The team behind the Exoplanet Explorers project has just published a Research Note of the American Astronomical Society announcing the discovery of 28 new exoplanet candidates uncovered by Zooniverse volunteers taking part in the project.
Nine of these candidates are most likely rocky planets, with the rest being gaseous. The sizes of these potential exoplanets range from two thirds the size of Earth to twice the size of Neptune!
This post is by Adina Feinstein. Adina is a graduate student at the University of Chicago. Her work focuses on detecting and characterizing exoplanets. Adina became involved with the Exoplanet Explorers project through her mentor, Joshua Schlieder, at NASA Goddard through their summer research program.
Let me tell you about the newly discovered system – K2-288 – uncovered by volunteers on Exoplanet Explorers.
K2-288 has two low-mass M dwarf stars: a primary (K2-288A) which is roughly half the size of the Sun and a secondary (K2-288B) which is roughly one-third the size of the Sun. The capital lettering denotes a star in the planet-naming world. Already this system is shaping up to be pretty cool. The one planet in this system, K2-288Bb, hosts the smaller, secondary star. K2-288Bb orbits on a 31.3 day period, which isn’t very long compared to Earth, but this period places the planet in the habitable zone of its host star. The habitable zone is defined as the region where liquid water could exist on the planet’s surface. K2-288Bb has an equilibrium temperature -47°C, colder than the equilibrium temperature of Earth. It is approximately 1.9 times the radius of Earth, which places it in a region of planet radius space where we believe planets transition to volatile-rich sub-Neptunes, rather than being potentially habitable super-Earth. Planets of this size are rare, with only about a handful known to-date.
The story of the discovery of this system is an interesting one. When two of the reaction wheels on the Kepler spacecraft failed, the mission team re-oriented the spacecraft to allow observations to continue to happen. The re-orientation caused slight variations in the shape of the telescope and temperature of the instruments on board. As a consequence, the beginning of each observing campaign experienced extreme systematic errors and initially, when searching for exoplanet transits, we “threw out” or ignored the first days of observing. Then, when we were searching the data by-eye for new planet candidates, we came across this system and only saw 2 transits. In order for follow-up observations to proceed, we need a minimum of 3 transits, so we put this system on the back-burner. The light curve (the amount of light we see from a star over time) with the transits is shown below.
Later, we learned how to model and correct for the systematic errors at the beginning of each observing run and re-processed all of the data. Instead of searching it all by-eye again, as we had done initially, we outsourced it to Exoplanet Explorers and citizen scientists, who identified this system with three transit signals. The volunteers started a discussion thread about this planet because given initial stellar parameters, this planet would be around the same size and temperature as Earth. This caught our attention. As it turns out, there was an additional transit at the beginning of the observing run that we missed when we threw out this data! Makennah Bristow, a fellow intern of mine at NASA Goddard, identified the system again independently. With now three transits and a relatively long orbital period of 31.3 days, we pushed to begin the observational follow-up needed to confirm this planet was real.
First, we obtained spectra, or a unique chemical fingerprint of the star. This allowed us to place better constraints on the parameters of the star, such as mass, radius, temperature, and brightness. While obtaining spectra from the Keck Observatory, we noticed a potential companion star. We conducted adaptive optics observations to see if the companion was bound to the star or a background source. Most stars in the Milky Way are born in pairs, so it was not too surprising that this system was no different. After identifying a fainter companion, we made extra sure the signal was due to a real planet and not the companion; we convinced ourselves this was the case.
Finally, we had to determine which star the planet was orbiting. We obtained an additional transit using the Spitzer spacecraft. Using both the Kepler and Spitzer transits, we derived planet parameters for both when the planet orbits the primary and the secondary. The planet radius derived from both light curves was most consistent when the host star was the secondary. Additionally, we derived the stellar density from the observed planet transit and this better correlated to the smaller secondary star. To round it all off, we calculated the probability of the signal being a false positive (i.e. not a planet signal) when the planet orbits the secondary and it resulted in a false positive probability of roughly 10e-9, which indicates it most likely is a real signal.
The role of citizen scientists in this discovery was critical, which is why some of the key Zooniverse volunteers are included as co-authors on this publication. K2-288 was observed in K2 Campaign 4, which ran from April to September back in 2015. We scientists initially missed this system and it’s likely that even though we learned how to better model and remove spacecraft systematics, it would have taken years for us to go back into older data and find this system. Citizen scientists have shown us that even though there is so much new data coming out, especially with the launch of the Transiting Exoplanet Survey Satellite, the older data is still a treasure trove of new discoveries. Thank you to all of the Exoplanet volunteers who made this discovery possible and continue your great work!
The paper written by the team is available here. It should be open to all very shortly.
This is the first of two guest posts from the Exoplanet Explorers research team announcing two new planets discovered by their Zooniverse volunteers. This post was written by Jessie Christiansen.
Hello citizen scientists! We are here at the 233rd meeting of the American Astronomical Society, the biggest astronomy meeting in the US of the year (around 3000 astronomers, depending on how many attendees are ultimately affected by the government shutdown). I’m excited to share that on Monday morning, we are making a couple of new exoplanet announcements as a result of your work here on Zooniverse, using the Exoplanet Explorers project!
Last year at the same meeting, we announced the discovery of K2-138. This was a system of five small planets around a K star (an orange dwarf star). The planets all have very short orbital periods (from 2.5 to 12.8 days! Recall that in our solar system the shortest period planet is Mercury, with a period of ~88 days) that form an unbroken chain of near-resonances. These resonances offer tantalizing clues as to how this system formed, a question we are still trying to answer for exoplanet systems in general. The resonances also beg the question – how far could the chain continue? This was the longest unbroken chain of near first-order resonances which had been found (by anyone, let alone citizen scientists!).
At the time, we had hints of a sixth planet in the system. In the original data analysed by citizen scientists, there were two anomalous events that could not be accounted for by the five known planets – events that must have been caused by at least one, if not more, additional planets. If they were both due to a single additional planet, then we could predict when the next event caused by that planet would happen – and we did. We were awarded time on the NASA Spitzer Space Telescope at the predicted time, and BOOM. There it was. A third event, shown below, confirming that the two previous events were indeed caused by the same planet, a planet for which we now knew the size and period.
So, without further ado, I’d like to introduce K2-138 g! It is a planet just a little bit smaller than Neptune (which means it is slightly larger than the other five planets in the system, which are all between the size of Earth and Neptune). It has a period of about 42 days, which means it’s pretty warm (400 degrees K) and therefore not habitable. Also, very interestingly, it is not on the resonant chain – it’s significantly further out than the next planet in the chain would be. In fact, it’s far enough out that there is a noticeable gap – a gap that is big enough to hide more planets on the chain. If these planets exist, they don’t seem to be transiting, but that doesn’t mean they couldn’t be detected in other ways, including by measuring the effect of their presence on the other planets that do transit. The planet is being published in a forthcoming paper that will be led by Dr Kevin Hardegree-Ullman, a postdoctoral research fellow at Caltech/IPAC.
In the meantime, astronomers are still studying the previously identified planets, in particular to try to measure their masses. Having tightly packed systems that are near resonance like K2-138 provides a fantastic test-bed for examining all sorts of planet formation and migration theories, so we are excited to see what will come from this amazing system discovered by citizen scientists on Zooniverse in years to come!
We are also announcing a second new exoplanet system discovered by Exoplanet Explorers, but I will let Adina Feinstein, the lead author of that paper, introduce you to that exciting discovery.