6 planets synchronized in perfect harmony

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Artist’s impression of the planetary system HD 110067. Credit: NASA
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Earth- to Neptune-sized planets, known as sub-Neptunes, are found in close orbit around more than half of all Sun-like stars. But their composition, formation and evolution remain a mystery.

The study of multiplanetary systems offers the opportunity to study how planets form and evolve, measuring initial conditions and environments..

Those in resonance (with their periods synchronized in integer ratios) are particularly valuable because they indicate that the system has practically never changed since its inception.

New research presented in the journal Nature presents observations of six planets transiting the star HD 110067. First, by using the three nearest planets to predict the other three, they found that the planets have resonant orbits.

Resonance systems

Although multiplanetary systems are common in our galaxy, astronomers observe those in a tight gravitational formation much less often known as “resonance”.

In this case, the planet closest to the star completes three orbits for every two orbits of the next planet (called a 3:2 resonance), a pattern that repeats itself between the four nearest planets.

Among the outermost planets, the pattern of four cycles for every three other planets (4:3 resonance) repeats twice.. The planets have probably been performing the same cosmic dance since the system formed billions of years ago.

This stability means that the system did not suffer from the bumps and jolts that normally occur when: collisions and collisions, mergers and breakups of planets competing for space. And that, in turn, could say something important about his education.

Their stability was like that from the beginning; the 3:2 and 4:3 planetary resonances are almost exactly as they were when they formed. More precise measurements of masses and trajectories will be needed to further refine the picture of system formation.

A cosmic mystery

The discovery of this system is something of a mystery. The first clues came from NASA’s TESS (Transiting Exoplanet Survey Satellite), which tracks the small eclipses (“transits”) that planets make when they cross the faces of their stars.

A combination of measurements Tess, separate observations two years apart showed different transits for the host star, call HD 110067. However, it was difficult to distinguish how many planets there were or what their orbits were.

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Finally, astronomers chose the two innermost planets with orbital periods (their year) of 9 days for the nearest planet and 14 days for the next. A third planet with a lifetime of about 20 days, was identified using data from CHEOPS, the European Space Agency’s exoplanet characterization satellite.

Then the scientists noticed something extraordinary. The orbits of the three planets matched what would be expected if they were locked in a 3:2 resonance. The next steps involved math and gravity.

Work with data

The scientific team led by Rafael Luque from the University of Chicagoexamined the well-known list of resonances that could be present in systems of this type and tried to compare them with the remaining transits detected TESS.

The corresponding string of resonances suggested a fourth planet in the system with an orbit of about 31 days. Two other transitions were observed, but their orbits were not taken into account as they were single observations.

More than one transit observation is needed to define a planet’s orbit.

The researchers looked again at the list of possible orbits to find two additional outer planets that fit the predicted string of resonances. The best result was found: the fifth planet with an orbit of 41 days and the sixth just under 55 days.

At this point, the science team was almost at a standstill. A portion of the TESS observations that had some chance of confirming the predicted orbits of the two outer planets were omitted during processing.

Last resort

The excessive light scattered from the Earth and Moon into the field of view appeared to hinder the observation. Scientist Joseph Twicken of the SETI Institute and NASA Ames Research Center realized the problem of scattered light.

He knew it scientist David Rapetti, also of Ames and the University Space Research Association, he was working on a new code to recover transit data believed to have been lost due to light scattering.

At Twicken’s suggestion Rapetti applied his new code to TESS data. He found two transitions to the outer planets, exactly where the science team led by Luque had predicted.

They finally found each other six sub-Neptunian planets with radii ranging from 1.94 to 2.85 Earth radii. Three of the planets have low-density masses, suggesting the presence of large hydrogen-dominated atmospheres. The most remarkable aspect is that they present resonances that have never been seen before.



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