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.

[AndyG]

And when you talk about hundreds of stars, you're no longer talking about "nearby."

You are, relatively: there are around 500 stars up to the 40 light year mark. That's practically next door.

Andy

[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?

[JRehling]

Some of TESS's targets will be closer, but the typical star observed will be a couple of hundred light years away.

It's an interesting question as to what the closest transiting planet is. Luck will figure large in that. There's no reason in principle why a very close one couldn't be, but the random orbital inclinations will bear whatever fruit they do.

[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.