New Study Says Primordial Asteroid Belt was Empty

Between the orbits of Mars and Jupiter lies a disk of rocks, small bodies and planetoids known as the Main Asteroid Belt. The existence of this Belt was first theorized in the 18th century, based on observations that indicated a regular pattern in the orbits of Solar planets. By the following century, regular discoveries began to be made in the space between Mars and Jupiter, prompting astronomers to theorize where the Belt came from.

For a long time, scientists debated whether the Belt was the remains of a planet that broke up, or remnants left over from the early system that failed to become a planet. But a new study by a pair of astronomers from the University of Bordeaux has offered a different take. According to their theory, the Asteroid Belt began as an empty space which was gradually filled by rocks and debris over time.

For the sake of their study – which recently appeared in the journal Science Advances under the title “The Empty Primordial Asteroid Belt” – astronomers Sean N. Raymond and Andre Izidoro of the University of Bordeaux considered the current scientific consensus, which is that the Main Belt was once much more densely packed and became depleted of mass over time.

Artist’s impression of how the Asteroid Belt could have become filled with C-type and S-type asteroids over time. Credit: Sean Raymond/planetplanet.net

As Dr. Raymond explained to Universe Today via email:

“The standard picture is that the building blocks of the Solar System — what we call planetesimals, generally thought of as 10-100 km-scale bodies — started off in a smooth distribution across the Sun’s planet-forming disk. The problem is, that puts a couple of times Earth’s mass in the asteroid belt, where there is now less than a thousandth of an Earth mass. The challenge in this picture is therefore to understand how the belt lost 99.9% of its mass (but not 100%).”

To this, Dr. Raymond and Dr. Izodoro considered the alternate possibility that perhaps the primordial belt started as an empty space. In accordance with this theory, there were no planetesimals – i.e. Ceres, Vesta, Palla, and Hygeia – orbiting between Mars and Jupiter as there are today. This began as a thought experiment which, as Dr. Raymond admits, sounded a bit crazy at first.

However, he and Dr. Izodoro soon realized that several protoplanetary disks like the one they were envisioning had already been discovered in other star systems. For example, in 2014, the Atacama Large Millimeter/submillimeter Array in Chile photographed a planet-forming disk of dust and gas (aka, a protoplanetary disk) in the HL Tauri system, a very young star located about 450 light years away in the Taurus constellation.

As the image (shown below) revealed, the dust in this disk is not smooth, but consists of several broad regions and less dense regions. “The exact explanation for the structure in this disk is still debated but pretty much all models invoke drifting dust,” said Raymond. “And planetesimals form when drifting dust piles up into sufficiently-dense rings. So, dust rings should (we think) produce rings of planetesimals.”

Image of the HL Tau planet-forming disk taken with the Atacama Large Millimeter Array. Credit: ALMA (ESO/NAOJ/NRAO)

To test this hypothesis, they constructed a model of the early Solar System which included an empty Main Belt region. As they moved the simulation forward, they found that the formation of the disk was related to the formation of the rocky planets, and would gradually become what we see today. As Raymond indicated:

“What we found is that the growth of the rocky planets is not 100% efficient. A fraction of planetesimals is gravitationally kicked outward and stranded in the asteroid belt. The orbits of captured bodies matches closely those of S-type asteroids. The efficiency of implanting S-types in the belt is quite low, only about 1 in 1000.  However, recall that the belt is almost empty.  There is a total of about 4 hundred-thousandths of an Earth-mass in S-types in the present-day belt.  Our simulations typically implanted a few times that amount. Given that some are lost during later evolution of the Solar System, this matches both the distribution and amount of S-type asteroids in the belt.

They then combined this model with previous work which looked at the growth of Jupiter and Saturn and how this would effect the Solar System. In this study, they showed the C-type asteroids would be deposited in the Belt over time, and that these asteroids would also be responsible for delivering water to Earth. When they combined the distribution of implanted C-type and S-type asteroids with their current work, they found that it matched the present-day distribution of asteroids.

Interestingly enough, this is not the first theory Raymond and Izodoro have come up with to address the Asteroid Belt’s missing mass. Back in 2011, Raymond was a co-author on the study that proposed the Grand Tack model, in which he and his colleagues proposed that Jupiter migrated from its original orbit after it formed. At first, the planet moved closer to Mars’ current orbit, then back out towards where it is today.

Diagram comparing two possible explanations for how the Asteroid Belt formed. Credit: Sean Raymond/planetplanet.net

In the process, the asteroid belt would have been cleared, and Mars would have been deprived of mass, thus leading to its diminutive size – relative to Earth and Venus. This resolved a key problem with classical theories of Asteroid Belt formation, which was known as the “small Mars problem”. In short, all previous simulations of Solar planet formation tended to produce Mars analogs that were far more massive than Mars is today.

However, the Grand Tack hypothesis still contained theoretical uncertainties, which prompted Raymond and Izodoro to consider the the Empty Primordial Belt theory. “Our new result lends credence to an alternate model in which planetesimals never formed in the asteroid belt at all,” he said. “Different pieces of this new alternative model have been developed in recent years, and I think they add up to make a solid alternative to the Grand Tack model.”

Looking ahead, Raymond says that he and Izodoro hope to conduct further studies and simulations to see if either theory can be confirmed or falsified. “That’s the next step,” he said. “Until the next (seemingly-)crazy idea!”

Further Reading: Science Advances, PlanetPlanet

Huge Asteroid 324 Bamberga Makes a Return Visit to Earth’s Neighborhood on Friday the 13th

This week offers a fine chance to catch sight of a unique asteroid.

324 Bamberga reaches opposition this week in the constellation Pisces on (friggatriskaidekaphobics take note) Friday the 13th at 7AM EDT/11:00 Universal Time.

About 230 kilometres in size, 324 Bamberga reaches 0.81 astronomical units from the Earth this week. No other asteroid so large gets so close.

Discovered on February 25th, 1892 by Johann Palisa, 324 Bamberga only reaches a favorable opposition once every 22 years.

Shining at magnitude +8.1, 324 Bamberga is also one of the highest numbered asteroids visible with binoculars. Earth-crossing asteroids 433 Eros, which made a close pass last year, and 4179 Toutatis are two of the very few asteroids that possess a larger number designations that can regularly reach +10th magnitude.

Stellarium
Look east in mid-September about an hour after sunset. The inset covers the region that 324 Bamberga is currently traversing in the introductory graphic. (Created by the author using Stellarium).

So, why did it take so long for 324 Bamberga to be uncovered? One factor is its high orbital eccentricity of 0.34. This means that most of the oppositions of the asteroid aren’t favorable. 324 Bamberga orbits the Sun once every 4.395 years and only comes around to an opposition that lands near perihelion once every 22 Earth years. Perihelion this year occurs only 45 days after opposition on October 27th.

The resonance between 324 Bamberga and Earth is nearly five Earth orbits for every one circuit of the Sun for the asteroid and is offset by only 9 days, meaning that the 22 year window to see the asteroid will actually become less favorable in centuries to come. 324 Bamberga made its last favorable appearance on September 15th, 1991 and won’t surpass +10th magnitude again until September 2035.

Orbit
The orbit of 324 Bamberga. (Created using the JPL Small-Body Database Browser).

Observing asteroids requires patience and the ability to pick out a slowly moving object amidst the starry background. 324 Bamberga spends September west of the circlet of Pisces, drifting two degrees a week, or just over 17’ a day, to cross over into the constellation Pegasus in early October.

324 Bamberga will be moving too slow to pick up any motion in real time, but you can spy it by either sketching the field on successive nights or photographing the region and noting if the asteroid can be seen changing position against the background of fixed stars. Start hunting for 324 Bamberga tonight, as the Full Harvest Moon will be visiting Pisces later next week on the 19th.

Starry Night
A closeup of the path of 324 Bamberga for the week of September 10-17th. Decimal points for comparison stars are omitted. (Created by the author using Starry Night Education software).

324 Bamberga is also unique as the brightest C-type asteroid that is ever visible from Earth. The runner up in this category is asteroid 10 Hygiea, which can shine a full magnitude fainter at opposition.

It’s also remarkable that Palisa actually managed to discover 324 Bamberga while it was at 12th magnitude! Palisa was one of the most prolific visual hunters of asteroids ever, discovering 121 asteroids from 1874 to 1923. He accomplished this feat first with the use of a 6” refractor while based at the Austrian Naval Observatory in Pola (now the Croatian town of Pula) and later using the Vienna observatory’s 27” inch refractor.

The Great Refractor of the University of Vienna used to discover asteroid 324 Bamberga. (Credit: Prof. Franz Kerschbaum, Wikimedia Commons image under an Attribution-Share Alike 3.0 Unported license).
The Great Refractor of the University of Vienna used to discover asteroid 324 Bamberga. (Credit: Prof. Franz Kerschbaum, Wikimedia Commons image under an Attribution-Share Alike 3.0 Unported license).

324 Bamberga itself takes its name from the town of Bamberg in Bavaria, the site of the 1896 meeting of the Astronomische Gesellschraft.

An occultation of a star by 324 Bamberga on December 8th, 1987 allowed astronomers to pin down its approximate size. Searches have also been carried out during occultations for any possible moons of this asteroid, though thus far, none have been discovered.

It’s interesting to note that 324 Bamberga will also actually occult the star 2UCAC 3361042 tonight in the early morning hours at 8:59-9:10 UT for observers spanning a path from Florida to Oregon. The magnitude drop will, however, be very slight, as the star is actually 3 full magnitudes fainter than the asteroid itself. Dave Gee caught a fine occultation of a 7.4 magnitude star in the constellation Corvus by 324 Bamberga in 2007.

There’s also something special about this time of year and the region that 324 Bamberga is crossing. More visual discoveries of asteroids have been historically made in the month of September than any other calendar month. In fact, 344 of the first 1,940 numbered asteroids were found in September, more than twice the average. Palisa’s own track record bears this out, though 324 Bamberga was discovered in February.

One of the primary reasons for a September surge in discoveries is viewing direction. Astronomers of yore typically hunted for asteroids approaching opposition in the anti-sunward direction, which in September lies in the relatively star poor fields of Pisces.  In December and June —the months with the lowest numbers of visual discoveries at only 75 and 65 for the “first 1,940” respectively —the anti-sunward point lies in the star-rich regions of Sagittarius and Gemini. And by the way, the meteor that exploded over the city of Chelyabinsk on February 15th was sneaking up on the Earth from the sunward direction.

Be sure to catch a glimpse of this unique asteroid through either binoculars or a telescope over the coming weeks. The next chance to observe 324 Bamberga won’t roll around again until September 2035… it’ll be great to compare notes of the 2013 apparition on that far off date!