Transiting Exoplanet Survey Satellite Options

[Paolo]

a new Explorer satellite dedicated to exoplanets around near stars

NASA Selects Explorer Investigations for Formulation

[JRehling]

TESS will look for exoplanets around nearby stars. The catch with the transiting method is that geometry doesn't do us any favors: Any planets which don't transit can't be seen, and the farther the planet is from the star, the less likely a favorable alignment is.

The probability of a transit varies from over 20% for planets with periods of a few days to less than 1% for planets at a distance of about 1 AU. So even if 100% of stars have a planet at 1 AU, you'd have to monitor hundreds of stars to see a few such planets. And when you talk about hundreds of stars, you're no longer talking about "nearby."

Kepler addresses this by looking at many stars farther away -- Kepler stars are basically between about 500 and 6500 light years away.

TESS will monitor about two million stars looking for planets with periods of less than two months. Once such systems are identified, that'll provide many promising leads for follow-on studies with other instruments.

[vjkane]

Anyone know how TESS will identify transits since it will be scanning across the sky and most transits will occur outside the immediate field of view?

[alphasam]

Anyone know how TESS will identify transits since it will be scanning across the sky and most transits will occur outside the immediate field of view?

If I remember correctly It will scan the same three fields in turn every 30 minutes (i.e ten minutes integration each) for a couple of months, giving it a slightly shorter measurement cadence than Kepler, and then it will move on to the next set of three fields and repeat.

Sadly this "Step and Stare" method means it will only be sensitive to short-period planets, ones close to their stars. It may be possible to detect temperate worlds around M-dwarfs though.

[JRehling]

While more knowledge is always (?) better, there are losses in completeness due to geometry and signal-to-noise even when monitoring is allowed to run longer.

A major contribution will be to identify which systems have inner planets that are aligned favorably. Those will merit follow-on observations to look for the rarer outer planets that are aligned favorably. It's useful as triage so a later mission might focus on a smaller number of systems.

Also, note that outer planets may still show single transits even during a very short period of observation, and so given large enough numbers, many outer planets will appear as candidates in the data, with more loosely-bound parameters.

[JRehling]

As TESS gets closer to launch, more complete information about this exciting mission is online. A quick overview is here:

http://tess.gsfc.nasa.gov/science.html

In essence, TESS will observe stars that are, typically, about ten times closer and 100 times brighter in apparent magnitude than the stars that Kepler observed. In a two-year primary mission, it will observe almost every bright star in the sky for 27 days. It will observe roughly 10% of them for about 81 days, and about 3% of them for a year.

Compared to Kepler, it will observe more stars for a short period, but fewer stars for a long period (Kepler observed most of its targets for 3 years), and much closer stars, on average. Because it searches the whole sky, it will also possibly find systemic differences in, e.g., galactic latitude, that Kepler, aimed at one tiny portion of the sky, missed.

The geometry of TESS's search is based on the ecliptic, with a gap along the ecliptic, and the stars that it observes for the longest centered at the ecliptic poles, in Draco and Dorado. These stars, conveniently, may also be observed at any time by JWST, which is also restricted from viewing near the ecliptic because of bright things like the Sun and the Earth in the way.

TESS should provide an almost comprehensive survey of "hot" planets with favorable transiting geometry in the solar neighborhood. This is interesting in that it will identify a large number of systems that have long-period transiting planets in the same orbital plane as the hot planet, but with too long a period for TESS to identify.

It will also find many warm planets and probe the habitable zone of cooler stars that happen to be located relatively near the ecliptic pole. Many of its discoveries will be promising targets for follow-up science by the JWST.

It is not particularly well-suited to finding planets with ~1-year periods orbiting G stars like the Sun. An extended mission of two years may provide that possibility for stars near the ecliptic pole, but I'm not sure which options would be possible for the extended mission. Focusing on one celestial hemisphere (north or south) may open up more discoveries with a long period while neglecting the other hemisphere. Or, giving equal attention to both hemispheres may allow medium-period discoveries everywhere.

[JRehling]

The other night, I saw Sara Seager of MIT speak about exoplanet discovery programs, which spanned the work of Kepler in the recent past, to TESS in the near future, and the Starshade mission that she's working on for later on.

One takeaway that I found interesting was the idea that the search for "earthlike" planets, as she sees it, will be strongly focused on planets of Earth's size and larger orbiting red dwarfs. And this is for the inescapable factor that they are easier to find. Not only do detection methods favor such planets, as opposed to planets orbiting G stars with a period of about a year, but they are more numerous objectively speaking. In fact, virtually any survey designed to find planets orbiting G stars with a period of about a year would end up finding a much larger number of such planets orbiting red dwarfs much closer in anyway.

A planet orbiting, say, Proxima Centauri, would receive earthlike levels of energy from its star if it had a period of 6 days. Thus, the TESS survey, looking at all sufficiently bright red dwarfs for 27 days, will see all the planets that transit such stars in their HZs, given low enough noise (which is assured for larger planets).

This goes a long way to explain the importance of TESS. There's a excellent chance that whenever we are able to spectrally examine an earth-sized (or small super-earth-sized) planet in its star's HZ, that TESS will be the mission that will have discovered that planet in the first place so that the follow-up could take place.