NASA's successor to the Kepler mission, the Transiting Exoplanet Survey Satellite (TESS), is already paying dividends. The satellite was only launched in April and spent time undergoing commissioning and calibration. But it has now started its science mission, and researchers have already discovered two new planets.

These are expected to be the first of as many as 10,000 planets spotted by TESS. So we thought this was a good opportunity to take a careful look at the planet hunter's design, the goals that informed the design, and what its success should mean for our understanding of exoplanets.

Four eyes

The body of TESS is pretty simple, being composed largely of a fuel tank and thrusters. It has reaction wheels for fine control of its orientation and a pair of solar panels for power. The business end of TESS consists of a sun shield protecting not one but four telescopes. Instead of being able to focus on faint objects, the telescopes (each a stack of seven lenses above CCD imaging hardware) are designed to capture a broad patch of the sky.

TESS images a single area for roughly a month before moving on to the next. Over the course of a year, this will allow it to capture most of the sky in a single hemisphere; it will switch to the other hemisphere for its second year of observations. Should the hardware still be operational at the two-year mark, it will have imaged most of the sky, and a similar cycle will likely start again.

This cadence creates some trade offs. If a planet's orbit is such that it doesn't pass in front of its star during the month TESS happens to be pointing that way, we'll miss it (unless it's part of the small overlap between separate areas). This will bias us toward finding planets with short orbital periods, where a transit is guaranteed to happen whenever TESS gets around to pointing at it. Short enough orbits mean we can observe multiple transits during that month, confirming the planet's existence without the need for follow-on observations.

The upside is that we get the entire sky and very broad fields of view during each of those months. As a result, estimates are that TESS will find three to four times as many planets as Kepler did during its mission.

Kepler's status The now-venerable Kepler telescope is probably just about done doing science. Kepler requires fuel for thrusters it uses to periodically reorient itself, and back in early July the satellite notified its operators that fuel reserves were critically low. NASA stopped observations and put the hardware in a safe mode that used no fuel until the probe was in a position to send down all the data from its previously completed observations. The now-venerable Kepler telescope is probably just about done doing science. Kepler requires fuel for thrusters it uses to periodically reorient itself, and back in early July the satellite notified its operators that fuel reserves were critically low. NASA stopped observations and put the hardware in a safe mode that used no fuel until the probe was in a position to send down all the data from its previously completed observations. Once that data was safely sent down, Kepler was put back to work doing science on August 29. NASA isn't sure how much fuel remains, and one of its thrusters is acting a bit odd, which may slightly reduce its ability to remain pointed at a specific location. If the probe manages to complete this round of observations—its 19th such cycle—it will be a pleasant surprise.

While TESS' hardware was designed to pick up brighter stars similar in size to the Sun, it's sensitive to light on the redder end of the spectrum. This will allow it to image relatively nearby dwarf stars, which has a number of scientific advantages. For one, these are the most common stars in our galaxy, so this is a lot of targets. Their smaller size means that planets occlude a relatively larger fraction of the light from the star, making them easier to spot. Finally, the lower output of dwarf stars means that the habitable zone (where liquid water is possible) is closer to the star. Being closer to the star means a shorter orbital period, so planets in the habitable zone may make more than one orbit within a month, making them easier to spot.

The imaging hardware can take a snapshot of the field of view every two seconds, but there's not enough on-board storage to support constantly capturing images at that rate, and the bandwidth requirements for sending images back to Earth are too steep. Instead, a half an hour of images of the full field of view are combined to cut down on noise and small, random fluctuations; these are stored on board and transmitted all at once.

In addition, a variety of stars were chosen for a more detailed look, with averaging at two-minute intervals. In this case, the remaining pixels are cropped away, leaving a small field containing little more than the star, which helps make for more compact data. Stars chosen for this treatment are relatively bright or nearby, allowing easy follow-up with ground-based observations, and are also easy to separate from background objects that could interfere with observations.

It works

Exoplanet researchers have set standards for discovery that don't accept a single dimming of a star as a sign of the existence of a planet, as too many rare events could cause this kind of dimming. If the planet is detected by transits alone, then we have to see multiple dimmings at intervals that reflect a consistent orbit. Failing that, there has to be some other means of confirming the exoplanet's existence, such as its gravitational influence on its host star or other planets orbiting the same star.

TESS hasn't been working long enough to capture multiple orbits of individual planets. But conveniently, in at least two cases, we already had additional observations sitting in data that hadn't been thoroughly analyzed. As a result, the TESS team has already prepared two papers on new planets it has observed.

One of the two new discoveries is at π Mensae, about 60 light years from Earth. We already knew there was one planet there, a massive super-Jupiter in an eccentric orbit that takes six years to complete. TESS has now spotted π Mensae c, a super-Earth that orbits every 6.25 days. Its close-in orbit probably protects it from gravitational interactions with the giant planet we already knew about.

Conveniently, π Mensae had already been imaged with a HARPS telescope, which measures changes in the star's light as the star is tugged around its nearby planets. When analyzed, the additional data confirmed the existence of π Mensae c and indicates that it has about 4.8 times the mass of Earth. Combined with the TESS data, which indicates that the planet is 2.1 times the radius of Earth, we find ourselves able to calculate its density. This ends up being similar to that of pure water. It's more probable that it's an ocean planet with a rocky core and an atmosphere containing water vapor and perhaps other lighter gases.

We now also know that there's another planet about 50 light years away—this one orbiting the M dwarf star LHS 3844. This one is only slightly larger than Earth and orbits so close that it completes a full orbit in only 11 hours. That would make it a scorching 800 Kelvin, or even hotter than Venus. Its presence was confirmed by other observations of the star made during previous surveys that covered the region.

What next?

To a certain extent, we don't need to send something up to space to find new exoplanets simply to expand the catalog. There are plenty of ground-based instruments that are doing so, and the existing catalog of roughly 3,500 exoplanets provides us with a good perspective on how common planets in different size classes are. TESS' observation cadence also means we'll miss any planets that don't happen to pass in front of their star during its one-month window. That means TESS won't do much to fill in one of the biggest remaining gaps in our understanding: the frequency of planets that orbit at an Earth-like distance or beyond.

So why is TESS in space and not a ground-bound experiment? You can think of it as an investment in the future.

While Kepler gave us a sense of the size of typical planets, we don't have a very good sense of their composition. Even when radial velocity measurements give us their overall density, there are typically multiple solutions compatible with that value. To use the example of π Mensae above, the size of any rocky core that's compatible with its density will be directly related to the size of its atmosphere as well as the gases that comprise it.

Atmospheres are critical in an additional way. While we often talk about a habitable zone based on the light output of the host star, actual habitability will be extremely sensitive to the greenhouse gas composition of a planet's atmosphere. To paraphrase one researcher in the field, if you tweak the atmosphere's composition, you could take a planet in the habitable zone and turn it into a frozen wasteland, a scorching hellscape, or anything in between.

Fortunately, this is something we can study, although it is difficult. Each time a planet passes in front of its host star, a tiny fraction of the light it sends our way passes through the planet's atmosphere first, where it interacts with the gases present there. These gases can leave fingerprints in the light that reaches us, and, by imaging enough transits, these fingerprints can be read despite their small contribution to the overall light. The precise distance at which this works depends on the quality of the telescope, the brightness of the star, and the size of the atmosphere, but it is possible for stars relatively close to Earth.

What we'd like is the equivalent of the Kepler mission for planetary atmospheres—something that will give us a better sense of how common atmospheres are and if there are any typical collections of molecules we'd find in them. Unfortunately, that's not really possible. Kepler was a survey telescope that found the planets but didn't have the resolution to image their atmospheres. That's not Kepler's fault; the two tasks are somewhat contradictory, as sensing the atmosphere requires high-resolution imaging, while finding the planets works best at low resolution.

TESS can be thought of as the first half of a Kepler-like program. It will help find a lot of planets, including a few that may be close enough to be imaged by existing hardware. But it's mostly laying the groundwork for the giant telescopes that are currently under construction, as well as the James Webb Space Telescope, which continues to inch toward launch. These will greatly expand our reach out into the galaxy, vastly increasing the distance at which we can image planetary atmospheres. (Remember, the volume of space where imaging is possible increases with the cube of the imaging radius.)

To put this all in perspective, you can think of Kepler as the exploratory phase of a potential project, the one that answered the question of whether exoplanets are common enough to pursue this. TESS is the next phase, identifying exoplanets that are within the range of our existing or near-future technology. The payoff will be a decade from now, when we can say some concrete things about exoplanet atmospheres and what we'll learn about the possibility of life on them.

The arXiv. Abstract number: 1809.07242, 1809.05967 (About the arXiv).

Correction: Celsius/Kelvin screwup.