Two giant planets have been found orbiting a tiny star, defying our theories for how planets are formed.
To be entirely truthful, there is nothing new in an exoplanet discovery shredding our current ideas about how planets are built. The first extrasolar planets ever discovered orbit a dead star known as a pulsar. Pulsars end their regular starry life in a colossal supernova explosion that should incinerate or eject any orbiting worlds. This discovery was followed a few years later by the first detection of a hot Jupiter; a gas giant planet orbiting its star in just a few days, defying theories that said such planets should form on long orbits where there is more building material to make massive worlds. Exoplanet hunting is a field full of surprises and now, it has one more.
GJ 3512 is a red dwarf star with a luminosity only around a thousandth (0.0016L☉) of our sun. The small size of these stars makes it easier to detect the presence of a planet, and many of our most famous exoplanet discoveries have been found orbiting red dwarf stars, including Proxima Centauri b and the seven worlds in the TRAPPIST-1 system. But a notable attribute of these systems is that the planets are small. Unlike our own sun which boasts four gas giant worlds, planets around red dwarfs are typically smaller than Neptune.
This preference for downsized worlds is assumed to be due to the protoplanetary disk; the disk of dust and gas that swirls around young stars out of which planets are born. Protoplanetary disks around small stars tend to be low mass and puffy. This limits and spreads out the solid material, making it difficult for a young planet to grow.
Yet the two planets discovered around GJ 3512 are not small.
Led by Juan Carlos Morales at the IEEC Institute of Space Studies of Catalonia, the announcement of the discovery was published in the journal Science today.
The team detected these two new worlds using the radial velocity technique which measures the wobble in the position of the star due to the gravitational tug of the orbiting planet. The method provides a minimum mass for the planet, as if its orbit is inclined with respect to our view from Earth then only the part of the star’s motion that is in our direction will be detected.
This causes us to underestimate the planet’s effect. The minimum mass for the innermost planet, GJ 3512 b, is 0.46 Jupiter masses; half the mass of the largest planet in our solar system and potentially with a true mass substantially larger. Orbiting further out seems to be a second planet, although its properties are harder to pin down. Estimates suggest the outer world must have mass larger than 0.17 Jupiters, or just over half the mass of Saturn.
If two giant worlds still seems paltry, the orbit of GJ 3512 b suggests there may have once been a third giant in the system. The innermost planet is not on a circular orbit, but loops the star on a strongly elliptical path. The degree of ellipticity for an orbit is referred to as the orbital eccentricity. The Earth’s orbit has an eccentricity of just 0.017, reflecting its nearly circular path around the sun. Our most eccentric planet is Mercury, with a modest 0.21. These low values are expected as the gas in the protoplanetary disc drags any developing elliptical orbits back towards circular paths. In defiance of this, GJ 3512 b has an orbital eccentricity of 0.44.
The most likely avenue for creating a bent orbit is that the planet was given a kick after formation. Such kicks can come from strong gravitational tugs between neighboring planets. One planet is then flung out of the system, while the other is shoved onto an elliptical orbit. To have created such a large eccentricity, the ejected shover must also have been a large planet, indicating that the tiny star was originally orbited by three gas giant worlds.
So how did two or three giant planets form in a small protoplanetary disc circling a red dwarf?
Previous exoplanet discoveries have pointed towards a range of exotic paths for planet formation, but most did not quite fit the situation. Discussing these options in the Supplementary Material for the paper, the discovery team debated whether these planets might have been stolen from another star. This is thought to have been the case for an old giant planet nicknamed Methuselah, that orbits around a binary of two dead stars. This gaseous world likely formed around only one of the stars but then both the star and planet were captured during a close encounter with the second star.
The problem with this scenario for GJ 3512 is that Methuselah’s system resides in a dense collection of stars known as a globular cluster. Having closely packed cluster members significantly increases the chance of close flyby between stars, allowing new configurations of stars and planets. By contrast, GJ 3512 sits in a more regular pocket of space where neighbouring stars are rare visitors.
Calculations performed by the discovery team suggest that for GJ 3512 to capture a planet, the original planet-hosting star must come within 1 au of the red dwarf; the average distance of the Earth from the sun. This is very unlikely, even during the early years of the star’s life when it might have formed close to other stars. Two planets would also have had to have been captured, reducing the probability even further.
So what if the planet did not form like a planet at all, but as a very small star? While planets form in protoplanetary disks, stars form through the fragmentation of cold clouds of gas in the galaxy. This often leads to binary or triple systems, where two or three stars forming from neighbouring fragments become bound together by their gravity so that they orbit one another.
However, while GJ 3512 b is large for a planet, it is small for a star. Even assuming the minimum mass is much lower than the true mass, the planet is unlikely to even be classified as a brown dwarf; stars that are not massive enough to burn hydrogen in their core but can fuse the heavier version of hydrogen, known as deuterium. A triple system of such tiny stars would also be very difficult to form due to their mutual gravity being very weak.
While the creation of a planet from a fragmenting star-forming cloud is unlikely, how about fragmenting the protoplanetary disk? If the protoplanetary disk broke apart into clumps, then these clumps could become gas giant planets orbiting the star. This method avoids the need to accrete the dispersed rocky material and favours large planet formation.
Such an idea is not new. It was first proposed as a method for giant planet formation by Alan Boss in 1997. However, subsequent computer simulations suggested that protoplanetary disks do not fragment in the region where planets orbit, and the light disks around red dwarfs should not fragment at all.
But this changes if the protoplanetary disk is very young. During its early years, the protoplanetary disk around GJ 3512 is expected to have been much heavier than during the usual planet formation period. Models run by the discovery team suggest that this early, heavy-weight disk could fragment beyond about 10 au, roughly Saturn’s distance from the sun. The two planets around GJ 3512 are far closer than this, orbiting at 0.34 au and somewhere beyond 1.2 au. So for this mechanism to work, the planets must have formed further out and then migrated inwards to their current location. Such a motion is plausible, as the tug of the remaining gas in the protoplanetary disc can cause young planets to be dragged inwards towards the star.
If this formation route is true, fragmentation of the protoplanetary disc might be the main formation mechanism for giant planets around low mass stars.
Discoveries such as the worlds of GJ 3512 smash and then expand our understanding of how planets can form. It seems there may be no limit to how diverse and alien the planetary systems we discover can be.
