Illustration shows how the stars motion around the center of mass between it and the planet causes a wobble in its motion through space. The VLBAs ability to detect this minuscule effect revealed the presence of the planet. Credit: Bill Saxton, NRAO/AUI/NSF

Using the supersharp radio vision of the National Science Foundations continent-wide Very Long Baseline Array (VLBA), astronomers have discovered a Saturn-sized planet closely orbiting a small, cool star 35 light-years from Earth. This is the first discovery of an extrasolar planet with a radio telescope using a technique that requires extremely precise measurements of a stars position in the sky, and only the second planet discovery for that technique and for radio telescopes.

The technique has long been known, but has proven difficult to use. It involves tracking the stars actual motion in space, then detecting a minuscule wobble in that motion caused by the gravitational effect of the planet. The star and the planet orbit a location that represents the center of mass for both combined. The planet is revealed indirectly if that location, called the barycenter, is far enough from the stars center to cause a wobble detectable by a telescope.

Side-by-side animations of the star and planet orbiting their common center of gravity (barycenter), and of the pair moving through space as they orbit, creating the stars wobble that revealed the planet. Credit: Bill Saxton, NRAO/AUI/NSF

This technique, called the astrometric technique, is expected to be particularly good for detecting Jupiter-like planets in orbits distant from the star. This is because when a massive planet orbits a star, the wobble produced in the star increases with a larger separation between the planet and the star, and at a given distance from the star, the more massive the planet, the larger the wobble produced.

Starting in June of 2018 and continuing for a year and a half, the astronomers tracked a star called TVLM 51346546, a cool dwarf with less than a tenth the mass of our Sun. In addition, they used data from nine previous VLBA observations of the star between March 2010 and August 2011.

Artists conception of dwarf star TVLM-513-46546 and its newly-discovered planetary companion. Credit: Luis A. Curiel Ramirez

Extensive analysis of the data from those time periods revealed a telltale wobble in the stars motion indicating the presence of a planet comparable in mass to Saturn, orbiting the star once every 221 days. This planet is closer to the star than Mercury is to the Sun.

Small, cool stars like TVLM 51346546 are the most numerous stellar type in our Milky Way Galaxy, and many of them have been found to have smaller planets, comparable to Earth and Mars.

Artists conception of dwarf star TVLM-513-46546 and its newly-discovered planetary companion. Credit: Luis A. Curiel Ramirez

Giant planets, like Jupiter and Saturn, are expected to be rare around small stars like this one, and the astrometric technique is best at finding Jupiter-like planets in wide orbits, so we were surprised to find a lower mass, Saturn-like planet in a relatively compact orbit. We expected to find a more massive planet, similar to Jupiter, in a wider orbit, said Salvador Curiel, of the National Autonomous University of Mexico. Detecting the orbital motions of this sub-Jupiter mass planetary companion in such a compact orbit was a great challenge, he added.

More than 4,200 planets have been discovered orbiting stars other than the Sun, but the planet around TVLM 51346546 is only the second to be found using the astrometric technique. Another, very successful method, called the radial velocity technique, also relies on the gravitational effect of the planet upon the star. That technique detects the slight acceleration of the star, either toward or away from Earth, caused by the stars motion around the barycenter.

The Very Long Baseline Array is a continent-wide radio telescope system spanning the distance from Hawaii in the Pacific to St. Croix in the Caribbean. It provides astronomers with extremely high resolving power, the ability to see fine detail. Credit: J. Hellermann, NRAO/AUI/NSF

Our method complements the radial velocity method which is more sensitive to planets orbiting in close orbits, while ours is more sensitive to massive planets in orbits further away from the star, said Gisela Ortiz-Leon of the Max Planck Institute for Radio Astronomy in Germany. Indeed, these other techniques have found only a few planets with characteristics such as planet mass, orbital size, and host star mass, similar to the planet we found. We believe that the VLBA, and the astrometry technique in general, could reveal many more similar planets.

A third technique, called the transit method, also very successful, detects the slight dimming of the stars light when a planet passes in front of it, as seen from Earth.

Very Long Baseline Array (VLBA) astrometry detected a wobble in the proper motion of a small star caused by the gravitational effect of a Saturn-sized exoplanet. First radio discovery of an exoplanet by this technique. Credit: NRAO/AUI/NSF

The astrometric method has been successful for detecting nearby binary star systems, and was recognized as early as the 19th Century as a potential means of discovering extrasolar planets. Over the years, a number of such discoveries were announced, then failed to survive further scrutiny. The difficulty has been that the stellar wobble produced by a planet is so small when seen from Earth that it requires extraordinary precision in the positional measurements.

The VLBA, with antennas separated by as much as 5,000 miles, provided us with the great resolving power and extremely high precision needed for this discovery, said Amy Mioduszewski, of the National Radio Astronomy Observatory. In addition, improvements that have been made to the VLBAs sensitivity gave us the data quality that made it possible to do this work now, she added.

Curiel, Ortiz-Leon, Mioduszewski, and Rosa Torres of the University of Guadalajara in Mexico, reported their findings in the Astronomical Journal.

Reference: An Astrometric Planetary Companion Candidate to the M9 Dwarf TVLM 51346546 by Salvador Curiel, Gisela N. Ortiz-Len, Amy J. Mioduszewski and Rosa M. Torres, 4 August 2020, Astronomical Journal.DOI: 10.3847/1538-3881/ab9e6e

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

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Saturn-Sized Exoplanet Discovered by the Gravitational Wobble in the Small, Cool Star It Orbits - SciTechDaily