1. Overview

What is an Exoplanet? An exoplanet is a planet that orbits around a star outside solar system. An exoplanet can orbit around any stars or white dwarf or any binary star system.It is very hard to detect an exoplanet. The first evidence of an exoplanet was recorded in 1917 by astronomer Adriaan van Maanen, but it was not recognised until 2012. The first scientific detection started in 1988 and first confirmed detection was in

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The spectrum of van Maanen's Star, recorded on a glass plate in 1917. This is the first
ever recorded evidence of exoplanets. (CARNEGIE INSTITUTION FOR SCIENCE)

1992. As of 1 May 2019, there are 4,058 confirmed planets in 3,033 systems, with 658 systems having more than one planet. The Transiting Exoplanet Survey Satellite or TESS is a NASA-sponsored explorer mission to survey the sky to search for planets transiting nearby stars. Primarily, TESS’s objective is to detect the planets less than the size of Neptune.

Unlike Kepler mission which was a 2009-2013 four year statistical transit survey mission to determine the frequency of Earth-sized planet orbiting around other stars nearby, which only covered 0.25% of sky; TESS is designed to cover 85% of sky (an area of sky 400 times larger than covered by Kepler). Kepler detected Earth to Neptune size planets with it’s 115 square degree field of view camera within a distance ranged between 100 to 1000 persec (1 persec = 3.26156 lightyear). While TESS mission will primarily focus on the stars which are 30-100 times more brighter than Kepler studied and within approx. 200 persec. It will be then easy to determine mass, size, densities, chemical and atmospheric properties planets around these stars.

2. TESS Mission Organization

The TESS Mission is led by the Massachusetts Institute of Technology (MIT), which directs the overall mission. The TESS Science Office (TSO) which is a partnership between MIT's Physics Department and Kavli Institute for Astrophysics & Space Research and the Smithsonian Astrophysical Observatory, analyzes the science data and organizes the co-investigators, collaborators, and working groups.

NASA’s Goddard Space Flight Center (GSFC) is responsible for project management, system engineering, safety and mission assurance. The TESS spacecraft is led and operated by Orbital Sciences Corporation. The large data processing is done by Science Processing Operations Center (SPOC) at NASA’s Ames Research Center. And all the raw and processed data are made available for public by Mikulski Archive for Space Telescope (MAST), operated by the Space Telescope Science Institute (STScI).

3. How does TESS work?

It is very difficult to detect a planet even with the largest telescopes on earth. Because they are hidden in the bright light emitted by host star. We can detect such planet by closely observing the starlight variation. As an exoplanet passes or transits in front of its host star, the starlight for that moment dims. TESS records the variation of starlight with time known as lightcurve. So, as an exoplanet transits its host star the lightcurve dips for that moment. This method is known as Transit Method. But it can only detect object which passes between the TESS and the host star. Despite of this limitations this method is proven itself extremely effective for detecting exoplanets.

4. Different Parts & Sensors of TESS

  • Cameras: The four TESS cameras are equipped with custom f/1.4 lenses, providing each camera with a wide 24×24 degree field of view. The cameras have an effective aperture size of 10cm (about 4 inches) in diameter, which was determined by simulating the detectability of planets. High cadence is needed for the detection of exoplanets, so exposures of planet search targets and other stars of particular interest are obtained every 2 minutes with full-frame images (FFIs) of the entire field of view returned every 30 minutes, see full frame images. Each camera consists of a CCD detector assembly, a lens assembly, and a lens hood.

    The lens assembly is a custom design housing seven lenses mounted into two separate aluminum barrels that are fastened together. The lens assembly has a 10.5 cm diameter entrance pupil and a focal ratio f/1.4. All optical elements have antireflection coatings and one element has a long-pass filter coating to enforce a short-wavelength cutoff at 600 nm in the TESS bandpass. Each camera forms a 24x24 un-vignetted image on the detector in its focal plane. The lens assemblies were designed for consistent image spot size across the field-of-view (FOV) and to produce undersampled images similar to Kepler. Operating at nominal focus and a flight temperature of -75 degrees C, the 50% ensquared-energy half-width is 15 micron averaged over the FOV. This corresponds to 1 detector pixel or 21 arcseconds (approx. 0.35 arcmin) on sky. Along with an internal stray light baffle, each lens assembly aperture is equipped with a hood to reduce scattered light from the Earth and Moon.

    The detector assembly in each camera consists of a focal plane CCD array and associated electronics. Each CCD array contains four back-illuminated MIT/Lincoln Laboratory CCID-80 devices. The deep-depletion, frame-transfer CCDs consist of a 2048 x 2048 imaging array and a 2048 x 2048 frame-store region (for rapid shutterless readout 4 ms) with 15 x 15 micron pixels. The four CCDs in each array are separated by 2mm and create an effective 4096 x 4096 pixel detector that is operated at -75 degrees C to reduce dark current. The detectors are read out at 625 kHz with < 10 e- read noise. The detector electronics consist of two compact double-sided printed circuit boards seated beneath the CCD focal plane. The electronics transmit digitized data over a serial LVDS link to the Data Handling Unit. The four TESS cameras are bolted to a common plate such that their FOV's are aligned to form a total simultaneous FOV of 24x96 degrees.

  • Data Handling Unit: The TESS Data Handling Unit (DHU) provides the hardware, software, and firmware for camera control, on-board data processing, data storage, spacecraft avionics, and ground communications. The DHU is manufactured by SEAKR Engineering, Inc. and consists of an Athena-3 Single Board Computer, an RCC5 module, an FMC-Gen3 192 gigabyte solid state recorder (SSR), a low voltage power supply, and other ancillary components. During science operations, the four TESS cameras produce a continuous stream of images with an exposure time of 2 s. The DHU performs real time processing on these data to convert raw CCD images into data products responsible for ground postprocessing. This includes cosmic ray mitigation and collecting pixel sub-arrays for 2 min cadence targets and image stacks for the 30 min FFIs. The DHU also calculates photometric centroids from around 200 photometric guide stars from each 2 s image from each camera. These data are used to calculate offset quaternions for fine attitude pointing control by the Master Avionics Unit (MAU). Data downlink via the Ka-band antenna is also controlled by the DHU. Data stored on the SSR are downlinked every 13.7 days at orbit perigee.

5. Orbit of TESS

TESS is in a highly elliptical orbit around earth to maximize the amount of sky( ~ 85 % ) a spacecraft can image. TESS’s orbit is inclined at about 40 degrees with moon’s orbital plane. TESS orbit around the earth in a orbit a never-before-used lunar-resonant orbit known as P/2. It exactly takes half the time it takes moon to orbit around earth. This helps the spacecraft to stabilize against the perturbation due to moon’s gravity. TESS spends most of it 13.7 days orbit observing the sky and as it approaches near earth, it transmits all the gathered data to the ground station.

6. Some interesting facts about TESS

TESS is designed to observe a large number of M-dwarfs due to several reasons. M-dwarfs are very common near our solar system. Planets near M-dwarfs are easy to detect near these stars as the planets induce larger transit signals. Because M dwarfs are cool and red, the TESS bandpass will be more sensitive to red wavelengths. The TESS detector bandpass spans from 600 - 1000 nm. This wide range of bandpass helps to reduce photon counting noise and increase the sensitivity to detect small planets transiting cool, red dwarf.

TESS was launched on April 18, 2018 atop a Falcon 9 rocket. Final orbit was achieved around 60 days after launch and regular science operations began on July 25, 2018.

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