The first-discovered triple-binary sextuple star system and a Sun-like star with five exoplanets orbiting within the distance of Mercury are just two recent examples of the extraordinary systems being investigated and discovered by scientist using data from the Transiting Exoplanet Survey Satellite, or TESS.
The joint NASA-Massachusetts Institute of Technology (MIT) mission is the cornerstone of two recently presented discoveries that characterize two tantalizing systems and their benefit to understanding stellar system formation.
A sextuplly-eclipsing sextuple system
Finding a system with six stars is rare to begin with. Finding such a system with three sets of binaries, all of which are eclipsing binaries when viewed from Earth is even more rare.
In fact, TYC 7037-89-1 is the first star system ever discovered to fall into such a category — though 17 other sextuple star systems have already been observed.
The discovery from a team led by NASA’s Brian Powell (Powell et al.) stems from the primary nature of the TESS mission, which is to continuously observe a large portion of the sky for month-long intervals at most searching for the telltale change in brightness coming from stars.
This design is meant to find exoplanets transiting their host stars in relation to TESS’s field of view; the telescope is therefore perfectly primed to detect the changes in brightness that occur when two stars of the same system eclipse one another.
The TYC 7037-89-1 system had been observed previously, but its true nature had evaded detection until Sector 4 and 5 observations during Cycle 1 of TESS investigations which placed the system in the telescope’s field of view.
Based on information extracted from the test light curves and follow-up observations not just from TESS but other telescopes as well, scientists were able to show that the six stars are arranged in three binary groups, with an inner grouping of two binaries and an additional outer binary.
The outer binary is known as binary B, while the inner binary pair are known separately as A and C.
The inner binaries A and C orbit each other every 3.7 years while the outer binary, B, orbits the inner binaries every 2,000 years.
Within each binary, the orbital periods of the stars were calculated as:
- Binary A: 1.570 days
- Binary C: 1.306 days
- Binary B: 8.217 days
When investigating other systems, the letter/number convention can often appear confusing, as it does here where the inner binaries are A and C while the outer binary is B. This arises from the fact that alphanumeric designations are given in order of discovery and are not based on intra-system relationship or distance from the center of a system.
Follow-up observations from TESS and seven different observatories as well as the use of archival data from the All Sky Automated Survey for SuperNovae (ASAS-SN) and the Wide Angle Search for Planets (WASP) corroborated the data points from TESS and confirmed the eclipses observed were all coming from the TYC 7037-89-1 system.
In a first, astronomers find a system of six stars made of three eclipsing binaries. Stars orbit their partners and pairs circle each other. It's a star dance party in space! @NASA's TESS space telescope spotted this strange system.https://t.co/iVGdrwkUyy pic.twitter.com/SYpeadmpML
— NASA Exoplanets (@NASAExoplanets) January 27, 2021
What’s more, deeper investigation of the data revealed that the three eclipsing binaries are triplets, with the primary star in each group having a mass of 1.23-1.30 times that of the Sun, a radius between 1.46-1.69 that of the Sun, and a temperature of 6,350-6,400 K.
The secondary star in each binary was found to have a mass of 0.56-0.66 that of the Sun, a radius 0.52-0.62 of the Sun, and a temperature of 3,923-4,290 K.
While the system itself is approximately 1,900 light years from Earth, its unique triplet nature and the fact that all three binaries are eclipsing when viewed from Earth make this system a prime candidate for further study on the nature of stellar system development and how multi-star systems form and stabilize.
Indeed, one of the first candidates for further study is exactly how all three primaries and all three secondaries developed near identical properties as well as how the system became gravitationally bound.
Furthermore, observations of eclipsing binaries, like the three included in the TYC 7037-89-1 system, are of prime interest for astrophysicists as a detailed study of the eclipses can yield highly accurate measurements of each stars size, mass, temperature, and separation… and crucially, the binary’s distance from Earth.
The paper from Powell et al. detailing the discovery has been accepted for publication this year in The Astronomical Journal. A pre-print can be found here.
Four exoplanets extremely close to their Sun-like parent star
When you think of large, G-type main sequence stars like the Sun and the planets they host, you most likely think of our home solar system, where the closest planet to the Sun, Mercury, orbits at an average distance (semi-major axis) of 57,909,050 kilometers over the course of just under 88 days.
But no two systems are the same. Some aren’t even close. And around 210 light years from Earth lies HD 108236, a Sun-like, G-type main-sequence star with a system of planets very dissimilar to our home.
Four now-confirmed exoplanets in the system, discovered by a team led by Tansu Daylan of MIT using TESS data, all orbit far closer to their star than Mercury does the Sun.
In fact, the innermost planet, HD 108236 b, completes an orbit every 3.795 days. The outermost planet, HD 108236 e, takes 19.59 days to perform an orbit.
Based on semi-major axis measurements conducted by Daylan et al., HD 108236 b resides just 0.0469 AU from its parent star while HD 108236 e is 0.14 AU away.
For comparison, Mercury’s semi-major axis is 0.307 AU.
The innermost planet, b, is a super-Earth, 1.586 times the radius of Earth while the remaining three exoplanets are 2.06, 2.72, and 3.12 Earth radii, placing them in the “sub-Neptune” category of exoplanets.
What’s more, the compact system does not have observed resonance between the planets, either in dual or triple pairings, and is dynamically stable.
Intriguingly, the new data from TESS highlights the possibility of an additional exoplanet residing between the orbits of the c and d planets. Numerous transit-like signals were detected for an object with an orbital period calculated to be 10.913 days based on the data seen from TESS.
I am pleased to present today in #AAS237 our discovery of four small planets transiting the bright, Sun-like star HD 108236 (TOI 1233) and how the system is an opportune laboratory for comparative characterization of exoplanets and their atmospheres.https://t.co/CqGjyNzx8r pic.twitter.com/NIPkDJLexy
— Tansu Daylan (@TansuDaylan) January 13, 2021
However, the signals were observed above the threshold of signal-to-noise ratio needed to firmly dismiss the possibility of a false detection.
Additional observations of the system will have to be made to either confirm the exoplanet candidate or dismiss it.
Nevertheless, a completely different observation of the system from a different team using the European Space Agency’s CHEOPS planet hunting satellite did find a fifth planet in the system, though not the potential one identified by TESS.
This confirmed exoplanet, HD 108236 f, orbits every 29.5 days and is still well within the orbit of Mercury when compared to our solar system. The paper for this discovery can be found here.
Luckily, the system’s proximity to Earth and the parent star’s brightness makes this a highly accessible target for such observations, as well as those for mass measurement, photometric assessment, atmospheric characterization, and search for additional planets.
The paper from Daylan et al. on this discovery is published in The Astronomical Journal and is available here.
Lead image: TESS. Credit: NASA Goddard Space Flight Center/MIT