New observations of a youthful binary star system, reported today in the journal Nature, may help to explain one of exoplanetary science’s greatest unanswered questions – the peculiar orbits of so many recently found planets.
Twenty years ago we thought we understood how planetary systems formed, based on a total of one planetary system known (our own).
A disk of material, circling around a young star’s equator, would gradually accrete to form a family of planets, all orbiting in roughly the same equatorial plane – producing rocky “terrestrial” planets in the inner reaches and gaseous “giant” planets further out.
But then came the discovery of the first exoplanets, which immediately forced astronomers to revisit their best story of planet formation. Over the two decades since, we have come to realise that the structure of our solar system is far different to the layout of many other planetary systems.
Peculiar orbits, bizarre worlds
In recent years, many planets have been found moving on highly unusual orbits – some that are highly inclined to their host star’s equators and others on orbits so eccentric that they are more reminiscent of our solar system’s comets than the planets themselves.
This presents a problem, because although astronomers have refined their models of planet formation, one key thing remains constant.
For planets to form, a star must be encircled by a dusty disk, known as a protoplanetary disk, formed as a natural result of the conservation of angular momentum.
As material spirals inwards towards a protostar, it collapses to form a disk that, by its very nature, must be aligned with the star’s equator.
Herein lies the problem. If planets form from such a disk, how are they perturbed to orbits that are highly inclined to their star’s equatorial plane?
And since the material in the disk should follow near-circular orbits, how are the planets scattered to such great eccentricities?
Double trouble
Today’s release of new observations of the binary star system, HK Tauri (located in the constellation Taurus) offers fresh evidence in support of the leading theory put forth to explain such planetary oddballs.
The system consists of two very young stars, less than four million years old, that are in the process of building planets. Indeed, it was already well known that one of the stars, HK Tauri B, has a protoplanetary disk that lies almost edge on to our line of sight.
The alignment is so good that the star is appreciably dimmed, as the edge-on disk partially obscures it from our sight.
Astronomers using the Atacama Large Millimetre Array (ALMA), in northern Chile, have made the most detailed observations yet of the system, revealing that HK Tauri A also hosts a protoplanetary disk. Unlike that orbiting HK Tauri B, however, the disk is definitely not seen edge on.
The exquisite resolution available using ALMA shows conclusively that the two disks are orientated in very different directions. In fact, the disks are most likely misaligned by between 60 and 70 degrees – a huge discrepancy.
Therefore, at least one of the disks must be significantly inclined in relation to the orbital plane of the binary system. In fact, statistically speaking, it is highly likely that both disks are misaligned, compared to the orbits of their host stars about one another.
Unfortunately, it is not possible to directly determine the orbital plane of the binary system itself, because the two stars in the HK Tauri system are separated by at least 380 times the distance from the earth to the sun.
It therefore takes thousands of years for the stars to orbit around their common centre of gravity and the stars simply haven’t moved far enough since their discovery to reasonably estimate the plane of their orbit.
But if the disks are misaligned, then so too will be the planets that form within them. This sets the stage for a wonderful celestial dance that will play out in the coming millions of years.
The role of unseen companions
Modelling shows that if you have a widely separated binary star, planets can form around one or even both components (just as is happening around HK Tauri A and B).
A planet orbiting one star will continually feel the gravitational pull of the other, tweaking and nudging its orbit – just as the earth’s orbit is continually nudged and tweaked by the other planets in our solar system, driving the Milankovitch cycles.
But what about the case of HK Tauri, where a planet’s orbit around its star is not well aligned with the orbital plane of the binary system?
In this case, the nudges and tweaks of the distant companion star can force the planet’s orbit to gradually become ever more eccentric. At the same time, the planet’s orbit will be pushed ever closer towards matching the orbital plane of the binary system.
In other words, the planet’s orbit experiences an increase in eccentricity and a decrease in inclination (relative to the orbital plane of the two stars).
Eventually, the process reverses, driving the planet’s orbit back towards one that is more circular and pushing the plane of the orbit away from that of the two stars and back toward the plane of its host’s equator.
A smoking gun
Over periods of thousands or tens of thousands of years, the planet’s orbit can repeatedly tilt, as it also flexes back and forth – a process known as the Lidov-Kozai mechanism.
This provides a remarkably good explanation for exoplanets with highly inclined and eccentric orbits.
But until now the theory has been difficult to test, since the multiplicity of most planet host stars remains unknown. Faint, unseen companions are notoriously hard to find and their orbital planes are almost impossible to swiftly determine.
The misaligned protoplanetary disks of HK Tauri definitively show that conditions do exist to allow such mechanisms to come into play and modify a planet’s orbit.
Of even more interest, the observations show that such conditions are established early on, while the planets are still accreting, as part of the process of forming the binary system itself.
Over the coming millions of years – a heartbeat in the lifetime of stars – the young planets now forming in the HK Tauri system will undergo their chaotic celestial dance. Eventually, any survivors will likely move on orbits far different to those within our solar system and another celestial oddity will have been born.