Great news for everyone! Our paper describing the Planet Hunters NGTS project and presenting the first five planet candidates discovered by the project has been published in the Astronomical Journal.
The paper goes into great detail on how the project is run, how we combine all the responses we received to the initial dataset we uploaded (over 2.6 million individual classifications!), and how we select a list of planet candidates for further observation. The most interesting part of the paper is the section introducing the five new planet candidates discovered by you, our citizen scientists. These planet candidates were not previously identified as of interest by the NGTS team or TESS, so some of you may have been the first to suspect that they could be planets. The paper includes a link to the project results page with the names of all the citizen scientists who classified these candidates at any stage of the project, and you can also find that list here. These candidates are not yet “confirmed planets,” but we continue to collect and analyze more data on each of them, and hope to announce the confirmation of one or more of these planet candidates in the near future. Even at this stage, these candidates are interesting discoveries, and we will discuss them in a bit more detail below, especially the evocatively named TIC-165227846 and TIC-135251751.
TIC-165227846 is an interesting system because it is an unusual discovery. We believe it could be a giant planet (larger than Jupiter) orbiting very close to a low-mass star called an M dwarf. Only about 10 such systems have been found so far, and if confirmed, our system would be the lowest-mass star hosting a nearby giant planet ever found. These types of systems are interesting because, quite simply, we think they could not exist. Newly formed stars can be surrounded by what is called a “protoplanetary disk” – a large amount of dust and gas that provides the material needed to form planets. Currently, it is thought that most planets form by “core accretion”, where this dust and gas stick together to form planets over time. However, the amount of material available in a protoplanetary disk is a function of the mass of the star, so for such low-mass stars there is less material available to form such a large planet. Another barrier is the longer time it takes for planets to form around low-mass stars (low mass means objects move slowly around them). However, planet formation is time-limited, as protoplanetary disks disperse as stars evolve. Discovering intriguing systems like TIC-165227846 that challenge current theories of planet formation allows us to test and fine-tune models to better understand how planets come into existence.
TIC-135251751 is an interesting system, but there is an even more interesting story of how we found out (part of) its true nature. When we first discovered this system, we thought we were looking at a giant planet orbiting near an aging star called a subgiant. But don’t be fooled by the name. It seems that this star had a radius 2.4 times that of the Sun. Such a system is interesting because we don’t really expect to find a planet near a star that is evolving in the final stages of its life cycle. This is because these stars may start to expand and swallow any planets that are nearby. However, one of the key steps to verify if the transit signal is really due to a planet is to obtain a speckle image, which you can read more about here. In other words, a speckle image allows us to observe the vicinity of the host star to see if there are other stars nearby that could affect our measurements. When observing TIC-135251751 with the Zoro instrument on Gemini South, the amazing Zoro team discovered that it is actually two stars in a binary star system. It doesn’t seem like much, but the slightly elongated shape of the red dot in the inset of the figure below is evidence that this system is different to how it first appeared.
Although initially we were disappointed to discover that we were indeed observing two stars, making further observations and analysis difficult, the fact is that the transit signal we detected is most likely due to a planet and is not the result of two stars passing in front of each other (as in an eclipsing binary). These stars orbit with a period of about 50 years, but the transit signal occurs about every 4 days. We may be observing a planet orbiting one of the two stars in this close binary system. This will be another interesting discovery for the Planet Hunters NGTS, as these types of systems also raise interesting questions for theories about planet formation.
This paper and these discoveries are just the first step for Planet Hunters NGTS. We will continue to analyze these systems with the belief that we will soon be able to report confirmation of bona fide planets. In the meantime, we will continue to analyze new data sets as our classification progresses, including some of the newest data from NGTS that is now publicly available on the site and has never been seen before. This means that you could be the first person to find a new planet in the NGTS data! Keep on classifying, and one day you may be able to publish a paper on a discovery that you helped discover.
