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TESS, NASA’S Exoplanet Hunter, Isn’t Optimized for Finding Habitable Planets

by David Sims


NASA’s newest planet-hunting powerhouse, the Transiting Exoplanet Survey Satellite (TESS), leaped into orbit Wednesday evening (April 18) atop a SpaceX Falcon 9 rocket….

The satellite may be small, but it packs a major science punch. TESS is following in the footsteps of NASA’s famed Kepler space telescope and is expected to surpass its predecessor in the number of exoplanets detected.

Over the course of its two-year mission, TESS will monitor the brightness of more than 200,000 stars, waiting to observe tiny dips in starlight known as transits. When a planet orbits in front of its host star, it temporarily blocks a tiny portion of starlight, and these dips will be recorded by TESS’ four ultrasensitive cameras. Kepler has used this same strategy to find more than 2,600 confirmed alien worlds to date.

Some of the first images TESS’ cameras collect may resemble television static rather than discernable cosmic objects, but the photos will be jam-packed with data. The mission will rely on observations by other telescopes, both on the ground and in space, to confirm which of its detected “candidates” are bona fide planets. In addition, some confirmed TESS planets should be close enough to Earth to be studied in detail by other instruments, including NASA’s $8.8 billion James Webb Space Telescope, which is scheduled to launch in 2020.

TESS will observe 85 percent of the sky over its two-year prime mission and is expected to discover thousands of new worlds, as well as other astronomical objects like galaxies. We could see the first of those worlds later this year, NASA officials have said.

The time required for the observation of three successive transits of an exoplanet in front of its primary star is the minimum time necessary for the confirmation of the exoplanet. Three transits, not two, are needed to establish the periodicity of the transits. Given only two dips in brightness, you don’t know whether both of them were caused by the same transiting object, or by two different transiting objects.

The distance of an Earth-like exoplanet from its star is proportional to the square root of the ratio of the star’s bolometric luminosity to the sun’s bolometric luminosity.

R/au = (L/L๏)⁰·⁵

The mass-luminosity relationship at the lower end of the main sequence is, roughly,

L/L๏ = (M/M๏)³·²


R/au = (M/M๏)¹·⁶

The Newtonian form of Kepler’s Third Law,

(P/yr)² = (R/au)³/(M/M๏)

Doing the algebra,

P/yr = {(R/au)³/(M/M๏)}⁰·⁵
P/yr = { (M/M๏)⁴·⁸/(M/M๏) }⁰·⁵
P/yr = {(M/M๏)³·⁸}⁰·⁵
P/yr = (M/M๏)¹·⁹
M/M๏ = (P/yr)⁰·⁵²⁶³²

If the interval of time devoted to observation of a patch of sky is 27 days, then the largest period for an exoplanet, for which we might observe three transits within that interval, is 9 days.

If P=9 days, then
P/yr = 0.02464
M/M๏ = 0.1424 ← for luminosity class V, this is a spectral type M6 star.

That’s a maximum mass for a star having an Earthlike planet that can be confirmed by TESS.

TESS’ mission design does not seem adequate to its intended purpose. For stars of spectral types G and K, TESS will be able to find only hot Jupiters and other planets that are too near the star to be habitable.

Watch this video about TESS with Tony Darnell of Deep Astronomy.

Looking at the video past 54:20 – 54:33, where Mr. Darnell reads the a question about why there are only 27-day view periods: Dr. Thomas Barclay’s answer was that TESS was intended to find only planets with 10-day orbits or less, and that it was the intention all along to find habitable planets only around red dwarf stars.

I don’t think that the public was aware of that. Besides that, it was repeatedly stated that TESS was going to look at “the brighter stars.” These red dwarfs aren’t among the brighter stars.

I see that the veil is dropped at 55:47 and onward. We are told that TESS wasn’t optimized for exoplanets in the habitable zone. Too right, that! For the brighter stars, TESS just plain won’t discover any exoplanets except for the very hot ones. You might think that the discoveries of lots of hot Jupiters can be handed over to the users of the JWST (James Webb Space Telescope), so that it can find transiting exoplanets that are in bigger-than-9-day orbits, but this is a dubious assumption. Remember that the odds for transit diminish with increasing orbital distance for exoplanets. Just because you can see that very close, very hot exoplanet does not mean that you can also see a more temperate exoplanet orbiting in the same exo-ecliptic plane further out.

Probability of transit = (2/π) arcsin(R/a)

Where (R) is the star’s radius and (a) is the planet’s orbital radius. Although the relationship isn’t quite linear, it is almost so, and, roughly speaking, when you double the exoplanet’s orbital radius, you halve the odds for transit.

For exoplanets in 9-day orbits: Star Mass, Star spec, orbit distance (AU), Equilibrium bb Temp (K)

0.100 , M7 , 0.03930 , 237.7
0.200 , M4 , 0.04952 , 335.5
0.300 , M3 , 0.05669 , 344.8
0.400 , M3 , 0.06239 , 374.7
0.500 , M2 , 0.06721 , 452.8
0.600 , K9 , 0.07142 , 534.2
0.700 , K5 , 0.07519 , 633.2
0.800 , K2 , 0.07861 , 740.6
0.900 , G9 , 0.08176 , 846.1
1.000 , G2 , 0.08468 , 956.4
1.100 , G0 , 0.08741 , 1050.9
1.200 , F7 , 0.08998 , 1142.0

What would be ideal is for there to be an improved TESS orbiting the sun at 19.2 AU, probably in the Sun-Uranus L4 and L5 points, where it could devote 2,562 days to each of 12 rows of scanning sectors, with each row stretching from the North Celestial Pole to the South Celestial Pole. After 84 years, it would have found every transiting exoplanet orbiting within the habitable zones of every star (having a mass of 1.2 suns or less) for which the S/N ratio was acceptably high, in addition to those hotter exoplanets to which the current TESS is sadly limited. If we didn’t want to wait 84 years, we could launch twelve of those improved TESS probes and do the job in seven years.

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George Wright
George Wright
6 May, 2018 8:30 am

It seems to me that unless things change soon, it will be irrelevant if any habitable planets are found. If we continue to spiral towards extinction, I hope the dregs of humanity left won’t spread.

James Clayton
James Clayton
16 May, 2018 9:54 am
Kevin Alfred Strom
Kevin Alfred Strom
Reply to  James Clayton
16 May, 2018 11:28 am

That NASA article states “the [1965] tour at Marshall and other NASA sites did not inspire Disney to use the 1950s television series as a model for a new film about space exploration. No doubt, Wernher von Braun was well qualified to imagine what the show and the future American space program might have looked like if Disney had chosen to do so.” It might be a false conclusion to state that Disney wasn’t inspired to do such a film. He might have been — but simply been prevented by lack of time: A year and a half later he was dead of lung cancer.

16 May, 2018 1:36 pm

Long time NASA employees throughout the 1960s into the ’70s told my mother they were abruptly replaced. They said everything changed — 1970s.

National Socialist rocket scientists were rounded up at the end of the war during Operation Plunder and transferred to compounds at their new jobsites over here.

Axis Sally
Axis Sally
Reply to  cc
5 June, 2018 4:52 pm

Ja, cc, this is an interesting point. Von Braun was, in 1970, demoted from his position as director of the Marshall Space Flight center to a Washington desk job as Deputy Associate Administrator for Planning for NASA, even while the Apollo Program was underway. Seven years later he was dead of cancer. Arthur Rudolf “retired” in 1969, again during the Apollo Program, and was later hounded out of the U.S. as a “war criminal.” These are two of the most prominent, but it would interesting to know the experiences of the rest of the team.