Takayuki Muto of Kogakuin University discusses how ALMA has delivered images of the formation site of planetary systems similar to our Solar System.
Planets are thought to be formed in a disk orbiting around a young star, called a protoplanetary disk. Protoplanetary disks are composed of gas and dust. Somehow, micron-sized dust particles grow to 1,000km bodies, or planets, within the disk. This takes less than 10 million years, much shorter than the stellar age of 1 to 10 billion years.
Planet formation has been a long-standing question, and we had not been able to clearly see the site of planet formation before the Atacama Large Millimeter/submillimeter Array (ALMA) came to its full scientific operation, due to the relatively small size of the area we needed to observe. Observing 10 astronomical units (au; with 10 au corresponding to the diameter of Jupiter’s orbit around the Sun) at 500 light years (the typical distance to nearby young stars) is equivalent to seeing a person standing at 5,000 km away.
ALMA is the most sensitive astronomical ‘microscope’ of the Universe, using millimetre and sub-millimetre radio wave. It combines signals from up to 66 antennas to obtain the synthesised images of the Universe, providing us with the most detailed images of planet-forming regions around young stars.
Transitional disk
Among several classes of protoplanetary disks, a ‘transitional disk’ is one of the interesting targets that can be used to study planet formation. Transitional disks are a class of disk that have a dust-depleted inner region, or central cavity, around the star. Transitional disks are considered to be at the late stage of planet formation, where disk materials are being depleted. A planet forming in the disk is known to carve out a gap in the disk due to its gravity and therefore transitional disks are candidates of planet-forming disks, being in a transition from the early gaseous/dusty disk phase to a later, clearer planetary system.
The existence of a central cavity was inferred in the early 2000s by investigating the brightness of young stars at muti-wavelengths ranging from optical to infrared (Calvet et al. 2005). Some disk-bearing young stars were faint at short-wavelength (near infrared) emission but were bright at longer wavelengths. This means that the hot region of the disk, which is close to the central star, is missing while the cold outer component exists. The images of transitional disks were taken by pre-ALMA radio instruments, e.g., Submillimeter Array (SMA) at Hawaii, at low spatial resolution (several tens of au) in the early 2010s (Andrews et al. 2011), and indeed some of these images indicated there was an inner cavity.
DM Tau
One of interesting transitional disks is DM Tau, which resides about 500 light years away from our Sun. Early results of the observations of DM Tau system were puzzling. Multi-wavelength photometry indicated that the central cavity has approximately three au in radius, while SMA imagery indicates that the hole is as large as 20 au in radius. Yes, there is a hole, but of what size? If DM Tau hosts a planet, then where is it? Is the planet like our Earth, located at several au from the central star? Or is it like Neptune, located at several tens of au?
ALMA provides us with decisive observations of the DM Tau System (Kudo et al. 2018, Hashimoto et al. 2021). The data showed that both early predictions were correct – DM Tau is a multi-component system composed of three au cavity and 20 au ring, separated by a dust-depleted gap (see Fig. 1). Moreover, ALMA found tenuous radio emission spreading outside the 20 au ring, up to about 60 au from the central star. While the stellar mass is about half of our Sun, the structure of the protoplanetary disk around DM Tau shows striking resemblance to our Solar System, which has rocky planets and the asteroid belt at several au from Sun, icy planets at around 30 au, and the Edgeworth-Kuiper belt exterior to the icy planet region. DM Tau could be what our Solar System looked like several billion years ago.
Is the DM Tau system representative, or more generally, do many systems show structures similar to our Solar System? To answer this question, observations of structures at several au from the central star are the key. This means we need to push ALMA to its limit to observe many disks in the universe. So far, ALMA has imaged dozens of bright protoplanetary disks with high spatial resolution, including several tens of transitional disks.
An investigation of 38 transitional disks resolved with typically 10 au or less spatial resolution showed that about half of the sample have inner disks of 10 au in radius or smaller, in addition to a large outer ring of several tens to a hundred au in radius (Francis and van der Marel, 2020). Planet formation at au scale may be common, although some disks show very different picture of the inner disk from our Solar System. At least about half of the resolved inner disks are found to be significantly misaligned with the outer ring, unlike our Solar System where all the planets are almost aligned to the same plane. We need some mechanisms to have misaligned disks since rotating objects generally tend to keep their rotation axis in the same direction. Significant torque is required to make misaligned systems, but where does it come from? Is there already a planet in the system that exerts torque (e.g., Zhu 2019)? Or is it simply an observational bias since the sample is still biased towards bright disks with large outer rings?
The future
Observing au-scale structures of protoplanetary disks requires the further development of instruments. As can be seen in Fig. 1, the structure of the inner disk is blurred. Clear pictures of this region will answer how systems like our own Solar System are formed.
The future development of ALMA is currently being discussed, and increasing the spatial resolution is one possible direction (Carpenter et al. 2018). The Next Generation Very Large Array (ngVLA) is another future instrument that could potentially resolve Solar System scales of protoplanetary disks at wavelengths longer than ALMA, sensitive to dust particles down to about 1cm in size. New instruments and observations in the next generation will uncover the mysterious and particularly interesting region of au around young stars.
References
Andrews et al., 2011, ‘Resolved Images of Large Cavities in Protoplanetary Transition Disks’, The Astrophysical Journal, Vol. 732, id. 42
Calvet et al., 2005, ‘Disks in Transition in the Taurus Population: Spitzer IRS Spectra of GM Aurigae and DM Tauri’, The Astrophysical Journal, Vol. 630, pp. L185-L188
Carpenter et al., 2018, ‘ALMA Development Roadmap’, www.almaobservatory.org/en/publications/the-alma-development-roadmap
Francis and van der Marel, 2020, ‘Dust-depleted Inner Disks in a Large Sample of Transition Disks through Long-baseline ALMA Observations’, The Astrophysical Journal, Vol.892, id.111
Hashimoto et al., 2021, ‘ALMA Observations of the Asymmetric Dust Disk around DM Tau’, The Astrophysical Journal. Vol. 911, id.5
Kudo et al., 2018, ‘A Spatially Resolved au-scale Inner Disk around DM Tau’, The Astrophysical Journal Letters, Vol. 868, id.L5
Zhu, 2019, ‘Inclined massive planets in a protoplanetary disc: gap opening, disc breaking, and observational signatures’, Monthly Notices of the Royal Astronomical Society, Vol. 483, p.4221
ngVLA website: ngvla.nrao.edu
ALMA website: www.almaobservatory.org/en/home
