TESS, NASA’s new space telescope for detecting planetary transit events was successfully launched by a SpaceX Falcon9 rocket today, with the first stage successfully landing on its recovery barge for re-use.
Everyone familiar with FtB should also be familiar with Kepler, both the rabble-rousing white dude and the chrome-hot, super-cool, USD $700 million, solar powered camera. Kepler was initially expected to generate data for no more than 3.5 years, though it has exceeded that by a wide margin despite failures to its gyroscopic stability controls that were originally thought mandatory to complete the mission. It is true, however, that the mission’s data collection is much slower now.
Enter TESS: The Transiting Exoplanet Survey Satellite. TESS is a next-generation solar powered black tube with a mirror and a camera, and dang is it fine. Estimates of the number of exoplanets Kepler might find were rarely very precise, but its 2652 confirmed planets*1 and 2724 yet-to-be-confirmed planet candidates*1 clearly more than satisfied expectations for the telescope. But even cooler than that, the planets found compared with the known number of stars in the Kepler camera’s field of view allows much better and more precise estimates of TESS’s expected future discoveries. While depending on confirmation rate Kepler will ultimately be credited with 4000-5000 confirmed planetary discoveries, TESS should rack up many, many more if its components last for their expected lifetimes:
“The number of Earth-sized and super-earth planets that TESS should find over the course of its two-year primary mission will be in the range of 500 to 1,000 new planets, and overall the number of planets that will be established is likely to be in excess of 20,000 all together,” enthuses MIT’sGeorge Ricker, who is TESS’ Principal Investigator.
Note that “earth sized and super-earth planets” is probably an informal reference to planets of approximately earth-mass (at least 1/3rd of earth mass, with an upper bound varying depending on the source, but frequently approximately 2-3 earth masses) plus planets categorized as “super-earths” (having a mass exceeding that of an earth-mass planet, whose upper bound can vary as noted, but not exceeding 10 earth masses, which limit has more general agreement than the earth/super-earth mass dividing line).
Though Ricker obviously knows a heck of a lot more than I do, that number seems conservative to me. TESS has more sophisticated systems to measure brightness changes, and this Caltech archive notes that of the 734 confirmed planets discovered by Kepler and whose mass was later successfully measured, only 121 were determined to be of a mass placing them in the earth-mass or super-earth categories (caltech used 3x earth mass as the dividing line between earth and super-earth). Since only about 1 in 3.5 planets were successfully weighed, we should expect 20k/3.5 mass-evaluated planets from TESS, with a similar percentage of planetary masses in the earth/super-earth range giving us an estimate of somewhere between 900 and 950 planets. With TESS’s increased ability to detect smaller planets, it’s possible we could see many more than 1000 earth/super-earth planets being discovered on this new mission.
If I had to guess, not being a planetary scientist or any kind of astronomer, I’d say uncertainty over length of mission is responsible for the conservatism. As much as sensitivity, length of mission has the capacity to affect the number of discoveries. Remember that to discover a planet candidate requires 3 observed transits with two (nearly) identical periods between. To discover earth via a TESS- or Kepler-like mission orbiting a star in our neighborhood, even with more than enough sensitivity it would require a full 2 years of observations for earth to register as a candidate planet even if it happened to transit the first day of that telescope’s operation. If earth had just transited the day before the telescope began operating, it would require 3 years just for earth to get any initial notice.
Because of the unpredictability of space operations, TESS’s primary mission is scheduled to run for a mere 2 years. For solar systems like ours, that’s only enough to note the existence of Mercury and Venus, and the sensitivity may not even be enough to notice Mercury. Given that, our solar system to a TESS-like mission would appear to have only a single planet after two years. But if there are no mishaps, and if TESS holds up at least as well as Kepler, we could see a mission length long enough for our imaginary extra-solar system TESS-analogue to easily discover earth and even possibly to discover Mars. This doubles or triples the number of discoveries at the same sensitivity. While it’s still not possible to know how many stars have solar systems similar to ours (sensitivity hasn’t been good enough to detect a Mercury or even a Mars before TESS, and we haven’t been observing long enough to detect any Jupiters or longer-period planets), there are a huge number of planetary systems with a star’s satellites concentrated in close-in orbits, and so doubling or tripling mission length might generate less than double or triple the number of planetary candidates. On the other hand, if it turns out longer-period planets are ultimately more common, if mean-period is optimal a doubling or tripling of observation time could yield more than a doubling or tripling of candidates. We’ll have to wait and see.
In the meantime, let’s just be excited that this amazingly cool tube with a black interior, a mirror, a camera, and a few solar panels is now outside earth’s atmosphere and ready to push itself into its observational orbit.
*1: These numbers include all confirmed and candidate planets found using the Kepler instrument, though for reasons having to do with budget, the Kepler mission was shut down after the instrument was reduced to 2 gyroscopes. When a proposal proved viable to use innovative stabilization strategies to enable renewed data collection, congress granted money for a new mission using the same Kepler telescope. The new mission is named K2. In most venues, confirmed planets found during the Kepler instrument’s main mission – a mission also called “Kepler” – are tracked separately from those found during the K2 mission, despite the use of the same space hardware. I added available numbers together as I thought for most persons the Kepler/K2 distinction was largely irrelevant.