U- or V-shaped dip? How to spot the difference?

When searching for exoplanets, the shape of the transit can tell us a lot about what object we could be looking at. For the Planet Hunters NGTS exoplanet transit search, we ask you to identify if a transit is U-shaped or V-shaped, as well as whether there’s stellar variability, data gaps or no significant dip in the flux at all. An exoplanet transiting a star will typically produce a U-shaped dip, but there are situations where that isn’t the case (more on that below). Meanwhile an eclipsing binary (two stars orbiting each other) will produce a V-shaped dip most of the time.

The first plot (Figure 1) shows a clear V-shape produced by an eclipsing binary system. In this case, the transiting star only partially eclipses the target star, meaning that it passes across the edge of the disk of the target star but never passes fully in front. This means that the point of minimum flux doesn’t last long before the flux starts to increase again.

Figure 1: Partial eclipse in an eclipsing binary system

The defining difference between U- and V-shaped dips is the angle of the sides of the transit, or ingress and egress to give them their scientific names. Ingress is when the transit begins and the flux is decreasing to the minimum (position 1 to position 2 in Figure 2), while egress is when the transit is ending and flux starts to increase back to the normal level (position 3 to position 4 in Figure 2).

Figure 2: Diagram of the positions of an object eclipsing a star and the resulting light curve (brightness = flux). [Ref]

V-shaped dips have sides that are at an angle whereas U-shaped transits will have a steeper decrease and increase in the flux, so much so that the sides of the transit will be almost vertical. The reason V-shaped dips have angled sides is because the object blocking out the light is typically (but not always!) another star. The eclipsing star is large (compared to a planet) so takes more time to pass fully in front of the target star, therefore the decrease in flux happens over a significant time period and we get an angled ingress (likewise for the egress as the star stops blocking light).

The angled sides are more pronounced in Figure 1, but don’t be fooled by dips with a curved base like Figure 3 below! If the transit has angled sides then it’s still a V-shape! The curved base of the transit is caused by a phenomenon called ‘limb darkening,’ where the central disk of a star appears brighter than the edge. The eclipsing star in this system is not just grazing the limb of the target star either, which is why the minimum flux of the transit is sustained for a range of phases.

Figure 3: V-shaped eclipsing binary with a curved base

How vertical is vertical? Sadly there isn’t a clear answer to this, which is why we use human vetting rather than just a computer to check these light curves. The example below (Figure 4) is the light curve for confirmed exoplanet HATS-43b, which was classified as U-shaped by all 20 volunteers who viewed it. This is a clear example of the near vertical drop in flux for the sides of the transit. The small radius of the planet compared to its host star means that it almost instantly passes through the ingress and egress phases, compared to the time taken by a larger star in an eclipsing binary system.

Figure 4: HATS-43b. A known exoplanet spotted by Planet Hunters NGTS!

But wait! V-shaped dips can still be exoplanets too! Just like the partial eclipse that produced the sharp, V-shaped dip in Figure 1, an exoplanet can perform a grazing transit where it just crosses the limb of the star and doesn’t go over the centre of its host star’s disk. This will produce a very shallow V-shaped dip, therefore we will get round to searching these classifications for potential exoplanets too! The sides of the dip appear more angled due to the shorter total duration of a grazing transit; it’s very likely that the scale of the x-axis on the plots (the phase) will show a much smaller range of numbers due to how short these transits will be. The ingress and egress times will be similar to a regular transit but the central dip is much shorter. The limb darkening effect also has a more obvious effect on the shape of the dip, which we can see in the light curves below for a near-grazing transit by WASP-174b (Figure 5). The dip has angled sides due to the zoomed in x-axis and has a curved base due to limb darkening. This is a classic curvy V-shape, but it’s also a real exoplanet!

Figure 5: Transit light curves for WASP-174b showing a grazing transit. [Ref]

There isn’t a definitive answer for when a curvy V becomes a regular U-shape, but as always your intuition and best guess is what we want! We hope this blog post makes it easier to spot the differences between U- and V-shaped dips when you’re classifying light curves on the Planet Hunters NGTS site, and remember you can always check the Field Guide or ‘Need some help with this task?’ for more help. There’s also the team of researchers and moderators on the ‘Talk’ forums who will be happy to help!

Sean & the Planet Hunters NGTS Team

The Next Generation Transit Survey needs you

Today we a guest blog by Sam Gill. Sam Gill is a research assistant at the University of Warwick. He studies long-period planets discovered with TESS and leads the monotransit working group within NGTS. He also researches red dwarfs to empirically calibrate the physical properties of the latest-type stars and is a keen spectroscopist and binary star enthusiast. In his free time, Sam enjoys Brewing and hiking with his beagle, Bruce.

In 2015, more than half of all known exoplanets with masses determined to better than 20 per cent were found from ground based surveys for transiting exoplanets. One such survey which you may know from the successful Zooniverse project searching for variable stars is the Wide Angle Search for Planets (WASP), with installations in Tenerife and South Africa. Both camera arrays have taken over 430 billion measurements of 30 million stars and has found over 150 new planets with many more candidates.

The WASP telescope and cameras located in South Africa.

Typically, WASP found planets spanning the masses of Saturn to a few times of Jupiter, along with many low-mass stars which have radii similar to Jupiter. If we look at the mass-radius diagram for planets (with masses known to better than 20%) found around other stars, we see that there is a dearth of those with masses below Saturn. These planets are notoriously hard to identify around stars like our Sun because of their small comparative size. However, these planets should be detectable around much smaller stars with ground-based telescopes which is where the Next Generation Transit Survey (NGTS) comes in.

NGTS is located at the Paranal observatory and consists of 12 fully-robotic telescopes operating at red-optical wavelengths (520-890 nm). This maximises sensitivity to bright and relatively cool and small stars enabling us to find planets less massive than Saturn. With a field of view of 2 degrees per telescope and an average cadence of 13 seconds, NGTS can take over 200 GB of images each night which include many thousands of stars.

The 12 NGTS telescopes at Cerro Paranal.

Data from NGTS is processed so that the relative brightness of each star can be measured with the influence of the atmosphere mitigated. These data can then be searched using a box-fitting algorithm which searches for periodic dips in a stars brightness which is a characteristic trait of transiting exoplanets. The box-fitting algorithm is powerful and can find many potential planet candidates in our data. The NGTS consortium tries their best to find as many planet candidates as they can but some of these candidates turn out to be false positives. We are successfully finding planets in NGTS data, but the size of our datasets are so large the we may have overlooked some of the most promising planet candidates. That is were you come in.

Phase folded NGTS data of NGTS-2b (left panel) with the box least-squares algorithm power spectrum (right panel)

The aim of Planet Hunters NGTS is to find planets in NGTS data. To do this, we have created a variety of plots (described in a future blog post) embedded in multiple workflows which will help identify promising candidates and many other interesting systems. Today, we just launched two new workflows that you can check out the Odd Even Transit Check and the Secondary Eclipse Check to help vet the best candidates identified by the Exoplanet Transit Search workflow. With your help, we can focus our efforts on the most promising systems and work towards completing a broader consensus of exoplanets. We look forward to you classifications!

More about Planet Hunters NGTS

We were excited to announce yesterday a new chapter in the Planet Hunters project with Planet Hunters NGTS. Here’s a bit more background for those of you who are new to the exoplanet hunt. 

The Next-Generation Transit Survey (NGTS) has been searching for extrasolar planets (exoplanets) around other stars for over 5 years but we can’t be sure we’ve found them all without your help. Exoplanets are planets outside our own Solar System. We know of their existence from various techniques, most popularly the exoplanet transit method. This method works by observing a star and watching for regular dips in the amount of light emitted by the star. These dips can be caused by other stars (“binary companions”), exoplanets and other strange objects but we can use the size and shape of the dips to pick the most promising planet candidates. Smaller dips typically correspond to exoplanets, for example Jupiter causes a 1% dip in our Sun’s light for any aliens looking our way. The best planet candidates will be chosen for follow-up observations with different techniques to hopefully confirm whether they really are planets!

Planet Hunters and Planet Hunters TESS used data from NASA space telescopes to search for these dips. This project differs slightly as we’re using the NGTS facility which sits atop a mountain in Chile, surveying the sky every night with its 12 telescopes. Computers have searched the NGTS observations to find these dips but the NGTS team can’t check everything the computer finds as there are just too many candidates. Not every dip is caused by a planet but our handy tutorials will guide you through and turn you into an exoplanet spotting expert.

Finding more exoplanets will allow us to learn more about how planets are formed and how they evolve, which in turn will teach us more about our own Solar System and the Earth we live on. If we can understand the architecture of our own Solar System and other exo-planetary systems, then we can better approach the search for life beyond Earth.

We look forward to working together with you and hope you enjoy the project!

Sean & the Planet Hunters NGTS team

Welcome to a new Planet Hunters!

We’re extremely happy to welcome a new member of the PlanetHunters family. Planet Hunters: NGTS is our first project using data from a ground based survey – the Next Generation Transit Search based at Paranal in the Atacama Desert in Chile.The twelve telescopes of NGTS aim to find planets around the brightest stars; the hope is that, unlike the large number of planets found with NASA’s Kepler, which provided data for the original Planet Hunters, these will be targets that can be followed up with further observations designed to characterise their mass, composition and atmospheres.

ngts_scopes

As with the Exoplanet Explorers project we ran on Zooniverse a while back, the aim of this new project is to review candidates that have been selected by automatic searches. The hope is that this will make our hit rate higher, but it does mean the task is a little different. The project lives on its own webpage as ngts.planethunters.org, and is led by Meg Schwamb, Chris Watson and Sean O’Brien at Queen’s University Belfast, along with their colleagues. They’ll be responsible for looking at your data, with the help of the Planet Hunters:TESS team.

With this new project there should be enough data to keep you all searching for more than the few days each month TESS data is available. Happy hunting!

(Image: the NGTS telescopes in Paranal, with ESO’s Very Large Telescope visible in the background).

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.

Follow-up Observations

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.

Planet Hunters TESS II: results from the first two years

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.

Figure 1: Half of the TESS planet candidates presented in the Planet Hunters TESS method paper.

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.

Figure 2: Planet radius vs orbital period for the TOIs (blue) and the single (orange circles) and multi (pink squares) transiting PHT candidates. Use the slider in the figure to add/remove the PHT candidates.

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.

Figure 3: An example candidate multi-stellar system.

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.

Light curve of the week: RR Lyrae

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

Lightcurve of the week: reflection effect EB

LCOTW_reflection_eb

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.

 

W-shaped dips in M-dwarf lightcurves

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.

wstars1

Phase folded lightcurves of two M-dwarf stars as observed by TESS.

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.   

wstars2

Sections of the full TESS  lightcurves of the M-dwarf stars.

A circumbinary Planet from TESS, with the help of Planet Hunters TESS

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.

toi_1338_transit_still