Title: A hypothesis for the rapid formation of planets
Authors: Susanne Pfalzner and Michele T. Bannister
First Author Institution: Max Planck Institute for Radio Astronomy; Bonn, Germany
Status: Accepted to ApJL, open access
An artist’s impression of ‘Oumuamua, reflecting its odd shape. Credit: ESO/M
If you were watching the astronomical news back in 2017, you may have heard of ‘Oumuamua. That’s the weird interstellar object (ISO) that was found floating around in our solar system and is the first intruder we have ever seen. It was initially identified as a comet, then an asteroid, then neither, and then according to the media, an alien ship. Potential alien spacecraft or innocent space boulder, this was a big discovery.
‘Oumuamua sparked interesting theories in the minds of today’s authors, and ultimately lead to, you guessed it, a new hypothesis for the rapid formation of planets. How could ISOs have anything to do with planet formation? Well, that’s what Susanne Pfalzner and Michele Bannister set out to tell us.
The General Suspect
You may be asking yourself why we need a new hypothesis – do the ones we have not work? The authors would argue that they do, but not on the timescale that we see in some systems. The typically referenced theory of planet formation is dust accretion, where planets form from the protoplanetary disk as a result of dust particles sticking together. This process is thought to take about 1-10 million years for a terrestrial sized planet to form, and even longer if you want to form a gas giant. However, we have observed planets around stars on the ~million year old scale (for example V830 Tau and potentially HL Tau).
The proposed process of dust accretion has additional issues, like growth barriers that keep dust particles from sticking together and growing to a certain size (the bouncing, drift, and fragmentation barriers affect dust grains at different velocities and sizes, namely ~1mm and ~1m sized grains). These growth barriers can prolong the timescale of planet formation. How then could we have planets form so quickly around stars so young?
Figure 1 (in the paper): A schematic of how ISOs are formed from their host star over the course of its life.
ISOs to the Rescue
Today’s authors suggest that ISOs may be the culprit. ISOs from previously formed systems should already be floating around in the interstellar medium (ISM) – they were likely once planetesimals that got ejected from their home by interactions with a nearby star or giant planet. ISOs can be captured by molecular clouds (MCs), the birthplace of stars, if their velocity is not too fast relative to the cloud’s velocity. Eventually, molecular clouds form clumps of gas and dust that collapse under gravity, forming stars and their accompanying protoplanetary disks. The authors propose that ISOs in the molecular clouds should also collapse with the clumps, and be present in the disk with the star as it forms.
According to years of PanSTARRS data, the authors adopt a minimum ISO density of 1015 pc-3. The paper explores all the reasons why in an area of star formation, this density could be a lot larger, depending on velocity distributions of ISO stellar hosts (in relation to said MC), or how the ISOs collapse into the clump. However, they stick with the minimum constraint for safe measure.
The standard star formation efficiency is about 10-30%, so let’s say that 30% of the ISOs in a MC collapse with the clump and buddy-up with the forming star. The protoplanetary disk forms with the star, and it is suggested that ISOs should act similarly to the gas of the disk. From its protoplanetary disk phase, the final disk has about 1-10% of its initial gas mass – some get sucked into the star or ejected out of the system. If we assume the same for ISOs, that still leaves ~108 ISOs left in the final disk, available as planetesimal seeds. And even if only a fraction of these survive in the system… that’s still a lot of space rocks.
Could It Be So?
Figure 2 (in the paper): Estimates of ISOs available per star. The conservative limit is the density used in this paper’s calculations, and this figure shows how many more may actually be available.
Could these foreign ISOs really make any difference as a star system is whipping up planets? Based on calculations of Moro-Martın et al. (2009), the authors estimate the size distribution of ISOs in this final disk. They find that most ISOs should be large enough to survive in the disk long enough to aid in planet formation. From those 108 remaining ISOs, the authors expect size distributions of ~107 100m-sized ISOs, at least 104 km-size ISOs, and ~1,000 ISOs that reach a size of 100km. By starting with such large planetary seeds, the growth barriers are no longer an issue. Plus, these 100km sized ISOs could be the seeds for rapid pebble accretion, which have been modeled to form terrestrial sized planets in only thousands of years.
If ISOs are speeding up planet formation around new stars, this implies that planet formation was likely a much slower process for the first stars in the galaxy. These original stars didn’t have any ISOs to speed things up – this could cause the observed dearth of planets in globular clusters and other very old stars. As for second generation stars, we see that even with the lowest density estimations of ISOs present in the ISM (or a potential MC), there should still be plenty around to help form planets after a star is born.
The presence of these ISOs could be speeding up the planet formation process, bypassing growth barriers and forming the planets that we observe around very young star systems. This is not to say the growth barriers cannot be passed, or that the old theories of dust accretion need to be thrown out the window. Rather, if you want to form a planet very quickly, you likely need the help of ISOs. With this new hypothesis, it seems possible that even our own solar system could have been formed from alien building blocks.
