Astronomers Stunned by Six-Planet System Frozen in Time

six planet

Alongside the two stars of space telescopes, the James Webb Telescope and its older sibling, the Hubble Telescope, a whole array of other satellite telescopes serve their duty in space. Often, each works independently, but for some exceptional discoveries, they must collaborate.

The space telescope Tess, operational since 2018, detected a dimming of the star HD110067, located 100 light-years away, in 2020. Tess monitors changes in starlight that could indicate a planet subtly passing in front of a star, a method known as the transit method, commonly used for discovering planets in foreign solar systems.

In this case, the data hinted not at one but two planets. Over time, it became apparent that astronomy had stumbled upon a scientific stroke of luck, as reported in the journal Nature.

Confusion in the Second Examination

The uniqueness of the situation became clear when Tess revisited the star about a year ago. Tess has the peculiarity of looking in the same direction for only four weeks every two years. Although the accuracy achieved was higher than the first time, the results were astonishing. Again, two planets were recorded, but with entirely different characteristics than the first time. The data suggested very short orbital periods of about nine and 14 days, confirming three planets in total. However, hints of additional planets were present.

Astrophysicist Rafael Luque from the University of Chicago (not to be confused with the controversial chemist of the same name) decided to take a different perspective. He found it in the Cheops space telescope of the European Space Agency (ESA).

Unlike Tess, which, like its predecessor Kepler, aims to observe as many stars as possible simultaneously to discover new exoplanets, Cheops can control its viewing direction to examine individual planetary systems more closely. Cheops, like Tess, is a relatively new telescope in service for three years. “We went fishing for signals among all the potential periods that those planets could have,” explained Luque. Cheops has indeed confirmed another planet, this time with an orbital period of 20 days.

Like the Hands of a Clock

An explanatory graphic in English. It shows the ratios of the orbital periods to each other.
An explanatory graphic in English. It shows the ratios of the orbital periods to each other. Image: ESA, CC BY-SA 3.0 IGO.

Upon closer inspection, a pattern can be discerned in the three orbital periods: 14 is roughly one and a half times nine, while 20 is approximately one and a half times 14. Such regularities could be coincidental, but in the case of planets, they indicate an exciting phenomenon. Some planetary systems have highly ordered orbits. Their orbital periods are in resonance, much like the sound waves of an instrument. Such planetary systems resemble clocks: the hour, minute, and second hands of a clock move precisely in sync.

Therefore, it was crucial to learn as much as possible about the orbits of the other planets to see if they were also in resonance. Using existing observation data, various resonance ratios for the outer planets were excluded. What remained were possible orbital periods of 31, 41, and 55 days.

Confirmation of Calculations

Indeed, two of the planets could be found exactly at their precalculated positions. A reanalysis of Tess and Cheops’ old data eventually revealed the outermost planet. Luca Fossati’s group from the Institute of Space Research at the Austrian Academy of Sciences in Graz was involved in the model calculations. The analysis of the Tess and Cheops data as a whole was also carried out there. “It was my colleague Andrea Bonfanti who first succeeded,” emphasizes Fossati. As part of an international research collaboration, various methods were applied to study exoplanets. Not only does the dimming provide information, but the minimal movements of stars also reveal details about the celestial bodies orbiting them. Applying this method allowed for conclusions about the masses of the planets.

Resonances in the orbits of exoplanets are not uncommon. They naturally occur over time due to the gravitational interactions among the planets. According to the theoretical ideas of astrophysicists, this should happen relatively frequently. However, observations show that the phenomenon is much rarer in nature than expected. The fact that six planets are in resonance suggests a system that has largely evolved undisturbed over billions of years. It is estimated that this is only the case in about one percent of all planetary systems. Chaotic events in their lifetime are likely much more frequent than assumed, says Luque.

This makes the planets of the star HD110067 an exceptionally interesting study object, providing a glimpse into the distant past. “HD 110067 offers optimal conditions for modeling planetary atmospheres, as we have conducted at the Institute of Space Research and described in the study,” says Fossati.

Atmospheric Investigation by Webb

So far, little is known about the planets except their orbital periods. However, it is suspected that three of the planets have atmospheres with a high proportion of hydrogen. They belong to the class of Sub-Neptunes, resembling Neptune but orbiting their star on very narrow paths and being smaller, in this case, between 1.94 and 2.85 Earth radii. The origin and structure of Sub-Neptunes remain largely mysterious.

The James Webb Space Telescope is expected to clarify the composition of the planet atmospheres in the HD110067 system. Analyzing the atmospheres of foreign planets is one of the essential tasks of this telescope. The planets of HD110067 are “ideal candidates for investigating the composition of their atmospheres with the James Webb Space Telescope,” emphasizes Christiane Helling, the head of the Institute of Space Research in Graz. After Tess and Cheops, the current flagship among space telescopes will also turn its gaze to this extraordinary planetary system.