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TESS is designed to perform an all-sky transiting exoplanet survey.

TESS's survey will focus on nearby G, K and M type stars with apparent magnitudes brighter than magnitude 12. Approximately 500,000 stars will be studied, including the 1,000 closest red dwarfs.

Since its targets are very bright and its sensitivity is relatively worse than Kepler, I wonder whether it can be replaced by gound-based-small-telescope arrays. There can be no gaps in the time series too.

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    $\begingroup$ From the Wikipedia article you referenced: "Previous sky surveys with ground-based telescopes have mainly detected giant exoplanets. In contrast, TESS will examine a large number of small planets around the very brightest stars in the sky." So from this I would guess that the ideal observing conditions in space allow for much more precise photometry to extract fainter transits out of the background star light. $\endgroup$ – Dean Dec 5 '16 at 12:22
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There's several reasons why we might want a space-based exoplanet telescope over a ground-based telescope (or array of telescopes).

  1. As Carl Witthoft already said, atmospheric absorption limits what we can do from the ground. The atmosphere absorbs at important wavelengths and, more importantly, atmospheric disturbances introduce unwanted noise in your data. In a field where clean, noiseless data is paramount, putting something in space to remove those problems is necessary.
  2. Earth-based, exoplanet-hunting telescopes are limited by the sky they can see in a few ways. For one thing, no single Earth-based location can view the entire sky so if you really want full coverage, you need these telescopes in multiple locations around the world and that can get expensive. One example of a project like this is the MEarth Project, but there are plenty of other ones. On the whole, a single, multi-camera telescope in space can cover more of the sky, and do so more easily, than an array of telescopes scattered across the planet. Secondly, the idea of being "limited by the sky" also comes into play when you consider that ground-based telescopes are limited by weather and light pollution. A space-based telescope doesn't have these limitations and can more or less view any star at any time (so long as it is not occluded by the Moon/Earth or too close to the Sun).
  3. Ground based telescopes are also limited by another factor (that ties into the previous two) which is that they fairly magnitude limited. Most ground-based exoplanet-hunting telescopes are groups of small telescopes which either (a) specifically target nearby, bright stars, or (b) scan the sky, extracting data for only the brightest stars. Neither of these situations is ideal for really being able to look at 500,000 stars as TESS is proposing. For example the MEarth Project I cited above was originally only targeting 1,976 stars! That's far less than TESS's 500,000, and just like MEarth, I'm sure TESS will continue to expand it's operational parameters.
  4. Another factor to consider is that there's already numerous ground-based exoplanet-hunting telescopes. However, there's not nearly as many space-based telescopes dedicated to searching for exoplanets. Why limit ourselves and choose to do one or other when we could do both?
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  • $\begingroup$ Whilst your general points are correct, only #1 comes close to answering the question about TESS. Both WASP and HAT find planets over both hemispheres and at magnitudes as faint as12. They also cost a tiny fraction of a space mission. $\endgroup$ – Rob Jeffries Dec 8 '16 at 0:18
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Depends on the wavelength ranges to be studied as well. Quoting from the link you provided:

The sole instrument on TESS is a package of four wide-field-of-view CCD cameras. Each camera features a low-noise, low-power 16.8 megapixel CCD detector created by the MIT Lincoln Laboratory. Each has a 24° × 24° field of view, a 100 mm (4 in) effective pupil diameter, a lens assembly with seven optical elements, and a bandpass range of 600 to 1000 nm

Atmospheric absorption has some serious peaks near 750 and 920 nm, which is one reason to place these cameras in orbit above the atmosphere.

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I think the biggest issue is the precision of the photometry. The atmospheric turbulence limits ground-based photometric precision to 1%. In space, Kepler achieved about 0.001% on bright stars over a few hours. This precision is needed to detect an earth-sized planet transiting a sun-sized star.

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The raison d'etre for TESS is to do a precise and complete survey for exoplanets, that are smaller than Jupiter, around bright stars in a short period of time.

The main reason to go to space, is to do more precise photometry than is possible (in an efficient way) from the ground looking through the Earth's atmosphere. This allows the detection of Neptune and even Earth-sized planets, which are currently not detected by the all-sky ground-based experiments like WASP and HATnet.

The predicted yield and improvement is shown below (from the NASA TESS site.)

Predicted TESS yield

The best prospect from the ground now appears to be NGTS, which will probably have the precision to get down to the sub-Neptune sized planets, but which will be limited to the southern hemisphere. NGTS started last year, so as long as it can get results out very soon, then it will steal a march on TESS.

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