University of Oxford) and Serge A. Tabachnik (Princeton University Observatory) is too favorable to the existence of Vulcanoids. While their model was peer-reviewed and is now widely accepted, it is possible that our solar actually has no stable Vulcanoid zone.

• The early Sun may have been much more hostile to material in the inner solar system, ejecting or absorbing anything that Mercury did not sweep up.

• The Mars-sized planetoid that collided with the proto-Earth may have been in an elliptical orbit that accreted or ejected material in the Vulcanoid zone.

Question 2.6: Which is more likely: that Vulcanoids exist, or that Vulcanoids do not exist.

Based on the weight of the evidence, it is more like that Vulcanoids exist. Based on our current understanding of how our solar system was formed and evolved, and the weight of evidence on both sides, we would be very surprised if Vulcanoids do not exist.

Question 2.7: Why do you refer to Vulcanoids as asteroids? Why can’t a comet be a Vulcanoid?

A comet that stays close enough to the Sun to become a Vulcanoid would boil away within a short period. A comet that recently became a Vulcanoid would have a coma and tail that would certainly bright enough to be seen by SOHO, if not by observers during total solar eclipses. Therefore the surface of any Vulcanoid would lack any icy / frozen material.

An object that is purely rocky and/or metallic would be considered an asteroid or planet. Dr. Durda has established an upper bound of 60 km in diameter of Vulcanoids, so we may omit planets from the Vulcanoid category.

Having eliminated the possibility of Vulcanoid comets and Vulcanoid planets, the only category that remains is Vulcanoid asteroids.

 

Section 3: Searching for Vulcanoids

Question 3.1: Where is the most likely place to find Vulcanoids?

Vulcanoid asteroids are most likely to reside in the stable Vulcanoid zone (see Question 1.3: “What is the stable Vulcanoid zone?”), between 0.08 a.u. and 0.18 a.u. Dr. Durda has suggested that Vulcanoids are more likely to reside towards the outside edge of the stable Vulcanoid zone.

As seen from the Earth, the stable Vulcanoid zone is between 4.6° and 10.5° from the center of the solar disk. However, as Vulcanoids reach inferior conjunction or superior conjunction they can appear to be closer to the Sun than 4.6°. Therefore, from Earth, Vulcanoids in stable orbits reside in a disk that is up to 10.5° from the center of the solar disk.

Most of the mass in our solar system lies within a few degrees of the ecliptic. Vulcanoids are likely to favor the ecliptic. However, the planet Mercury, the closest planet to the stable Vulcanoid zone, is inclined from the ecliptic by about 7°. It is possible that Mercury can pull Vulcanoids out of the favored by as much as 10.5°, perhaps more. Stable Vulcanoid orbits with an inclination of 10.5° will appear to be within about 1.91° of the Ecliptic … close enough to call 2°.

During a total eclipse the solar corona, to well dark-adapted eyes has been known to extend out as far as 9 solar radii from the center of the solar disk. The glare of the solar corona makes it difficult to view objects fainter than magnitude +8.0 (the limit set by Dr. Druda’s SOHO search team). While the corona does extend 9 solar radii, beyond 2.5° it may be regarded as faint enough to not be a significant hindrance. Therefore as a rule of thumb, the zone within 2.5° of the center of the solar disk may be regarded as too dominated by the corona to be practical for Vulcanoid searching.

The following diagram gives the prime search area for Vulcanoids relative to the center of the solar disk:

 

Question 3.2: Why wait for a total solar eclipse to search for Vulcanoids?

The glare of the Sun makes it very difficult to search for faint objects close to the Sun. During the brief period of totality during a total solar eclipse, the moon blocks most of the glare from the Sun.

It is not mandatory to wait for totality to search for Vulcanoids. However, without very special equipment that can shield a detector from the Sun’s glare, is extremely difficult to search for Vulcanoids outside of totality. Even with special shielding, one must overcome the problem of daytime sky glow and/or the problem of the thick dusty light-extinguishing atmosphere near the horizon. For most people, a total solar eclipse is needed to view faint objects within 10.5° of the center of the solar disk

See Question 3.1: “Where is the most likely place to find Vulcanoids?”.

Question 3.3: How does one take pictures of Vulcanoids during an eclipse?

Step 0: Select a total Eclipse. See http://sunearth.gsfc.nasa.gov/eclipse/solar.html for detailed eclipse information. You need to select an observing site close the centerline of the shadow of totality. In some cases, it helps to join a group such as one organized by TravelQuest International (https://web.archive.org/web/20220331230304/http://www.tq-international.com/).

Step 1: Perform a “zenith test” well ahead of the eclipse making sure that you can image objects fainter than +8.0 magnitude under lighting conditions similar to that of a total solar eclipse. See Question 3.4: “How can one know if your setup can image a faint Vulcanoid?” for more information.

Step 2: Select your target position relative to the center of the solar disk. See Question 3.1: “Where is the most likely place to find Vulcanoids?” for more information on where to look.

Step 3: You need to practice finding your target position without resorting to the classic &”star-hop” method. Find the same position relative to the center of the disk of the full moon. You need to be able to reliably find your position under the stress and drama of the onset of totality. As the expression goes, “ractice makes perfect!”

Step 4: Pack your equipment so that it can survive intact and travel to your eclipse site. Bring along spare equipment, tools and supplies just in case.

Step 5: Arrive at your eclipse site well ahead of time to be able to setup and test your equipment carefully.

Step 6: Starting recording image(s) in your target area 4 seconds after the start of totality. Stop recording image(s) in your target area 4 seconds before the end of totality. One strategy is to take at least two images on either side of totality, as this will allow a single observer to look for Vulcanoid movement against the background of the stars. While your equipment is doing its work during totality, spend the time look at the eclipse and the world around you. It is also important to record:

• When the image was taken (to the second UTC)
• Where the Latitude and Longitude where the image was taken (to the arc second, or better if possible)
• The exposure time and other image exposure details

It is highly recommended that you become a minor planet observer. At a minimum, you need to be able to acquire known asteroids and know how to report their position. See Question 3.8: “How can one learn to make accurate verifiable observations?” for more information.

Step 7: Analyze your images looking for Vulcanoids. Consider publishing your images on the web so that others can look for possible Vulcanoid images.

Question 3.4: How can one know if your setup can image a faint Vulcanoid

Before you go to the eclipse, you need perform a “zenith test‚” A “zenith test” will allow you to practice taking images of faint objects against a totality-like sky without having to wait for a total solar eclipse. By taking images during the correct time at twilight, you can simulate the typical sky conditions present during most total solar eclipses.

When the center of the Sun’s disk is 4° and 6° below the ideal horizon, then the zenith will be as bright the sky is during your typical totality. The brightness of totality during a total solar eclipse can vary. When the SunSun’s center is 4° below the ideal horizon, then the zenith is as bright the some of the brightest totalities. When the Sun’s center is 6° below the ideal horizon, then the zenith is as dark the some of the darkest totalities.

Of course, you should conduct your “zenith test” in a cloudless sky that is not impacted by the light of the moon. Your local test site should have a similar level of light pollution to that of your intended eclipse site.

You should use an astronomical ephemeris software package such as Xephem (http://www.clearskyinstitute.com/xephem), Kstars (http://edu.kde.org/kstars), TheSky (http://www.bisque.com), RedShift (http://www.redshift.de/us/_main/index.htm), or other quality planetarium-like software to determine when the center of the Sun’s disk is 4° and 6° below the ideal horizon for your current location on a given day.

It is important to note that the ideal horizon has a special meaning (see the “ideal horizon” glossary entry). The ideal horizon ignores obstructions and takes into account the bending of light as it passes through the atmosphere.

Your exposure during a “zenith test” should not exceed the length of totality during which you plan to observe. To be safe, you should leave a 4 second margin of safety after totality starts and before totality, ends to avoid having the light from the Sun over-expose your image. So if totality will last 4m 3s at your eclipse site, then you should not take an exposure longer than 3m 55s. Depending on your setup, you may need to take an exposure that shorter than the full totality to avoid over-exposing / over-saturating your imager.

If your setup can record star images fainter than +8.0 magnitude during a “zenith test”, then you may have a setup that can image a Vulcanoid!

It is very important to note that one needs more than a setup that is capable of acquiring faint objects during totality. You must possess the skills needed to make accurate and verifiable observations.