Six Planets Found Orbiting an Extremely Young Star

The field of exoplanet study continues to grow by leaps and bounds. As of the penning of this article, 5,572 extrasolar planets have been confirmed in 4,150 systems (with another 10,065 candidates awaiting confirmation. Well, buckle up because six more exoplanets have been confirmed around TOI-1136, a Sun-like star located roughly 276 light-years from Earth. This star is less than 700 million years old, making it relatively young compared to our own (4.6 billion years). This system will allow astronomers to observe how systems like our own have evolved with time.

The six-planet system was confirmed by the TESS Keck Survey, an international team of astronomers that searches for exoplanets by combing data obtained by the Transiting Exoplanet Survey Satellite (TESS) and the W.M. Keck Observatory (of which UC Riverside planetary astrophysics professor Stephen Kane is the principal investigator). The details of the six-planet system were presented in a series of papers that appeared in The Astronomical Journal. In the seventeenth and latest paper in the series, the survey team presented precise mass measurements of the six exoplanets, details about their orbits, and the characteristics of their atmospheres.

To date, most of the exoplanets observed by astronomers have been either individual discoveries or one of just a few planets. But in some cases, such as Kepler-90 and TRAPPIST-1, astronomers have observed many planets in a single system (8 and 7, respectively). Depending on the age of their parent star, these systems present astronomers with the opportunity to observe how multi-planet systems formed and evolved. In the case of TOI-1136, its age sets it apart from many known systems, being merely 700 million years old.

Artist’s impression of the planetary system around Kepler-90, a Sun-like star 2,545 light years from Earth. Credits: NASA

Tara Fetherolf, a visiting assistant professor of astrophysics at Cal State San Marcos and co-author of a new paper, explained in a UC Riverside News release:

“Because few star systems have as many planets as this one does, it’s getting close in size to our own Solar System. It’s both similar enough and different enough that we can learn a lot. This gives us a look at planets right after they’ve formed, and solar system formation is a hot topic. Any time we find a multi-planet system it gives us more information to inform our theories about how systems come to be and how our system.”

Initial observations of the system were made in 2019 using TESS, which was followed up with observations using the High-Resolution Echelle Spectrometer (HIRES) at the W.M. Keck Observatory and the Automated Planet Finder (APF) at the Lick Observatory. The latter observations allowed the team to precisely constrain the mass of the planets using the Radial Velocity measurements (where slight variations in the star’s motion indicate the gravitational forces acting on it). This yielded estimates of about 3.5 (TOI-1135 b) to 9.7 (TOI-1135 f) Earth masses, placing them between Super Earths to Mini-Neptunes.

The team also used Transit Timing variations, where dips in a star’s luminosity are used to determine the presence of planets (and their size). They then created computer models where the velocity measurements were layered over the transit data, yielding more information about the system. Typically, young stars are difficult to study because they are so active, possessing powerful magnetic fields, sunspots, and powerful solar flares that influence their planets by affecting their atmospheres. Since all the planets observed around TOI-1136 are of a similar age, they likely formed under similar conditions.

An amusing rendition of the TOI-1136 system if each body in the system were a duck or duckling. Credit: Rae Holcomb/UCI

And since the planets of this system are relatively close to each other, the team was able to measure something hard to gauge in other systems. As Kane summarized:

“Young stars misbehave all the time. They’re very active, just like toddlers. That can make high-precision measurements difficult. This will help us not only do a one-to-one comparison of how planets change with time but also how their atmospheres evolved at different distances from the star, which is perhaps the most key thing.”

The results of this study could have far-reaching implications for exoplanet research and the search for life in the cosmos (astrobiology). According to the most recent fossilized evidence, life emerged on Earth during the Archaean Eon (ca. 3.9 billion years ago), almost immediately after it formed. While many of TOI-1136’s planets orbit too closely and are subject to too much radiation to make life likely, the team hopes that observations of this system will ultimately answer questions of how our planet and life as we know it came to be.

“Are we rare?” said Kane. “I’m increasingly convinced our system is highly unusual in the Universe. Finding systems so unlike our own makes it increasingly clear how our Solar System fits into the broader context of formation around other stars.”

Further Reading: UC Riverside, The Astronomical Journal