TESS After One Year

Following the first year of observations by the Transiting Exoplanet Survey Satellites (TESS), the Massachusetts Institute of Technology (M.I.T.) held a three-day conference to discuss the findings from July 29 through August 2. In the first year, scientists working with TESS images identified 1,075 “objects of interest” they believe would be worth following up with additional study. From this list, additional observations have confirmed the discovery of 29 exoplanets. More are being identified each month.

Large Earth-based telescopes like this 8.1 meter Gemini telescope in the Chilean Andes are critical to validating TESS objects of interest.

 None of the planets discovered to date are places where most people would want to live. Nor are there likely to be any. That’s because TESS is not designed to identify Earth-sized planets orbiting G-type stars like our Sun.

In response to our April 16, 2018 article about the launch of TESS, reader Michael Earle commented, the “range [of TESS’s cameras] is limited to the infrared wavelength, and its target planets are around M class (red dwarf) stars. These stars are by far the most common stars in the galaxy, but, despite the discoveries around Proxima Centauri, about the least likely to host life” compatible with humans.

We sent Michael’s comments to Dr. Sara Seager, Deputy Director of Science for the TESS project and a professor of planetary science, physics, and aerospace engineering. She agreed in part with Michael’s comment. “TESS is not designed to find Earth-sized transits of G-type stars,” she wrote in e-mail.

“While the [TESS cameras] are red-optimized, this may not in and of itself preclude transits being discovered around G-type or hotter stars. The G-type stars are still very bright and there is enough light…to make detections for larger-than-Earth planet transits.

“The main reason TESS cannot find Earth-sized transits of G-type stars is that TESS wasn’t designed to be sensitive enough to find them. Recall that red dwarf stars are much smaller than G-type stars, making the drop in brightness due to an Earth-size planet (e.g. 1/100) transiting a small star a larger signal than the same size planet transiting a G type star (1/10,000).”

However, she ended on an optimistic note. “One subtlety is that some small regions at the ecliptic poles will be covered for multiple TESS sectors, which might just barely enable an Earth-sized transit detection around a G star with multiple transits binned together.”

TESS was not designed to find Earth-sized planets orbiting stars like our sun because such stars are relatively rare. The project’s focus, red dwarf stars, are overwhelmingly more common. George Ricker, TESS Principal Investigator, designed the experiment to maximize the number of exoplanets discovered. He was not primarily interested in planets where people might conceivably live.

Yet misconceptions remain in both scientific literature and popular astronomy websites and magazines. A paper by Max Gunther and 59 co-authors about a TESS object of interest opens with this sentence. “One of the primary goals of exoplanetary science is to detect small, potentially habitable planets orbiting stars that are sufficiently nearby and suitable for detailed characterization.” However, the purpose of TESS is not, and has never been, to identify “potentially habitable planets” for human beings. Its purpose is to find as many exoplanets in the Sun’s galactic neighborhood as possible.

A diverse neighborhood

The discoveries made by TESS to date have already met Ricker’s expectations. They exhibit a diverse range of sizes, orbits, and planetary system architectures that could not have been imagined a couple of decades ago.

Unlike our solar system, which has just eight large planets orbiting at large distances, many of the planets discovered by TESS orbit very close to their primaries. The longest orbital period of the TESS planets is 36 days, one tenth of an earth year. That planet orbits a type K4.5 dwarf called HD 21749 and is estimated to be 2.6 times Earth’s diameter and nearly 23 times Earth’s mass. It lies about 52 light years from Earth. The discoverers of this planet also claim to have spotted a second planet orbiting the same star whose diameter is 90 percent that of Earth’s. It orbits the primary in less than 8 days. (See the “Further reading” section below for details.)

Only five planets among the 29 have a diameter smaller than 1.4 times Earth’s. The orbital periods of those five range from just under 8 days to as short as 11 hours. The rest are larger and more massive, ranging in size from 1.6 Earth diameters to more than 60.

Even if the larger planets were cool enough to support water-based life, they would still not make suitable homes for people. If a rocky planet had the same density as Earth, a planet 1.4 times Earth’s diameter would have a gravitational field at its surface 1.7 times that of Earth. Future space pioneers would find it an effort to stand up, let alone get any work done.

A complete table of the planets discovered to date can be found on the TESS website. Links in the first column direct readers to abstracts of papers with more data about each exoplanet.

What’s next?

The great value of the TESS observations has been and will continue to be discovering the enormous diversity of planetary architectures. Forty years ago, the only solar system we had observed (beside binary- or triple-star systems) was our own Sun. Today 3,000 systems with exoplanets have been at least partially mapped by Kepler, TESS, and Earth-based observatories.

Diagram showing sectors of the southern sky surveyed by TESS (from a presentation “TESS Observations in Year 2” by Roland Vanderspek at the 2019 TESS conference)

The distribution of planet sizes varies dramatically. Some are Jupiter-sized planets orbiting close to their stars. Others are smaller, with diameters less than four times Earth’s. Many planets fall between these two extremes. Contrary to expectations, TESS has discovered some relatively small exoplanets orbiting stars that are larger and brighter than the Sun.

To date, TESS has surveyed only 13 of the 26 sectors of the sky planned for its first mission. In its second year of operation, the remaining sectors will be observed at least once. Assuming that the spacecraft keeps working, the observable sectors will be photographed again in extended missions. These additional observations will increase the chances of finding more planets. Some may orbit within the theoretical “habitable zones” of various red-dwarf stars. These planets may support some types of water-based life such as bacteria. But none seem likely to be suitable for long-term human settlements.

Further reading

For the most part, reading papers from the TESS conference and website is not fun. They are artlessly written and redolent with jargon. Sometimes two or more papers report the same planet discovery.

Nevertheless, readers who want to get a sense of the range of discoveries being made by the TESS science team and the difficulties of the tasks they face, might want to scan at least the abstracts of the following recommendations.

TESS Delivers Its First Earth-sized Planet and a Warm Sub-Neptune — This paper (described above) announces an exoplanet discovered by TESS that is most nearly the size of Earth along with a Neptune-sized companion that has the longest orbital period of the 29 TESS discoveries.

TESS’s first planet. A super-Earth transiting the naked-eye star π Mensae— The π Mensae system is a G0V type star less than 60 light-years from the sun. In 2002, astronomers working at the Anglo-Australian observatory discovered a planet 10 times the mass of Jupiter orbiting the star in a highly elliptic orbit that takes 5.7 solar years. In comparison, Jupiter takes 12 years to orbit the Sun. This paper announces discovery of a second planet roughly twice Earth’s diameter orbiting the star in just six days and seven hours. For comparison, Mercury, which is unbelievably hot, takes 88 days to orbit our Sun.

HD 202772A b: A Transiting Hot Jupiter around a Bright, Mildly Evolved Star in a Visual Binary Discovered by TESS — The system described in this paper illustrates the extremes of planet system architectures being found by TESS. A planet 1.5 times the radius of Jupiter orbits a hot star 1.7 times as massive as Earth’s Sun in just 3.3 days! The temperature of the primary is 6,272 degrees K, putting it at the low-temperature end of spectral class F. (Our Sun’s temperature is 5,778 K, placing it squarely in the mid range of class G.) Hotter and more massive stars are believed to be younger than smaller, cooler stars. So this system is probably much younger than our Sun.

HD 2685 b: a hot Jupiter orbiting an early F-type star detected by TESS — A similar short-period Jovian-sized planet was found orbiting another class F star. Expect more of these.

TESS Spots a Compact System of Super-Earths around the Naked-eye Star HR 858 — HR 858 is another relatively cool F-type star whose mass is slightly larger than Earth’s Sun. It is 104 light years away. Instead of a single gas giant, this system has three exoplanets whose diameters are roughly twice that of Earth. Their orbital periods range from 3.5 to 11.2 days, suggesting they are so close to the primary that they would make Mercury seem frigid.

The L 98-59 System: Three Transiting, Terrestrial-size Planets Orbiting a Nearby M Dwarf — TESS is likely to identify lots of exoplanets orbiting M-class red dwarf stars. The star described in this paper is relatively close (less than 35 light years) and has three planets ranging in size from 0.8 to 1.6 earth diameters. Unfortunately for space pioneers, these exoplanets orbit very close to the dwarf star, completing their orbits between 2.25 and 7.45 days. At these distances the chancing of supporting water-based life are zero. Astronomers want to study exoplanets in red-dwarf systems for signs of life, have already found a far better candidate in Proxima Centauri b. It orbits in the right temperature zone and is only 4.3 light-years from the sun. (See “Almost Wyzdom” for discussion.)

A Super-Earth and two sub-Neptunes transiting the bright, nearby, and quiet M-dwarf TOI-270 — This M-dwarf star is more than 73 light-years from the Sun and has three exoplanets. One is small enough that its surface gravity could be just 1.5 g. Unfortunately, that planet’s orbital period is less than three days and eight hours and its distance from the primary is just 4.5 million kilometers. TOI-270 is hotter and more massive than Proxima, so its belt where liquid water could be found is even further out.

A Revised Exoplanet Yield from the Transiting Exoplanet Survey Satellite — Using stars from the TESS Input Catalog Candidate Target List and Monte Carlo simulation, the authors have estimated the number of exoplanets TESS is likely to find during its two-year mission. The bottom line is that the authors expect TESS to identify 4,372 exoplanets. Of these 1,218 will orbit type G stars like our Sun. Of these, only 150 are expected to be verifiably smaller than two earth diameters. Around 70 planets will be discovered in the habitable zones and all of these will orbit M-dwarf stars, which offer poor conditions for human life.

Stellar Flares from the First Tess Data Release: Exploring a New Sample of M-dwarfs — Not all of the TESS papers are about exoplanets. This study of 24,809 stars taken in the first two months of observations identified 763 flaring stars. Of these, 623 are class M dwarfs. M-dwarf stars have long been believed to have more solar flares than their more massive K- and G-class cousins. This and similar studies to come may provide further insight into this phenomenon.

Asteroseismology of massive stars with the TESS mission: the runaway β Cep pulsator PHL 346 = HN Aqr — Asteroseismology, like its earthbound counterpart, is the study of oscillations within the body of a star. TESS data can be used for such studies as well as for discovering exoplanets. This paper is a sample of the sort of work on pulsating energy sources being done with TESS data. There are lots of papers on the TESS website discussing asteroseismology.

The hyperlinks above refer to abstracts in the Astrophysics Data System at Harvard University. Full texts of the papers can generally be downloaded by selecting the link arXiv Open Access in the upper left hand corner of the screen.

The best source of data about exoplanets and their primaries remains the NASA Exoplanet Archive hosted by the California Institute of Technology. Enter the star’s name, such as HD 2685, and the data will pop up. As of August 29, 2019, 4,044 confirmed exoplanets in 3,001 star systems have been entered into the catalog.

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