Object 108: PDS 70
Podcast release date: 02 October 2023
Right ascension: 14:08:10.2
PDS 70 is a reddish protostellar object (or, in other words, a star still in the process of forming) located at a distance of 366.6 light years (112.4 pc) in the direction of the constellation Centaurus [1, 2]. The star is still forming, but its current mass is estimated to be 0.76 times the mass of the Sun , which means that, when it finishes forming, it will probably be very similar to the Sun.
PDS 70 belongs to a class of stars called T Tauri stars, which was named after the star T Tauri. These protostellar objects are just beginning to emerge from the disks of gas and dust that they formed from, but their outer layers are still in the process of collapsing, and they are still surrounded by large amounts of gas and dust. T Tauri stars are generally found together in molecular clouds and star formation complexes; PDS 70 itself lies within a region named the Scorpius-Centaurus Association [3, 4].
One way to identify T Tauri stars is to look for stars that appear to produce too much infrared emission, which would potentially be coming from dust within the disk of material surrounding the protostar. PDS 70 was identified as a potential T Tauri star in 1992 by people working with data from the Infrared Astronomical Satellite (IRAS) . IRAS was a spacecraft that produced blurry infrared maps of the entire sky in the 1980s, and while almost everything observed by IRAS (including PDS 70) looked like smudges, the spacecraft was still quite valuable for identifying bright infrared objects, and indeed, it identified that PDS 70 was a potential T Tauri star based on the excess infrared emission it produced. In the decade afterwards, multiple other surveys and follow-up observations confirmed the PDS 70 was a T Tauri star with an extended disk of gas and dust. By the way, the letters PDS in the name of PDS 70 come from the Pico dos Dias Observatory, which was the first telescope used to make some of these follow-up observations .
However, PDS 70 turned out to be rather unusual for a T Tauri star. First of all, multiple follow-up observations, including observations at near-infrared wavelenegths with the Very Large Telescope and Subaru Telescope and at millimeter and submillimeter wavelengths with the Submillimeter Array and the Atacama Large Millimeter/submillimeter Array, showed that the disk had a very large gap on the inside [3, 6, 7, 8, 9]. In other words, the disk looked like a flattened ring. This gap has a diameter of roughly 40 AU, where 1 AU is defined as the distance from the Earth to the Sun . Note that this edge is not really that well defined, but even so, everything in our Solar System within the orbit of Neptune would easily fit inside this gap in PDS 70's dust disk.
Additionally, in images from the Very Large Telescope published in 2006, an object that looked like a brown dwarf was found orbiting at a distance of 22 AU from the protostar within the inner gap of the dust disk [1, 10]. This brown dwarf was given the name PDS 70b because the International Astronomcial Union requires astronomers to give boring names to these types of objects. Brown dwarfs are basically objects with masses between multiple times the mass of Jupiter and 0.08 times the mass of the Sun that are too small to fuse hydrogen into helium in their cores and too large to really count as planets. Although they are expected to be fairly common, they are difficult to find, mainly because they are relatively cool and produce little electromagnetic radiation. So, PDS 70 looked like a star system where a brown dwarf is in the process of forming around a larger star, which would make it quite exciting as an astronomical object to study.
And that is my summary of why PDS 70 is so interesting to astronomers, or it would be except that the initial estimates of the mass of PDS 70b turned out to be way too high. Multiple follow-up observations indicate that PDS 70b is somewhere between 2 and 10 times the mass of Jupiter [10, 11, 12, 13], which would make it large compared to planets in our Solar System but which makes it much moere like a planet than a brown dwarf. So, PDS 70b is actually a gas giant in the process of forming. Additionally, it looks like this gas giant has its own disk of gas and dust surrounding it . In other words, it looks like PDS 70b is still absorbing (or, to use the technical term, accreting) gas, and some of the leftover gas in this circumplanetary disk might even go on to form moons. This protoexoplanet provides astronomers with a unique opportunity to see how gas giants form and how they affect other objects in the planetary system in which they are forming.
And that is my summary of why PDS 70 is so interesting to astronomers, or it would be except that another protoexoplanet was discovered in the PDS 70 system in 2019 in follow-up observations of the star system by the Very Large Telescope . This object, named PDS 70c, orbits at a distance of 34.5 AU from the protostar , which places it much closer to the outer edge of the gap in the dust disk. The mass of this second protoplanet has not been very well constrained, but it looks like it's between the mass of Jupiter and 12 times the mass of Jupiter [12, 14]. PDS 70c also has its own circumplanetary disk and is also in the process of gaining matter from this disk .
So, to summarize, PDS 70 contains two protoexoplanets that are still in the process of forming, and both were discovered through direct imaging. This is rare; most exoplanets are discovered through indirect methods, such as how they gravitationally interact with the star that they are orbiting or how the brightness of the star drops slightly when the exoplanets pass in front. Both PDS 70b and PDS 70c are examples of planets forming, which is something that a lot of people have spent a lot of time searching for but have had problems finding. Additionally, both PDS 70b and PDS 70c may have very likely been responsible for creating the inner gap in the larger disk of gas and dust surrounding the protostar. This has been hypothesized as causing the formation of ring-shaped features in a lot of protoplanetary disks, but PDS 70 is one of the really rare cases where the exoplanets have actually been found. This system is going to attract a lot of attention from astronomers as a unique example of where exoplanets can be seen in the process of formation and where those exoplanets themselves are altering the rest of the protoplanetary system.
And that is my summary of why PDS 70 is so interesting to astronomers. This time, I'm serious. This podcast is going be published in October 2023, and while it's possible that someone might make another dramatic discovery about the PDS 70 system sometime later, all I have for now is a protostellar object with a disk of gas and dust and two expolanets.
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