Long Live TESS!

J. Tumlinson (tumlinson[at]stsci.edu) and S. Mullally (smullally[at]stsci.edu)

The Transiting Exoplanet Survey Satellite (TESS), launched in April 2018, is NASA's current planet-hunting mission. TESS uses a set of four large-format CCDs to image a 24° by 96° field of view. This large sky footprint will, by the end of the 2-year prime mission in mid-2020, have scanned almost the entire sky in 26 "sectors." Figure 1 shows how TESS mapped the southern sky in its first 13 sectors. As of April 2020, the first 22 sectors, including the entire South and most of the North, are completed and released through STScI's Mikulski Archive for Space Telescopes (MAST).

the milky way as viewed by TESS
Figure 1: The Southern TESS sky. TESS sectors 1–13 were observed continuously for a month each and then patched together to create this mosaic. The dust-lined disk of the Milky Way runs from bottom to top, with the Large Magellanic Cloud in the center of the image. Each TESS sector FOV is composed of four cameras, each with four CCDs separated by small gaps that appear as black lines around the edges of the mosaic. Credit: NASA/MIT/TESS

To discover exoplanets, TESS monitors its 200,000 bright target stars at a cadence of 2 minutes over the 27-day monitoring period in each sector. Every CCD image is stored and downloaded, enabling photometric monitoring of anything on the sky at 30-minute cadence and 20 arcsecond pixel sampling. This extremely large and rich dataset is available from MAST in bulk download, but also through a sophisticated suite of query tools such as the MAST Astroquery API, exo.MAST, and TESSCut. Users can also run analysis code on the TESS data without downloading it by accessing it directly on Amazon Web Services (see https://archive.stsci.edu/tess for help). These innovative features were designed to enable flexible and fast access to the large and very rich TESS dataset. MAST download statistics show that TESS is an extremely popular dataset, with 1–2 TB of data pushed out every day to 50–100 unique users around the world, with large peaks at the time of each Sector release (Figure 2).

download volume and IP address
Figure 2: TESS download statistics to date for 2020. The green bars show daily data volume transfer, averaging between 1 and 2 TB per day. These data are transferred to somewhere between 50 and 100 unique users, with large peaks lining up with the monthly sector releases. TESS is one of the most downloaded datasets in the history of MAST.

Science highlights

As of April 2020, TESS is most of the way through its 2-year, exoplanets-focused prime mission. It has been a year since TESS announced its first Earth-sized planet discovery, HD 21749c, which has R = 0.9 Rearth and orbits its K star host every 8 days (Dragomir et al. 2019). This planet is probably too hot to support life, but it proved TESS can find Earth-sized planets. The first habitable-zone planet from TESS was announced in January 2020: TOI-700d has R = 1.2 Rearth and orbits its M2 dwarf host star every 27 days (Gilbert et al. 2020). Both of these planets are in multi-planet systems, bringing new insights into how planetary systems are assembled. Many more such discoveries are anticipated as TESS works toward its goal of finding 50 Earth-sized planets with measured masses. In a great example of non-exoplanet transient science, the tidal disruption of a star 115 megaparsecs away was discovered by the ASAS-SN supernova survey, but in fact its early brightening was visible in the TESS FFI data more than 8 days before the ASAS-SN trigger (Holoien et al. 2019), so that the early rise of the light curve could be precisely characterized.

The extended mission

In 2019, NASA approved the TESS mission for a 2-year extended mission that will start in July of 2020. Some exciting changes will be introduced into TESS's observing strategy and data products. The TESS team has discovered that there is enough space on its on-board disk drives to increase the cadence of the 24° by 96° FFIs from 30 to 10 minutes. This will enable more precise measurements of exoplanet transit times and enable the mission to measure the variability of tens of thousands more stars. Also, TESS will provide a 20-second-cadence observing mode for a few hundred stars every month. This is this first time TESS will observe a large number of targets continuously for one month at such short exposures, enabling astronomers to model the interiors of some of the smallest stars. In the fourth year (mid-2021 to mid-2022), TESS will be turned on its side to observe along the ecliptic, overlapping several of the K2 fields and filling in a larger gap of the sky currently unobserved by TESS. Finally, peer-reviewed guest investigators will chose almost the entire selection of targets for the rapid-cadence modes.  

In the extended mission, TESS's science emphasis will no longer be focused on finding the best nearby transiting planets. Instead, its program will be chosen by the astronomical community and will be a balance across all astrophysical topics that can be performed with rapid-cadence brightness measurements, ranging from Kuiper Belt objects in our own solar system to supernova explosions in distant galaxies. All of these new science cases will be supported for the end user by new tools currently under development. Watch this space for new ways to use TESS, coming soon!