TESS Discovers Jupiter-Sized Exoplanet around Ancient Binary Star

The newfound circumbinary planet, named TIC 172900988b, is slightly larger than Jupiter and transits both of its host stars, according to a paper to be published in one of the AAS journals. “Long before NASA’s Kepler space telescope’s discovery of transiting circumbinary planets, astronomers discussed an unusual observational signature such planets would have — the occurrence of multiple transits during one conjunction,” Dr. Veselin Kostov of NASA’s Goddard Space Flight Center and his colleagues wrote in their paper. “This effect is caused by the planet transiting one or both stars of the host eclipsing binary several times over the course of a fraction of its orbital period.” “The configuration of such transits depends on the relative sky-projected velocities of circumbinary planets and the stars being transited. Importantly, the orbital period of these planets can be estimated from such transits provided the host system is a double-lined spectroscopic binary.” “Several groups of astronomers attempted to detect transits of circumbinary planets from the ground before the turn of the century — single-conjunction or otherwise — but were ultimately hampered by the limited time sampling,” they added. “Fortunately, thanks to its long dwell time and high photometric precision, the Kepler mission enabled the discovery of a dozen transiting circumbinary planets and also demonstrated that the occurrence of pairs of transits during one conjunction is common.”

“Four of the eleven known Kepler circumbinary systems exhibit such transits: Kepler-16, Kepler-34, Kepler-35, and Kepler-1647.”

TIC 172900988b is 1.01 times larger than Jupiter and 11.07 times larger than Earth.

With a mass of 2.9 Jupiter masses, it is the most massive transiting circumbinary planet known, a factor of two times larger than the next most massive planet, Kepler-1647b.

The planet has an orbital period between 189 and 204 days and is too hot to be in the habitable zone.

It was detected in data from NASA’s Transiting Exoplanet Survey Satellite (TESS) using the multiple-transits-in-one-conjunction technique.

The analysis of the TIC 172900988 binary system’s light curve revealed one primary and two secondary eclipses with depths of 40% and 35%, respectively.

“TIC 172900988 was observed in a single sector and the planet produced just two transits — one across each star — during the same conjunction,” the astronomers wrote.

“The planet transited the primary star and then 5 days later it transited the secondary star.”

“The binary star is itself eclipsing, with a period of 19.7 days and an eccentricity of 0.45.”

TIC 172900988 is located approximately 824 light-years away in the constellation of Cancer.

Otherwise known as TYC 2483-160-1 and 2MASS J08343881+3133147, the system is 3.1 billion years old.

“TIC 172900988 demonstrates the discovery potential of TESS for circumbinary planets with orbital periods greatly exceeding the duration of the observing window,” the researchers wrote.

Sun is less active than similar stars

The extent to which solar activity (and thus the number of sunspots and the solar brightness) varies can be reconstructed using various methods - at least for a certain period of time. Since 1610, for example, there have been reliable records of sunspots covering the Sun; the distribution of radioactive varieties of carbon and beryllium in tree rings and ice cores allows us to draw conclusions about the level of solar activity over the past 9000 years. For this period of time, scientists find regularly recurring fluctuations of comparable strength as during recent decades. "However, compared to the entire lifespan of the Sun, 9000 years is like the blink of an eye", says MPS scientist Dr. Timo Reinhold, first author of the new study. After all, our star is almost 4.6 billion years old. "It is conceivable that the Sun has been going through a quiet phase for thousands of years and that we therefore have a distorted picture of our star," he adds. Since there is no way of finding out how active the Sun was in primeval times, scientists can only resort to the stars: Together with colleagues from the University of New South Wales in Australia and the School of Space Research in South Korea, the MPS researchers investigated, whether the Sun behaves "normally" in comparison to other stars. This may help to classify its current activity. To this end, the researchers selected candidate stars that resemble the Sun in decisive properties. In addition to the surface temperature, the age, and the proportion of elements heavier than hydrogen and helium, the researchers looked above all at the rotation period. "The speed at which a star rotates around its own axis is a crucial variable", explains Prof. Dr. Sami Solanki, director at MPS and co-author of the new publication.

A star's rotation contributes to the creation of its magnetic field in a dynamo process in its interior. "The magnetic field is the driving force responsible for all fluctuations in activity," says Solanki.

The state of the magnetic field determines how often the Sun emits energetic radiation and hurls particles at high speeds into space in violent eruptions, how numerous dark sunspots and bright regions on its surface are - and thus also how brightly the Sun shines.

A comprehensive catalogue containing the rotation periods of thousands of stars has been available only for the last few years. It is based on measurement data from NASA's Kepler Space Telescope, which recorded the brightness fluctuations of approximately 150000 main sequence stars (i.e. those that are in the middle of their lifetimes) from 2009 to 2013.

The researchers scoured this huge sample and selected those stars that rotate once around their own axis within 20 to 30 days. The Sun needs about 24.5 days for this. The researchers were able to further narrow down this sample by using data from the European Gaia Space Telescope. In the end, 369 stars remained, which also resemble the Sun in other fundamental properties.

The exact analysis of the brightness variations of these stars from 2009 to 2013 reveals a clear picture. While between active and inactive phases solar irradiance fluctuated on average by just 0.07 percent, the other stars showed much larger variation. Their fluctuations were typically about five times as strong.

"We were very surprised that most of the Sun-like stars are so much more active than the Sun," says Dr. Alexander Shapiro of MPS, who heads the research group "Connecting Solar and Stellar Variabilities".

However, it is not possible to determine the rotation period of all the stars observed by the Kepler telescope. To do this, scientists have to find certain periodically re-appearing dips in the star's lightcurve. These dips can be traced back to starspots that darken the stellar surface, rotate out of the telescope's field of view and then reappear after a fixed period of time.

"For many stars, such periodic darkenings cannot be detected; they are lost in the noise of the measured data and in overlying brightness fluctuations," explains Reinhold. Viewed through the Kepler telescope, even the Sun would not reveal its rotation period.

The researchers therefore also studied more than 2500 Sun-like stars with unknown rotation periods. Their brightness fluctuated much less than that of the other group.

These results allow two interpretations. There could be a still unexplained fundamental difference between stars with known and unknown rotation period. "It is just as conceivable that stars with known and Sun-like rotation periods show us the fundamental fluctuations in activity the Sun is capable of," says Shapiro. This would mean that our star has been unusually feeble over the past 9000 years and that on very large time scales phases with much greater fluctuations are also possible.

There is, however, no cause for concern. For the foreseeable future, there is no indication of such solar "hyperactivity". On the contrary: For the last decade, the Sun has been showing itself to be rather weakly active, even by its own low standards. Predictions of activity for the next eleven years indicate that this will not change soon.

New method for exoplanet stability analysis

Exoplanets revolving around distant stars are coming quickly into focus with advanced technology like the Kepler space telescope. Gaining a full understanding of those systems is difficult, because the initial positions and velocities of the exoplanets are unknown. Determining whether the system dynamics are quasi-periodic or chaotic is cumbersome, expensive and computationally demanding. In this week's Chaos, from AIP Publishing, Tamas Kovacs delivers an alternative method for stability analysis of exoplanetary bodies using only the observed time series data to deduce dynamical measurements and quantify the unpredictability of exoplanet systems. "If we don't know the governing equations of the motion of a system, and we only have the time series - what we measure with the telescope - then we want to transform that time series into a complex network. In this case, it is called a recurrence network," Kovacs said. "This network holds all of the dynamical features of the underlying system we want to analyze." The paper draws on the work of physicist Floris Takens, who proposed in 1981 that the dynamics of a system could be reconstructed using a series of observations about the state of the system.With Takens's embedding theorem as a starting point, Kovacs uses time delay embedding to reconstruct a high-dimensional trajectory and then identify recurrence points, where bodies in the phase space are close to each other.

"Those special points will be the vertices and the edges of the complex network," Kovacs said. "Once you have the network, you can reprogram this network to be able to apply measures like transitivity, average path length or others unique to that network."

Kovacs tests the reliability of the method using a known system as a model, the three-body system of Saturn, Jupiter and the Sun, and then applies it to the Kepler 36b and 36c system. His Kepler system results agree with what is known.

"Earlier studies pointed out that Kepler 36b and 36c is a very special system, because from the direct simulation and the numerical integrations, we see the system is at the edge of the chaos," Kovacs said. "Sometimes, it shows regular dynamics, and at other times, it seems to be chaotic."

The author plans to next apply his methods to systems with more than three bodies, testing its scalability and exploring its ability to handle longer time series and sharper datasets.