Exoplanets, Brown Dwarfs, and Semantics. In the show this time, Mél Azombo interviews Dr. Ed Gillen about the evolution of stellar and planetary systems, we review the status of each of the next-generation optical/near-infrared telescopes currently under construction, and George Bendo answers an ask an astronomer question about the difference between exoplanets and brown dwarfs.

The News

Three next generation ground-based optical/near-infrared telescopes are at various stages of being constructed at the moment. Each of these telescopes are going to have segmented primary mirrors of between 25 and 39 m in size, and the design and construction of each of these telescopes has been technically challenging, taking years of development and billions of US dollars in funding. We thought it would be interesting to review where each project stood at the moment.

The Giant Magellan Telescope, which is the smallest of the three telescopes with a primary mirror with a diameter of 25 m, will be built at an altitude of about 2500 m on Las Campanas Peak in Chile, which is in the Atacama Desert inland from the city of La Serena. This location already has several other telescopes operated by the Carnegie Institution for Science, making it a straightforward place to place the Giant Magellan Telescope. The telescope very recently passed its final design review, which means that construction work on this telescope could start imminently, or at least after some final funding issues are taken care of. This should be quite exciting for the people at Carnegie as well as the other institutions that are part of the consortium building this telescope.

The Extremely Large Telescope (ELT), which is a European Southern Observatory (ESO) project, is currently under construction on Cerro Amazones, a mountain with an elevation close to 3050 m that is located in the northern part of Chile's Atacama Desert near Antofagasta. Even though a substantial fraction of the telescope's dome has been built, the ELT recently reached a notable milestone. As I mentioned before, each of these next-generation telescopes will have segmented primary mirrors, and the 39 m diameter primary mirror for the ELT will consist of 798 hexagonal segments, each of which is 1.5 meters across. However, if that did not sound like enough segments, the telescope will have 133 spare segments, bringing the total number of segments needed to 949. Well, back in June, ESO received the last of the 949 special glass blanks needed to create the 949 segments for the primary mirror. These blanks still need to be polished and cut into hexagons, but the assembly of the telescope is still underway.

As for the Thirty Meter Telescope (TMT), it's still in the planning and design phase. The current plans call for the TMT to be build it on Maunakea in Hawaii, but the observatory has faced numerous legal challenges and protests over the past several years regarding its construction, with some of the strongest opposition coming from Native Hawaiians, who consider various areas near the summit of Maunakea to be culturally or spiritually important. Additionally, the National Science Foundation, which had previously committed funds to both the Giant Magellan Telescope and the TMT, has been reviewing funding for both telescopes for a few months now and may choose to cut funding for one of them. Given the opposition that the TMT is facing as well as the results from Google searches on the TMT and Giant Magellan Telescopes, which seem to mention the funding issue primarily when I search for the TMT, it would not be surprising if the TMT is the telescope to get cut. This would not necessarily end the project, but the TMT would need to try to locate funds from somewhere else if they want to build it.

Interview with Dr. Ed Gillen

In this episode, Mél Azombo interviews Dr. Ed Gillen about the evolution of stellar and planetary systems. Dr. Gillen discusses how he shifted the focus of his research from binary star systems to the origins of exoplanets, why new telescopes and instruments are needed for making further advances in studying exoplanets, and in particular the challenges and potential rewards from working with data from new transiting surveys.

Ask An Astronomer

QUESTION: T. K. Arispe wrote, "What, if anything, is the difference between a small brown dwarf and a large planet? When NASA put out a news article about JWST discovering a very small free-floating brown dwarf, I found it a bit odd that they kept insisting it was a brown dwarf and not a planet, basing that judgment on the fact that it apparently formed by itself. This is despite the fact that the object in question is 3-4 Jupiter masses, when the commonly accepted lower limit for brown dwarfs is 13 Jupiter masses."

ANSWER: So, the International Astronomical Union has not officially adopted a definition of the difference between a small brown dwarf and large planet, but two common definitions are used. This topic was mentioned during the interview, but I thought it would be a good idea to go into more detail here.

The first method some people use to differentiate between a brown dwarf and a large planet is based on whether the object seemed to form by itself. The idea is that things that are more star-like would not be gravitationally bound to other objects, and those things should be called brown dwarfs, while things that are more planet-like would have formed out of the circumstellar disks surrounding stars and would be gravitationally bound to those stars, and those objects should be called planets. I personally think this definition is, to use the technical term, bogus. First of all, we would have to trust that free-floating things are not objects that got ejected from the star systems that they were orbiting. This could be impossible to do in a situation like what T. K. Arispe mentioned. We also know that hydrogen-fusing stars can form in orbit around other hydrogen-burning stars in binary star systems or multiple star systems, so it's not necessarily true that anything gravitationally bound to another star is going to be planet-like. I also can't imagine identifying a massive brown dwarf very close to being hydrogen-burning but not quite there as an exoplanet on the basis that it could be orbiting something else.

The second method some people use to differentiate between a brown dwarf and an exoplanet is whether the object has enough mass to trigger the fusion of deuterium. While objects need to be 0.075 times the mass of the Sun (or 78.5 times the mass of Jupiter) to trigger the fusion of protium (which would be hydrogen atoms with one proton and no neutrons), the mass needed to trigger the fusion of deuterium (which would be hydrogen atoms with one proton and one neutron) is about 13 Jupiter masses. In my opinion, this is a much clearer distinction between the two classes of objects; brown dwarfs can actually generate energy more effectively than gas giants. The cutoff between fusing deuterium burning and not fusing deuterium is not exact, but it's still better than trying to distinguish between brown dwarfs and large exoplanets based on where they formed.

As for the press release that T. K. Arispe mentioned, I find it a bit odd that people are sometimes really hesitant to mention discovering exoplanets. In episode 44 of George's Random Extragalactic Object, I mentioned a similar situation where an exoplanet directly imaged by the Spitzer Space Telescope at infrared wavelengths was discovered orbiting a white dwarf, but for some reason, the science paper calls it a very very cool brown dwarf and not an exoplanet, even though other people citing that paper called it an exoplanet. I think that situation as well as the free-floating 3-4 Jupiter mass exoplanet found by the JWST are cases where people are so worried about wrecking their careers by overstating their discoveries that they just go the opposite direction and severely understate their discoveries, which, to some degree, does not help, as these really small objects are much more functionally like gas giants than like brown dwarfs and should be treated in scientific analyses as such.

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NAME: Anne
Greeting from Bellvue, Washington, USA. Glad you are back. I hope you do a podcast about moons in the Solar System. Take care. Jod on.

NAME:David Ault
Great to see the Jodcast continuing under new management! Keep up the good work!

NAME: Philip Le Riche emails

Disappointed there wasn't an August edition, but I'm sure you all deserve a great Summer break - Enjoy!

Interview : Dr. Ed Gillen and Mél Azombo
Presenters : George Bendo and Honor Harris
Editors : George Bendo
Website : George Bendo and Lily Correa Magnus
Producer : George Bendo
Cover Art : An artist's illustration of two exoplanets passing in front of TRAPPIST-1. CREDIT:NASA/ESA/STScI/J. de Wit (MIT).