Artist’s rendition of the super-eccentric, retrograde giant exoplanet orbiting TIC 241249530. Image credit: NOIRLab/NSF/AURA/J. da Silva (Spaceengine)
Introduction
Today, the NEID Science Team announced the discovery of the exoplanet TIC 241249530b, a giant exoplanet with some truly remarkable properties. The planet follows an oblong, highly eccentric orbit around its star; if we compare it to objects in the Solar System, its orbit more closely resembles that of a comet than those of our planets. Furthermore, its orbit is almost exactly backwards from what we would expect based on theories of planet formation. Beyond being an interesting oddity in itself, TIC 241249530b offers a unique glimpse into the process of planetary orbital evolution, helping to explain the existence of “hot Jupiter” exoplanets. Read on for more details!
Discovering TIC 241249530b
As is the case for other NEID discoveries discussed here, our first indication of a planet orbiting the star TIC 241249530 was the detection of the planet transiting the star by the TESS satellite. However, a transit signal often tells us little more than the radius of the planet and its distance from the star. In the case of TIC 241249530, we didn’t even get that much information; the planet completes an orbit every 166 days, which is so long that TESS only caught one transit, leaving the orbital period ambiguous.
In order to fully characterize its orbit, we conducted a multi-facility observational campaign to find additional transits of the planet, and to measure radial velocities of the star. From September 2021 to March 2024, we monitored TIC 241249530 with NEID and its sister spectrograph, the Habitable-zone Planet Finder, in order to confirm the planetary nature of the signal seen by TESS, and to measure the mass and orbital period of the planet. At the same time, we used the diffuser-assisted camera ARCTIC on the 3.5-meter ARC Telescope at Apache Point Observatory to observe an additional transit from the ground.
A Highly Eccentric Planet
Our observations fully confirmed the existence of the planet TIC 241249530b, and firmed up its basic properties. Specifically, the planet is truly giant, with 5 times the mass of Jupiter, and it orbits its star at an average distance of 64 percent the Earth-Sun distance. However, in this case, stating the average distance may be misleading, as the actual planet-star separation varies wildly over the course of an orbit.
The orbital path of TIC 241249530b, shown in comparison to the orbits of Mercury and Earth. TIC 241249530 has the highest eccentricity ever observed for an exoplanet. Image credit: NOIRLab/NSF/AURA/R. Proctor
We know from the basic physics of gravity that all orbits must be elliptical in shape, rather than perfectly circular. Many orbits, like those of the 8 planets in our Solar System, are very close to circular. Others, like the comets that populate the outer Solar System, have highly oblong orbits, which explains why these distant, icy bodies occasionally swing in close to the Sun. We use the quantity called eccentricity to describe how circular an orbit is, with eccentricity of 0 corresponding to a perfect circle, and 1 describing an orbit so elongated it becomes a flat line.
From our radial velocity observations of TIC 241249530, we learned that its giant planet resides on the most eccentric orbit ever seen for an exoplanet, with an eccentricity value of 0.94! This means that over an orbit, the planet swings from a distance similar to that of Earth in our Solar System all the way in to just 6 times the radius of the star itself. Clearly, TIC 241249530b is an exotic and unusual world, but it turns out the orbital eccentricity is just part of its extreme nature.
A video showing the extreme orbit of TIC 241249530b. As the planet makes its closest approach to the star, it becomes brighter due to the increase in incident stellar radiation. Video credit: Abigail Minnich, Penn State University
It’s Retrograde, Too!
Since TIC 241249530b is a transiting planet, we can use the Rossiter-McLaughlin technique to measure the inclination of its orbit relative to the rotational axis of the star. NEID is the perfect instrument for the job, and we measured the RM effect of the planet during two separate transits.
Measurements of the Rossiter-McLaughlin effect for TIC 241249530b from NEID. Our model to the signal is shown as a solid black line, while our expectation for the signal of a planet aligned with the star’s rotational axis is shown as a dashed line. This measurement shows that the planet is in a retrograde orbit relative to the stellar spin axis
As it turns out, TIC 241249530b orbits its star in a direction that is almost exactly opposite to the direction of the star’s rotation. This is, again, unusual: for most of the planets where we have been able to measure the RM effect, we have seen that their orbits are well aligned with their stars’ spin axes. Furthermore, planets are expected to have orbits aligned with stellar rotation based on our current theories of planet formation.
Dynamical Violence
Between its record-setting eccentricity and its retrograde orbit, TIC 241249530b is unique among known exoplanets. How did it come to have such an extreme orbit?
A likely possibility is that TIC 241249530b is in an intermediate stage of a process known as planetary migration. Briefly, a planet may form at one location in its stellar system, but gravitational interactions with the disk of material from which it formed, or other bodies in the system, will cause it to move to a different orbit. TIC 241249530 actually has a second star in the system, which could have dynamically impacted the orbit of the planet over the system’s nearly 4 billion year history.
Computers can simulate the effects of a planet like TIC 241249530b being disturbed by a massive companion such as the secondary star in this system. What results is called the Lidov-Kozai mechanism, wherein the orbital eccentricity and inclination of the planet can be driven to extreme values. Over many orbits–each of which includes a close-in pass to the host star–the gravity of the star will “dampen” the planet’s orbit, causing it to eventually settle in to a circular orbit near to the star.
It would appear, then, that in the TIC 241249530 system we are bearing witness to the high-eccentricity phase of planet migration, and that the planet will eventually become one of the “hot Jupiter” exoplanets that so surprised astronomers when they became the first exoplanets ever discovered. It is difficult to explain the formation of giant planets like Jupiter in the hot regions close to a star; instead, astronomers believe they probably form far from their host stars, and later move inwards through mechanisms such as Lidov-Kozai oscillations. In that regard, TIC 241249530b will serve as a valuable case study for further refining our theoretical understanding of exoplanet formation and migration. It also provides a stark reminder that the Galaxy is not a calm, unchanging environment; the orbits of stars and their planets do not always play nicely with one another, and the results can be extreme!
Find Out More
This post is based on a research manuscript published today in Nature, led by NEID Science Team member Arvind Gupta. If you would like to learn all of the details behind the discovery, we encourage you to check out the paper.