Planet Hunters TESS finds an exciting two-planet system
We have some exciting news – you helped discover another exciting planet system: TIC 349488688 (also known as HD 152843). This exciting discovery follows on from our validation of the long-period planet around an evolved (old) star, TOI-813, and from our recent paper outlining the discovery of 90 Planet Hunters TESS planet candidates, which give us encouragement that there are a lot more exciting systems to be found with your help!
The new exoplanetary system, TIC 349488688, consists of two planets that are similar in size to Neptune and Saturn in our own solar system, orbiting around a bright star that is similar to our own Sun. Planet b is around 3.4 times the size of the Earth, and takes around 12 days to complete an orbit around the star. The outer planet, planet c, is around 5.8 times the size of the Earth and has an orbital period somewhere in the range of 19 to 35 days. The paper has been published by the Monthly Notices of the Royal Astronomical Society (MNRAS) journal and you can find a version of it on arXiv at: https://arxiv.org/abs/2106.04603
Figure 1: the arXiv version of the published paper.
Multi-planet systems, like this one, are very exciting as they offer a wealth of information. In particular, they allow for comparative planetology: the study of two planets that necessarily formed at the same time and out of the same material, but which have evolved in different ways over time resulting in different planet properties that we observe today. Studying these two planets together, therefore allows us to test theories of planet formation and evolution.
Figure 2: TESS lightcurve showing the transits of planet b in blue and the single transit of planet c in pink.
Detection and Validation of the planets
The target was observed in Sector 25 of the TESS data only and the light curve displayed three transit events belonging to the two different planets (see Figure 2). These events were flagged on the talk discussion forums and brought to the attention of the PHT science team. Once it was flagged, we ran a large number of vetting tests to validate it as a planet. First, we made sure that the signal wasn’t caused by a jolt in the TESS satellite or a background event. Next, we ruled out ‘astrophysical’ false positives – signals caused by other astrophysical phenomena such as two stars orbiting around one another, known as an eclipsing binary.
After ruling out a large number of astrophysical and instrumental false positive scenarios, we were confident that the signals were real! However, in order to truly confirm a planet you have to measure its mass. One of the ways to do that is to use what is known as the radial velocity method. As a planet orbits around it’s host star, the gravitational pull between the two bodies causes the star to ‘wobble’ back and forth, meaning that the star is sometimes moving towards us and sometimes moving away from us. As the star moves towards us, the light that it gives off is ‘squished’ and appears more blue, whereas when it’s moving away from us the light is ‘stretched’ and appears more red. The amount of these red and blue shift scales with the mass of the planet.
In order to measure these red and blue shifts we used two ground-based telescopes: HARPS-N located in La Palma, Canary Islands; and EXPRES located at Lowell Observatory, Flagstaff, Arizona. These two telescopes allowed us to obtain spectroscopic observations – observations that split the light of the star up into its individual wavelengths, similar to how a prism splits light into a rainbow. Careful analysis of this split light allowed us to detect the tiny shifts from red to blue and back to red, which were caused by the two planets orbiting around TIC 349488688. We obtained enough racial velocity measurements to estimate the mass of planet b to be around 12 times more massive than the Earth, and to place an upper mass limit of 28 times the mass of the Earth on planet c.
Why is this system so interesting?
Even though there are now hundreds of confirmed multi-planet systems, the number of multi-planet systems with stars that are close enough such that we can observe and study them using ground-based telescopes remains exceedingly small. The proximity and brightness of HD 152843 is one of the properties that makes this new system stand out. To date we have been able to constrain the masses of the two planets and we are currently continuing to monitor the system to confirm them.
The masses that have already been derived suggest that both planets have low densities, and therefore are likely to have extended gaseous atmospheres. Combined with the brightness of the stars these properties offer exciting prospects for probing the atmospheres and chemical composition of both planets in the future, for example with upcoming space telescopes such as NASA’s James Webb Space Telescope.
Last but not least this system is interesting because it was discovered by you! With this find you have once again shown that with visual vetting we are able to detect exciting planet systems that the automated computer algorithms struggled to find. Thank you to everyone who helps out with the search for distant worlds on Planet Hunters TESS and who help to further our understanding of our Galaxy. A special thanks also to Safaa Alhassan, Elisabeth M. L. Baeten, Stewart J. Bean, David M. Bundy, Vitaly Efremov, Richard Ferstenou, Brian L. Goodwin, Michelle Hof, Tony Hoffman, Alexander Hubert, Lily Lau, Sam Lee, David Maetschke, Klaus Peltsch, Cesar Rubio-Alfaro, Gary M. Wilson who are now coauthors of the discovery paper.
We have some very exciting news: our paper summarising the results from the first two years of Planet Hunters TESS has been accepted for publication! Check out the paper here.
The paper outlines the ins and outs of planet Hunters TESS project and presents 90 new planet candidates from the first two years of the TESS mission (sectors 1 to 26). These planets wouldn’t have been found without the help of all of the citizen scientists taking part in the Planet Hunters TESS project. The paper includes a link to a site that lists all of the citizen scientists who identified each of these 90 planet candidates mentioned in the paper. This page can also be found here.
The majority (81%) of the planet candidates outlined in the paper only exhibit a single transit event in the TESS lightcurve, meaning that they tend to have longer orbital periods (where the orbital period corresponds to the duration of a ‘year’ on this planet) than the average duration of the planets found by the TESS automated algorithms. This is because automated pipelines often require two or more transit events in order to be able to detect the signal. However, with visual vetting, we are equally sensitive to a single transit event as we are to planets that transit multiple times within the duration of one light curve.
You can see that in the figure below, where the orange and pink points show the PHT candidates, and the blue points show the automated pipeline found Tess Objects of Interest (TOIs). The figure highlights that the planets identified with PHT tend to have longer orbital periods than the TOIs, and therefore allowing us to study the characteristics of a different ‘set’ of planets, and maybe even of planets that are more similar to the planets within our own solar system.
Even though the majority of the planet candidates outlined in the paper are not yet confirmed planets, we are following them up using ground-based telescopes which are situated around the world, including in Australia, Chile, USA and the Canary Islands. Hopefully these observations, including both photometric and radial velocity observations, will allow us to confirm the planetary nature of these objects, and even derive masses for some of them which will allow us to infer their densities and therefore bulk compositions. This is ongoing work and we hope to share some of it with you in the near future.
In addition to the 90 new planet candidates, the paper presents some of the most interesting stellar systems that have been discussed on the Planet Hunters TESS Talk discussion forum. An example of a potential multi stellar system is shown below.
Multi stellar systems not only provide very interesting and pretty lightcurves, they also allow us to probe stellar evolution theories in more detail, as all the stars in one system must have necessarily formed at the same time and out of the same material. This highlights some of the other exciting science that results from Planet Hunters TESS and from the continued work of so many citizen scientists.
Since the launch of the Planet Hunters TESS project, almost exactly 2 years ago, we have had over 25.5 million classifications completed by over 25 thousand citizen scientists from around the word. This huge global effort can help us understand what kind of planets exist within our galaxy, how planets form and evolve over time, as well as bring to light some of the other interesting and bizarre astrophysical phenomena that TESS observed over the last two years.
The above light curve, with its periodic increase and decrease in brightness, has the clear signature of an RR Lyrae stars. The pulsations, caused by increases and decreases in the radii and temperatures of these types of stars, typically vary on times-scales ranging from a few hours to and a couple of days. These stars are also evolved stars, meaning that they tend to be older than the Sun with typical ages of around 10 billion years.
RR Lyraes are not only nice to look at, they are also very important for the field of astronomy, as they allow us calibrate the ‘distance ladder’ and thus help us determine the distance to far away objects. They can do that because the time between the pulsations depends on the mass, temperature and intrinsic brightness (the brightness if you were right next to the star) of the star. When we compare the intrinsic brightness to the brightness that we see from Earth, we can calculate how far away the RR Lyrae star is using the inverse-square law.
This target was discussed on the PHT discussion forums at: https://www.zooniverse.org/projects/nora-dot-eisner/planet-hunters-tess/talk/2107/1550146
This week we have an exotic EB, explained to us by Dr. Cole Johnston, where the primary star is a subdwarf which is the stripped helium-burning core of a star. The temperature of this star is so high that it illuminates the much cooler secondary star, causing the surface of the secondary star that is facing the primary to heat up and appear much brighter than the side that is facing away. This causes a dramatic increase in brightness approaching and receding from the secondary eclipse (the small dip at the top of the ‘wave’ in the above lightcurve). The two stars are so close together that they complete one orbit in just a few hours! The above light curve is phase folded to emphasise the brightening which is known as the ‘reflection effect’.
Studying these systems is important because these primary stars are thought to be the tracers of a very strange evolutionary path, whereby the entire hydrogen envelope of an evolving star is stripped away by some mechanism (probably by a binary or high mass planetary companion), just at the point were helium burning starts in the core of the star.
Planet Hunters TESS users have identified two very interesting lightcurves of M-dwarf stars that show repeating patterns that are difficult to explain with well-known astrophysical phenomena such as star spots, eclipsing binaries or stellar pulsations.
Both of these lightcurves exhibit two dips, a shallow one and a deeper one, which could be explained as the primary and secondary eclipses of an eclipsing binary. However, in addition to these ‘eclipses’, we can also see a w-shaped dip after or after the deeper eclipse. Both lightcurves repeat with a period of less than 1 day and the patterns remain stable over the course of the available TESS observations, as shown in the phase folded figure below. The target stars are both M-dwarfs, with temperatures of around 3000 K and radii of around 0.3 Solar radii.
Similar lightcurves had previously been seen in the Kepler data, see Stauffer et al. 2017 (https://iopscience.iop.org/article/10.3847/1538-3881/aa5eb9/pdf) for more detail. This paper suggests that the pattern could be caused by clouds of dust orbiting around one of the stars. Could that also explain the w-shaped dips in the TESS data, or are we seeing an entirely different phenomenon here?
In order to study these systems in more detail we want to see if we can identify more of them in the TESS data.
If you see any of these please tag a member of the researcher team or use the hashtag #wstars to help us find and study these elusive systems.
A new paper released this week and accepted by the Astronomical Journal (https://arxiv.org/abs/2004.07783) announced the discovery of TOI-1338, a planet in an eclipsing binary system. An eclipsing binary consists of two stars in orbit around each other, and this new planet orbits both stars. The paper was led by Veselin Kostov and his team at NASA’s Goddard Space Flight Center and elsewhere, but they were first tipped off to the presence of this interesting system by reading comments on Planet Hunters Talk. Several citizen scientists appear as authors on the paper – congratulations to everyone involved!
This isn’t the first time Planet Hunters have found a circumbinary system – PH1b, discovered way back in 2014, was found in Kepler data by volunteers, and is still the only planet known in a four-star system.
Many stars are not alone, but instead form part of a multi-stellar system of two or more stars that are gravitationally bound together. Even though the prime science goal of TESS is to find exoplanets, it also observes a plethora of eclipsing binaries that allow us to study these systems in more detail.
The light curve of TIC 8153514, observed in Sector 21, shows a sharp eclipse superimposed on top of the signal of a variable star.
If you see eclipsing binaries on PHT you can tag it with #EB or #eclipsingbinary to let others known what you have found!
Asteroids are small rocky bodies that are left over from the time of the formation of the Solar System. They range in sizes from small rocks that you could see on the side of a lake to hundreds of kilometers across. There are as many as hundreds of millions of these within our own Solar System, so it comes as no surprise that we often see them in the TESS light curves, manifesting themselves as spikes or dips or anything in between.
The above figure shows the light curves of six bright stars observed by TESS, sorted by their distance with reference to the top object (the red light curve). As you can see, almost all of them show a ‘strange’ signal, which is very likely caused by an object such as an asteroid passing through the field of view.
This next figure shows an approximation of a possible path of the asteroid, which was determined by matching the times of the events with the locations of the target stars. The target stars are highlighted with circles of the same colours as the light curves in the first image, and the times are the times in hours since the first event (in the orange light curve). Based on a very simple estimations of the times and projected distances, we can approximate the projected speed of the asteroid to be around 0.6 arcseconds per hour.
Asteroids are fascinating objects that allow us to probe the conditions of the Solar System during the time of its formation (4.5 billion years ago!). If you’re interested in making your own light curves, such as those in the top figure, you can download a program called LATTE to have a go.
The pattern seen in this light curve could be due to the variability of a young stellar object (YSO), which is often related to a disc obscuring some of the stars light and/or material accreting onto the star. Studies of the symmetries in the LCs of YSOs can help us understand the environment around the star.
Some stars are quiet,
Some stars pulsate,
Some stars are single,
Some have a mate.
This is a pulsating star in a binary system. Thank you to the citizen scientists who brought this fun system to our attention and suggested it as a lightcurve of the week. https://www.zooniverse.org/projects/nora-dot-eisner/planet-hunters-tess/talk/2112/1030293?comment=2048847&page=1