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Maunakea Telescopes Confirm First Super-Planet Discovered by Radio Observations



Dr. Michael Liu
UH Institute for Astronomy
Office: +1 808-956-6666

Dr. Harish Vedantham
ASTRON and University of Gröningen

Dr. Roy Gal
Media Contact
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Artist impression of radio-discovered brown dwarf Elegast
Artist's impression of Elegast. The blue loops depict the magnetic field lines. Charged particles moving along these lines emit radio waves that LOFAR detected. Some particles eventually reach the poles and generate aurorae similar to the northern lights on Earth.
Credit: ASTRON / Danielle Futselaar

A collaboration between the LOw Frequency ARray (LOFAR) radio telescope in Europe and two telescopes on the summit of Maunakea, the Gemini Observatory and the NASA InfraRed Telescope Facility (IRTF), has led to the first direct discovery of a cold brown dwarf from its radio emission. In addition to demonstrating a new way for future brown dwarf discoveries, this result is an important step towards applying radio astronomy to the exciting field of exoplanets.

For the first time, astronomers have used observations from the LOFAR radio telescope, the NASA IRTF, operated by the University of Hawaiʻi, and the international Gemini Observatory to discover and characterize a cold brown dwarf. The object, designated BDR J1750+3809 (also dubbed "Elegast" by the discovery team), is the first substellar object to be discovered through radio observations — until now, brown dwarfs have largely been found from infrared sky surveys. Directly discovering these objects with sensitive radio telescopes such as LOFAR is a significant breakthrough, because it demonstrates that astronomers can detect objects that are too cold and faint to be found in infrared surveys, and perhaps even detect free-floating gas-giant exoplanets.

"This work opens a whole new method to finding the coldest objects floating in the Sun's vicinity, which would otherwise be too faint to discover with the methods used for the past 25 years," said Michael Liu, an astronomer at the University of Hawaʻi's Institute for Astronomy (IfA) and a co-author on the discovery paper.

Brown dwarfs are substellar objects straddling the boundary between the largest planets and the smallest stars1. Occasionally dubbed 'failed stars', brown dwarfs lack the mass to trigger hydrogen fusion in their cores, and instead glow at infrared wavelengths with leftover heat from their formation. Also dubbed 'super-planets', their gaseous atmospheres resemble the gas-giant planets in our solar system more than they resemble any star. While brown dwarfs lack the fusion reactions that keep our Sun shining, they can emit light at radio wavelengths. The underlying process powering this radio emission is familiar, as it occurs in the largest planet in the Solar System. Jupiter's powerful magnetic field accelerates charged particles such as electrons, which in turn produces radiation — in this case radio waves2 and aurorae.

The fact that brown dwarfs are radio emitters allowed the international collaboration of astronomers behind this result to develop a novel observing strategy. Radio emissions have previously been detected from only a handful of cold brown dwarfs, which were discovered and catalogued by infrared surveys before being observed with radio telescopes. The team decided to flip this strategy, using a sensitive radio telescope to discover cold, faint radio sources and then perform follow-up infrared observations with Maunakea telescopes to categorize them.

"We asked ourselves, why point our radio telescope at catalogued brown dwarfs?," said Harish Vedantham, lead author of the study and astronomer at ASTRON in the Netherlands. "Let's just make a large image of the sky and discover these objects directly in the radio."

Having found a variety of tell-tale radio signatures in their observations, the team had to distinguish potentially interesting sources from background galaxies. To do so, they searched for a special form of radio waves which were circularly polarized3 — a feature of light from stars, planets, and brown dwarfs, but not from background galaxies. Having found a circularly polarized radio source, the team then turned to archive imagery, the Gemini-North Telescope, and the NASA IRTF to provide the measurements required to identify their discovery.

NASA IRTF is equipped with a sensitive infrared spectrometer, called SpeX, which has been a workhorse for studying brown dwarfs for the past 20 years, including an upgrade 5 years ago funded by the National Science Foundation (NSF). The team used SpeX to obtain a spectrum of BDR J1750+3809, which revealed the characteristic signature of methane in the atmosphere. Methane is the hallmark of the coolest brown dwarfs, and also is abundant in the atmospheres of our solar system's gas-giant planets.

"These observations really highlight the increased efficiency of SpeX following its NSF-funded upgrade with state-of-the-art infrared arrays and electronics in 2015," said John Rayner, IRTF Director and astronomer at the University of Hawaii's Institute for Astronomy.

As well as being an exciting result in its own right, the discovery of BDR J1750+3809 could provide a tantalizing glimpse into a future when astronomers can measure the properties of exoplanets' magnetic fields. Cold brown dwarfs are the closest things to exoplanets that astronomers can currently detect with radio telescopes, and this discovery could be used to test theories predicting the magnetic field strength of exoplanets. Magnetic fields are an important factor in determining the atmospheric properties and long-term evolution of exoplanets.

"Our ultimate goal is to understand magnetism in exoplanets and how it impacts their ability to host life," concluded Vedantham. "Because magnetic phenomena of cold brown dwarfs are so similar to what is seen in Solar System planets, we expect our work to provide vital data to test theoretical models that predict the magnetic fields of exoplanets."


[1] The first unambiguous observation of a brown dwarf did not occur until 1995, after more than 30 years of theoretical predictions. The name of these objects was coined by the American astronomer Jill Tarter in reference to their expected color.

[2] The radiation emitted by the acceleration of charged particles in a magnetic field is referred to as cyclotron radiation. The name comes from the cyclotron, an early type of particle accelerator.

[3] Circularly polarized light is also used to create 3D movies.

More information

This research is presented in the paper Direct Radio Discovery of a Cold Brown Dwarf, to appear in The Astrophysical Journal Letters.

The team is composed of H. K. Vedantham (ASTRON and University of Groningen), J. R. Callingham (Leiden Observatory and ASTRON), T. W. Shimwell (ASTRON and Leiden Observatory), T. Dupuy (University of Edinburgh and Gemini Observatory/NSF's NOIRLab), William M. J. Best (University of Texas), Michael C. Liu (University of Hawaiʻi), Zhoujian Zhang (University of Hawaiʻi), K. De (California Institute of Technology), L. Lamy (LESIA, Observatoire de Paris), P. Zarka (LESIA, Observatoire de Paris), H. J. A. Röttgering (Leiden Observatory), and A. Shulevski (Leiden Observatory).

Founded in 1967, the Institute for Astronomy at the University of Hawaiʻi at Mānoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakalā and Maunakea. The Institute operates facilities on the islands of Oahu, Maui, and Hawaiʻi.