It is widely accepted that our Solar System was created about 4.6 billion years ago based on dating of meteorites. Scientists believe the Solar System formed when a cloud of dust was disrupted by the shockwave from the explosive death of a nearby star initiating its collapse into a stellar nursery. One of the protostars in the nursery, imbedded in its spinning protoplanetary disc, would one day be our own Sun. However what exactly happened next? How did the planets, so familiar today form?
A recent hypothesis, which describes how once upon a time the Solar System would have looked very different from it does now, is known as the “Grand Tack theory” and is becoming popular among scientists. The paper theorizing this idea was written by Avi Mendall, Allessandro Morbidelli, Sean Raymond and David O’Brien. This theory was developed to explain the Solar System we observe now. This has the four terrestrial planets orbiting between 0.4 and 1.5 astronomical units of the Sun (1au=150 million km). Mars, half the diameter of Earth, is the outermost of these rocky worlds. Between Mars and outer gas giant planets lies the assorted debris of the Asteroid Belt, densest at 2.8 au, while Jupiter now steadily orbits about 5.2au from the Sun. Unfortunately simulations of the early Solar System had been failing to replicate the reality, commonly predicting a solar system at odds with reality. Common feature predicted in computer models include additional planets inside the orbit of Mercury, an Earth-sized planet where the diminutive Mars ought to be and an Asteroid Belt full of Mars-sized worlds. The Grand Tack Theory offers another explanation more in harmony with reality.
The most important planet in this tale of the distant past is Jupiter, which danced a planetary do – si – do, aiding the formation of its fellow planets and the Asteroid Belt. The theory is known as the Grand Tack as according to it, during the first 10 million years of the Solar system’s existence Jupiter migrated inwards toward the Sun before turning and coming back, sweeping away all debris that fell into its path. This change in direction is similar to the sailing manoeuver calling “tacking”.
According to this theory, Jupiter originally formed in a region more than three times the distance from the Sun as the Earth is now (3au from the Sun). Jupiter had been the first planet to form and lots of gas and dust still remained around the Sun from the protoplanetary disc. In fact the mass of material was much greater than the mass of the planets, so dense was it that Jupiter was subject to hydrodynamic effects as well as gravity it orbited the infant Sun, so Jupiter was caught in the currents of the swirling gas. Drag from this tenuous medium caused it to spiral inwards towards the Sun, just as our planet’s atmosphere causes the orbital decay of satellites in low Earth orbit. Giant Jupiter manoeuvred towards the Sun, its mighty influence disrupting any inner Solar System planets forming at the time. Jupiter absorbed their debris, growing to approach its current size in the process.
The giant planet’s fate might have been to fall into the Sun, but it was saved by mighty outflows of gas in the protoplanetary disc. Jupiter eventually settled into an orbit roughly where Mars is now at around 1.5 au from the Sun. None of the current terrestrial planets can have formed in this early period.
According to Avi Mendell, a planetary scientist at NASA’s Goddard Space Flight Center and co-author of the Grand Tack paper, Jupiter stopped moving towards the Sun and avoided a fiery demise because of Saturn’s formation. Saturn too was gradually falling sunwards after its formation, but more slowly than its larger sibling. Eventually Saturn reached an orbit a couple of astronomical units from the Sun. Eventually the two giants came to a stop and their motions were reversed. Now the planets were moving outward until Jupiter reached its current position of 5.2au from the Sun and Saturn reached its position of 9.5au. Behind them they left an inner Solar System depleted of the materials for building planets, but with just enough to form the four terrestrial planets (and Earth’s Moon) that we find there today.
This theory not only explains the formation of Jupiter but can also support the idea that Jupiter helped to form the Asteroid Belt. Jupiter’s gravitational influence prevented the rocky material from forming together to make a planet. Instead Jupiter swept the area of objects as it moved closer to the Sun, the asteroid belt was pushed further out and Jupiter made more of a switching motion with these loose materials. As Jupiter moved further out and into the colder regions of the Solar System, this gas giant deflected some icy materials towards the Sun which also joined the Asteroid belt.
One major justification for developing the Grand Tack Theory is the unexpectedly small size of Mars. Theoretically Mars should be bigger as it formed further out from the Sun than the Earth and Venus. However until now it has seemed inexplicably smaller, but if Jupiter did spend time in the inner Solar System then it would have scattered the material available for making planets, thus explaining the undersized Mars we see today.
Venus and Earth would have had the best pickings when it came to material for forming a planet as they are relatively close to the Sun. Another widely-accepted theory of the formation of the Solar System, the delightfully-named Nice model neatly ties in with the Grand Tack theory as they both support each other. The Nice model (so called as it was developed by researchers at the French city of that name) claims a close relationship between Jupiter and Saturn, which led to the rearrangement of the Solar System 600 million years after its formation, the production of the Oort cloud and the existence of smaller bodies in the Kuiper Belt. The Nice model also looks as at the dissipation of the protoplanetary disc whilst incorporating the Late Heavy Bombardment, all of which fits nicely with the Grand Tack theory and further supports its claims.
With new planetary systems being discovered in the universe, the Grand Tack theory is supported further. Large gas planets are being found close to their host stars, even closer than Mercury is to the Sun and these are being nicknamed ‘hot Jupiters.’ These new discoveries are comforting to the authors and supporters of the Grand Tack theory as it helps confirm what they believed happened in those dramatic early days of the Solar System.
(Article by Samantha Steed, Education Support Officer)