A Third Planet Orbits the 25th Nearest Star

Title: Teegarden’s star revisitied: A nearby planetary system with at least three planets

Authors: S. Dreizler et al.

First Author’s Institution:  Institut für Astrophysik, Georg-August-Universität

Status: ArXiv [open access]

An international collaboration of exoplanet astronomers has announced the discovery of a third planet in the Teegarden’s star system! But wait…Teegarden’s star? If you haven’t heard of this star, you’re not alone!

Teegarden’s star

Teegarden’s star is very small, only about 10% of the mass and radius of our Sun, and very cool in temperature, only ~3000 K which makes it an M dwarf. Because of its small size and cool temperature, it is very dim in visible light. In fact, it is so dim, that despite being the 25th closest star to Earth, it was not discovered until 2003! At the time, a team led by NASA astrophysicist Bonnard J. Teegarden was sifting through some archival data sets taken as part of an asteroid tracking survey. These kinds of asteroid tracking surveys generally operate by taking wide-field images of the sky and then looking for any dots of light that move on an hourly/weekly/monthly basis. So they are very good at finding things that “appear” to be new in their region of the sky. Therefore, this was the perfect data set to find a “new” star, which is exactly what the team found: one star was out of place. With further follow up, they determined it was in fact a previously undiscovered star and it was subsequently named after Teegarden.

Back in 2019, a team announced the discovery of two exoplanets orbiting Teegarden’s star.  This was big news because it made Teegarden’s star the coolest (in temperature!) star to be known to host a planetary system. The two planets are on 5 day and 12 day orbital periods, which, at first glance might suggest that they are very hot in their close-in orbits, but in fact the star is so low temperature that they are in the optimistic habitable zone! This is the range of distances from the host star where the temperature is amenable to having water in a liquid form. 

A Third Planet

Now using new data from 4 different observatories, a third planet is announced in the system on a 26 day orbit and with a mass of just a little less than our own Earth’s mass! The team combined archival and new data from the CARMENES spectrograph at Calar Alto Observatory in southern Spain, the ESPRESSO spectrograph at Paranal Observatory in northern Chile, the Maroon-X spectrograph at Gemini North Observatory in Hawaii, and the Habitable-zone Planet Finder (HPF) spectrograph at McDonald Observatory in Texas. Together, they amassed 355 total observations, an impressive data set. These observations are all spectra of the star, allowing the team to compute the radial velocity (RV) of the star at timestamp to search for planets using the gravitational wobble, and associated Doppler shift, imparted on the star from the planet. Through a rigorous model selection analysis, the team’s preferred model is one that includes the original two planets and the third planet announced here. Additionally, they found two additional signals, one at 98 days and another at 172 days, that correlated with similar periodic signals in metrics that track stellar activity of the star, therefore leading the team to conclude that these signals most likely represent stellar activity. Through this analysis, they affirm a stellar rotation period of 98 days, in line with previous measurements and estimates. 

Additional Signals?

The team also finds two tantalizing signals, at 7.7 days and 17 days, but cannot confirm that they are a statistically significant signal nor that they are even planetary signals. They go on to discuss that a 4th and/or 5th planet would fit nicely into this system at these orbital periods as simulations show the 4 and 5 planet systems would be dynamically stable. They also point out that such planets would create period resonances with the 5 and 12 day planets, which would make the system have a very similar architecture to one of the most famous M dwarf multi-planet systems, TRAPPIST-1, see Figure 1. Despite all this, they do not claim a planet at 7.7 days nor at 17 days. 

The authors go on to describe an exhaustive transit search. Using public data from the TESS mission, the authors review the light curves for possible transit events of the star. Unfortunately, they do not find any transits, despite running a test where they simulated fake planets in the real data set and tried to detect them with the same methods as they used to try to detect a real planet, this is sometimes called an “injection/recovery test”. This test showed that they should be sensitive to the expected sizes of the known planets in the system. 

The paper concludes by putting this system into context with the known population. Here is a very low mass and temperature star that has at least 3 small, likely rocky, planets orbiting it. This gets added to only a small sample of other multi-planet systems with well-measured masses of small planets around M dwarfs. They also end on an optimistic note: there is much more to explore in this system! A possible 4th planet signal, interesting stellar activity signals, and perhaps an opportunity to directly image the 26 day planet with future space missions like the Habitable Worlds Observatory

Figure 1: A selection of similar systems to Teegarden’s star based on stellar host, multiplicity, and dynamics. The authors include a hypothetical Teegarden’s star system (denoted with an “h”) which includes the two non-confirmed signals. Credit: Figure 6 in the paper.

Astrobite edited by Pranav Satheesh

Featured image credit: UNIVERSITY OF GÖTTINGEN, INSTITUTE FOR ASTROPHYSICS

About Jack Lubin

Jack received his PhD in astrophysics from UC Irvine and is now a postdoc at UCLA. His research focuses on exoplanet detection and characterization, primarily using the Radial Velocity method. He enjoys communicating science and encourages everyone to be an observer of the world around them.

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