We have some exciting news! We have validated the first Planet Hunters TESS planet, TOI-813b, where validated means that we can say, beyond reasonable doubt, that it is a planet! TOI-813b is around 7 times larger than the Earth, on an 84 days orbit around a 3.7 billion year old star. The paper has been submitted to 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/1909.09094
So how did we go from detection to validation?
You initially spotted the transit events that occurred in sector 5 and brought it to our attention via Talk. This put the candidate on our ‘to watch’ list until a second transit was discovered in sector 8 a couple of months later. Additional transits were also identified in sectors 2 and 11. With multiple transit events we could be much more certain that the signal was real and, therefore, began to invest more time looking into studying it.
Initial validation checks
Full detrended light curve showing data from sectors 1 to 12 (with the exception of sectors 3 and 7). The times of the transits are indicated by the black dashed lines.
We carried out a number of vetting tests on the TESS data in order to validate it as a planet. First we made sure that the signal wasn’t caused by a jolt in the satellite or a background event. Next, we verified that it wasn’t an ‘astrophysical false positive’ – signals caused by other astrophysical phenomena such as an eclipsing binary (two stars orbiting around one another). To do this, we first compared all four transit events to make sure that the shape and depth of the signals were consistent with one another, as alternating transit depths are characteristic of eclipsing binaries. Next, made sure that when extracting the lightcurve with different aperture sizes the depth of the transits didn’t change. A change in depth could indicate that the signal is caused by an eclipsing binary in the background. Furthermore, to make sure that the light wasn’t coming from a background object, we subtracted images from when the star was in transit to when it was out of transit in order to make sure that the change in light (causing the dip in the light curve) is centred on the star. Finally, we looked at nearby stars in order to make sure that their light curves did not have transit-like dips at the same time as TOI-813.
Each observed transit.
Spectroscopy. In order to determine whether TOI-813b really is a planet, we had to find out as much as possible about the host star. This can be done using spectroscopy, whereby we split light up into its individual wavelengths, much like a prism splits light into a rainbow. From this we can derive properties such as temperature and composition of the host star. We obtained these observations using the Wide Field Spectrograph instrument on the 2.3-m Australian National University telescope and the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph on the ESO 3.6-m telescope in Chile.
High Resolution Imaging. We also wanted to check for nearby stars that could contaminate the light curve. This was done using a technique known as ‘speckle’ imaging, which takes thousands of consecutive images with extremely short exposure times. When the images are combined in a particular way, we are able to essentially ‘freeze’ our the effects of the atmosphere (which usually makes our images blurry) and obtain high resolution images. This was done using the Zorro instrument on the 8.1-m Gemini South telescope in Chile.
With this data in hand, we were able to run a program, known as VESPA, to statistically validate the planet. This clever program models the data assuming a number of different astrophysical scenarios and returns the likelihood of each one being true. Based on this analysis, we can be 99.7% sure that he signal is caused by a planet! Yay!
Why is TOI-813b so great?
TOI-813b is interesting for many reasons. First because it is orbiting around an evolved, subgiant star. The subgiant phase of a star occurs when a star runs out of its nuclear fuel source and, in a desperate attempt to find another source of energy, expands its outer layers and contracts its core. Our Sun has not yet reached this stage of its life – it still has around half of its fuel source left – but it will undoubtedly one day also become a subgiant star. There is a noticeable lack of well-studied planets around these types of stars, however, they may be able to help us predict what will happen to the planets within our own solar system in the (very very distant) future. Do planets survive this stage of a star’s life? And if so, how do their characteristics change? We investigated this further for TOI-813 by modelling the size of the star over time. This analysis showed that our newly discovered 7 Earth radii planet will sadly be engulfed by its evolving host in approximately 780 million years (mark your calendars)!
Planet radius vs orbital period plot where the black cross shows the properties of TOI-813.
TOI-813 is also interesting as it currently has the longest orbital period (84 days) of any validated TESS planet. This is largely because most of the TESS targets are only observed for ~30 days, making the discovery of longer period planets challenging. Nonetheless, the PHT project has already shown to be extremely good at finding these longer period planets. Even though TOI-813b is currently the longest period planet found by TESS, this will likely change very soon as TESS continues to observe thousands of stars every night.
Finally it’s a great planet because it was discovered by you. The discovery and validation of TOI-813b has shown that we are able to find planets that the pipelines miss! This is the first validated PHT planet, but we are actively following up more targets that you have helped us identify! Thank you so much to everyone for helping us search for unique and exciting new systems that will help us understand the Universe that we live in. Also a special thank you to Frank Barnet, Stewart J. Bean, David M. Bundy, Zbigniew Chetnik, Jamie L Dawson, Judy Garstone, Andrés Guillermo Stenner, Marc Huten, Scott Larish, Larry D. Melanson, Thomas Mitchell, Christopher Moore, Klaus Peltsch, David John Rogers, Claudia Schuster, Dean J. Simister, Daniel Shane Smith, Christopher Tanner, Ivan Terentev and Alexander Tsymbal, the PHT volunteers who helped us find TOI-813b and are now co-authors of the validation paper.