Catching Planets in the Womb

Young stars have a disk of gas and dust around them called a protoplanetary disk. Credit: NASA/JPL-Caltech

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Awhile ago I wrote on the difficulty of finding young planets. There, I mentioned one team announcing the potential discovery of a planet a mere 1-5 million years old. But what are astronomers to do if they want to find even younger planets?

The chief difficulty in this instance is that such planets would still be hidden in the circumstellar disks from which they formed, hiding them from direct observation. Additionally, depending on how far along the process had advanced, they may not yet have accreted sufficient mass to show up in radial velocity surveys, if such surveys could even been conducted with interference from the disc.

One way astronomers have proposed to detect forming planets is to observe their effects on the disc itself. This could come in a number of ways. One would be for the planet to carve out grooves in the disc, clearing its orbit as it sweeps up matter. Another possibility is to look for the “shadows” caused by the local overdensity an accreting planet would cause.

But recently, another new method caught my eye. In this one, proposed by astronomers at the Crimean National Observatory in the Ukraine, astronomers could potentially look for again turns to the characteristics of the parent star. Earlier, astronomers had made a link between the properties of the disc around classes of protostars (such as T Tauri and Herbig Ae stars) and the variable luminosity of the star itself.

The authors suggest that, “[t]wo different mechanisms can be involved in interpretation of these results: 1) circumstellar extinction and 2) accretion.” In either scenario, a body present in the disc itself concentrating the material would be necessary to explain these results. In the first case, a protoplanet would draw a swarm of material around it again creating a local overdensity in the disc which would be dragged around with the planet, creating a dimming of the star as it passed near the line of sight. In the second, the planet would draw out tidal structures in the disc in much the same way tidal interactions can draw out spiral structure in galaxies. As these veins of matter fall onto the star, it feeds the star, temporarily causing an outburst and increasing the brightness.

The team conducted an analysis of periodicity in several protostellar systems and found several instances in which the periods were similar to those of planetary systems discovered around mature stars. Around one star, V866 Sco, they discovered, “two distinct periods in light variations, 6.78 and 24.78 days, that persist over several years.” They note that the shorter period is likely “due to axial rotation of the star” but could not offer an explanation for the longer period which leaves it open to the possibility of being a forming planet and they suggest that spectral observations may be possible. Other systems the team analyzed had periods ranging from 25 – 120 days also hinting at the possibility for young planetary systems.

The advantage to this method is that finding candidate systems can be done relatively easily using photometric systems which can survey great numbers of stars at once whereas radial velocity measurements generally require dedicated observations on a single object. This would allow astronomers to discriminate against candidates unlikely to harbor forming planets. Ultimately, finding young systems with forming planets will help astronomers understand how these systems form and evolve and why our own system is so different than many others found thus far.

Astronomy Without A Telescope – No Metal, No Planet

The spiral galaxy NGC 4565, considered a close analogue of the Milky Way and with distinctly dusty outer regions. Credit: ESO.

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A Japanese team of astronomers have reported a strong correlation between the metallicity of dusty protoplanetary disks and their longevity. From this finding they propose that low metallicity stars are much less likely to have planets, including gas giants, due to the shorter lifetime of their protoplanetary disks.

As you are probably aware, ‘metal’ is astronomy-speak for anything higher up the periodic table than hydrogen and helium. The Milky Way has a metallicity gradient – where metallicity drops markedly the further out you go. In the extreme outer galaxy, about 18 kiloparsecs out from the centre, the metallicity of stars is only 10% that of the Sun (which is about 8 kiloparsecs – or around 25,000 light years – out from the centre).

This study compared young star clusters within stellar nurseries with relatively high metallicity (like the Orion nebula) against more distant clusters in the outer galaxy within low metallicity nurseries (like Digel Cloud 2).

The study’s conclusions are based on the assumption that the radiation output of stars with dense protoplanetary disks will have an excess of near and mid-infra red wavelengths. This is largely because the star heats its surrounding protoplanetary disk, making the disk radiate in infra-red.

The research team used the 8.2 metre Subaru Telescope and a procedure called JHK photometry to identify a measure they called ‘disk fraction’, representing the density of the protoplanetary disk (as determined by the excess of infra red radiation). They also used another established mass-luminosity relation measure to determine the age of the clusters.

Graphing disk fraction over age for populations of Sun-equivalent metallicity stars versus populations of low metallicity stars in the outer galaxy suggests that the protoplanetary disks of those low metallicity stars disperse much quicker.

Left image - The Subaru Telescope in Hawaii. Credit: NAOJ. Right image - the relationship between disk persistence for low metallicity stars (O/H = -0.7, red line) and stars with Sun-equivalent metallicity (O/H = 0, black line). The protoplanetary disks of low metal stars seem to disperse quickly, reducing the likelihood of planet formation. Credit: Yasui et al.

The authors suggest that the process of photoevaporation may underlie the shorter lifespan of low metal disks – where the impact of photons is sufficient to quickly disperse low atomic mass hydrogen and helium, while the presence of higher atomic weight metals may deflect those photons and hence sustain a protoplanetary disk over a longer period.

As the authors point out, the lower lifetime of low metallicity disks reduces the likelihood of planet formation. Although the authors steer clear of much more speculation, the implications of this relationship seem to be that, as well as expecting to find less planets around stars towards the outer edge of the galaxy – we might also expect to find less planets around any old Population II stars that would have also formed in environments of low metallicity.

Indeed, these findings suggest that planets, even gas giants, may have been exceedingly rare in the early universe – and have only become commonplace later in the universe’s evolution – after stellar nucleosynthesis processes had adequately seeded the cosmos with metals.

Further reading: Yasui, C., Kobayashi, N., Tokunaga, A., Saito, M. and Tokoku, C.
Short Lifetime of Protoplanetary Disks in Low-Metallicity Environments

Astronomy Without A Telescope – The Nice Way To Build A Solar System

When considering how the solar system formed, there are a number of problems with the idea of planets just blobbing together out of a rotating accretion disk. The Nice model (and OK, it’s pronounced ‘niece’ – as in the French city) offers a better solution.

In the traditional Kant/Laplace solar nebula model you have a rotating protoplanetary disk within which loosely associated objects build up into planetesimals, which then become gravitationally powerful centres of mass capable of clearing their orbit and voila planet!

It’s generally agreed now that this just can’t work since a growing planetesimal, in the process of constantly interacting with protoplanetary disk material, will have its orbit progressively decayed so that it will spiral inwards, potentially crashing into the Sun unless it can clear an orbit before it has lost too much angular momentum.

The Nice solution is to accept that most planets probably did form in different regions to where they orbit now. It’s likely that the current rocky planets of our solar system formed somewhat further out and have moved inwards due to interactions with protoplanetary disk material in the very early stages of the solar system’s formation.

It is likely that within 100 million years of the Sun’s ignition, a large number of rocky protoplanets, in eccentric and chaotic orbits, engaged in collisions – followed by the inward migration of the last four planets left standing as they lost angular momentum to the persisting gas and dust of the inner disk. This last phase may have stabilised them into the almost circular, and only marginally eccentric, orbits we see today.

The hypothesized collision between 'Earth Mk 1' and Theia may have occurred late in rocky planet formation creating the Earth as we know it with its huge Moon of accreted impact debris

Meanwhile, the gas giants were forming out beyond the ‘frost line’ where it was cool enough for ices to form. Since water, methane and CO2 were a lot more abundant than iron, nickel or silicon – icy planetary cores grew fast and grew big, reaching a scale where their gravity was powerful enough to hold onto the hydrogen and helium that was also present in abundance in the protoplanetary disk. This allowed these planets to grow to an enormous size.

Jupiter probably began forming within only 3 million years of solar ignition, rapidly clearing its orbit, which stopped it from migrating further inward. Saturn’s ice core grabbed whatever gases Jupiter didn’t – and Uranus and Neptune soaked up the dregs. Uranus and Neptune are thought to have formed much closer to the Sun than they are now – and in reverse order, with Neptune closer in than Uranus.

And then, around 500 million years after solar ignition, something remarkable happened. Jupiter and Saturn settled into a 2:1 orbital resonance – meaning that they lined up at the same points twice for every orbit of Saturn. This created a gravitational pulse that kicked Neptune out past Uranus, so that it ploughed in to what was then a closer and denser Kuiper Belt.

The result was a chaotic flurry of Kuiper Belt Objects, many being either flung outwards towards the Oort cloud or flung inwards towards the inner solar system. These, along with a rain of asteroids from a gravitationally disrupted asteroid belt, delivered the Late Heavy Bombardment which pummelled the inner solar system for several hundred million years – the devastation of which is still apparent on the surfaces of the Moon and Mercury today.

Then, as the dust finally settled around 3.8 billion years ago and as a new day dawned on the third rock from the Sun – voila life!