my third paper: is it a planet? (no)

My third academic paper (that is, the third peer-reviewed research article I’ve written as the lead author) was accepted for publication today in The Astronomical Journal, and is publicly available on arXiv. Similar to my second paper, this result describes our observations of a failed star (a.k.a. a “substellar object”, or “brown dwarf”) that is orbiting a much larger, brighter star. Previous work estimated this particular substellar object, HD 136164 Ab (or HIP 75056 Ab), to be about 25 times more massive than Jupiter, but also estimated that the ratio between its mass and the mass of its parent star was small. Astronomers generally define planets to be objects that do not undergo any nuclear fusion, which limits how big a “planet” can be to below around 13 times the mass of Jupiter. That makes HD 136164 Ab a “failed star” and not a planet, but because the ratio between the star and the substellar object pair was so small, a previous paper speculated that this substellar object might have formed like a planet. It would have had to form from little asteroids in orbit around the bigger star that stuck together, and gobbled up enough asteroids to become about 10 times as big as Neptune in our own solar system, and then subsequently accrete so much hydrogen gas that it would grow beyond the definition of a planet and ignite fusion briefly at its center. It seems unlikely, but there are some models that can be fine-tuned to produce an outcome like this.

An image of the HD 136164 Ab system (HIP 75056 Ab). This image, taken using the Very Large Telescope and the SPHERE camera, was published originally in Wagner et al. 2020, but I reprocessed the data in my recent paper and have labeled my reprocessed image here. The host star (about 2 times the mass of the Sun) is hidden by an optical tool called a coronagraph. The resolution of the VLTI (which I used to make new measurements) fits within a single pixel of this image, but interferometry is hard to explains so I’ll save that for another post. If you want to see what the “images” from the VLTI look like, check out Figure 1 in my paper.

Often, when we first discover star systems like this, we are confused, because from a single picture of two points of light alone, we cannot tell exactly how massive either source is, or what it is made out of. The two points of light in our image could be two stars orbiting one another (a-la my first paper), or a star and a substellar object (very similar to a binary star, except that one object is fusing hydrogen, while the other has run out of fuel, a-la my second paper), or even a star and a planet (this is the most interesting case, in my opinion, and I was involved in a recent paper as the second author where we observed a planetary system with the same telescope I used in this paper). We have to take multiple pictures, look at the system in different colors of light, and make sure to dot all our i’s and cross all our t’s when it comes to understanding our telescopes and instruments in order to determine what kinds of solar systems we are seeing in our images. Even then, it can be very difficult to estimate the fundamental properties of space stuff (the mass, or density, or composition) just by looking at the light it gives off. We have imperfect models to compare our observations to that we can use to translate the light we observe into, for instance, an estimate of how massive it is. You could imagine that, yes, the more massive something is, the more light it gives off, but the exact details can be difficult to sort out, and so these estimates of physical properties from our images can often be inaccurate.

So, HD 136164 Ab is a thing too big to be a planet anymore, but (depending on who you asky) maybe too small to have 100% without a doubt formed like a star. Did it get its start as a bunch of pebbles, or did a gaseous forming star fragment into smaller pieces and create this “failed star”? The original papers seemed to lean towards the former explanation, but they estimated the mass of HD 136164 Ab from the amount of light it produced. In my paper, I measured very precisely how the substellar object moved in its orbit around the massive star. I was able to tell, from the speeds at which we observed the binary pair to be moving, how much mass was in the system. Even better, I was able to measure how much the star moved in response to the gravity of the substellar object, and how much the substellar object moved in response to the gravity of the star (remember, the Sun also orbits the Earth, and the Earth also falls towards the apple). So, I made a “dynamical” measurement of the masses of both objects (star and substellar object) in this binary orbit. Ironically, I made these measurements at the Very Large Telescope Interferometer (VLTI) in Chile using a camera called “GRAVITY“.

a figure from my paper, showing the various measurements of the motions of the host star-substellar object pair. The orbit is plotted as straight lines that run through the observations.

It turns out, my dynamical mass estimate is about 35 (plus or minus 5) times the mass of Jupiter. The most credulous could imagine a planet that grows past the planetary definition and becomes 15, or even 20 or so times the mass of Jupiter, but 35 is out of the picture. That mass is impossible to explain with any reliable model of bottom-up planetary formation, but it is possible to explain with top-down models of forming stars (or pancakes of gas swirling around forming stars) that fragment into smaller “failed stars”. Hence the title, VLTI/GRAVITY Provides Evidence the Young, Substellar Companion HD 136164 Ab formed like a “Failed Star”.

I was also able to measure some of the colors of the failed star, which contain information about its atmosphere. I found that it is relatively cloudless (compared to, for instance, Jupiter, which we observe has lots of swirly clouds). Its atmosphere contains lots of hydrogen, helium, water vapor, and carbon monoxide gas (these are all to be expected by simple chemistry if you put a bunch of gas in a big ball, and it doesn’t ignite fusion in its core). The failed star is also about as bright as you’d expect it to be, if it was composed entirely of gas; if it started with a rocky core, it would likely be much fainter.

a figure from my paper that shows the brightness of the substellar object HD 136164 Ab versus the color (in infrared light). water and carbon monoxide make these bite marks in the data that we can fit with models of the atmosphere of this object.

That’s the paper! It was a lot shorter and easier to write than my last one, and was more focused on the observations rather than the modeling (which I prefer). I’m working right now on some very exciting observations of a planet (a real planet, this time) with VLTI/GRAVITY and some images of multiple planets with JWST. I’m not sure which paper I’ll finish writing sooner, but until then, clear skies.


One response to “my third paper: is it a planet? (no)”

  1. Benjamina Avatar

    I feel you write these pieces just so I can understand what I didn’t understand in my Physic classes in school. Thanks for making it enjoyable.

Leave a Reply

Your email address will not be published. Required fields are marked *