Exoplanet made almost with iron

Newswand: Researchers have found an exoplanet made almost with iron. Over the past decades, astronomers have found several thousand extrasolar planets. Extrasolar planets orbit stars outside our solar system.

The next frontier in this research field is to learn more about their composition and internal structure, in order to develop a better understanding of how planets form.

Elisa Goffo, Ph.D. student at the Physics Department of the University of Turin (Italy) and at the Thüringer Landessternwarte (Germany), together with an international research team, has made a unique discovery about the planet GJ 367 b that raises interesting questions about how planets are born.

She is the lead author of the article Company for the ultra-high density, ultra-short period sub-Earth GJ 367 b: discovery of two additional low-mass planets at 11.5 and 34 days published in “The Astrophysical Journal Letters”.

Elisa Goffo is part of the international KESPRINT collaboration, which confirmed that the ultra-short period exoplanet GJ 367 b, whose orbital period is only 7.7 hours, is also ultra-dense.

The density of a planet can be determined from its mass and radius. GJ 367 b is ultra-dense because the researchers found its density to be 10.2 grams per cubic centimeter. That is almost twice the density of Earth, suggesting that this extrasolar planet consists almost entirely of iron.

An unusual composition

Such a composition of a planet is very rare, raising questions about its formation. “You could compare GJ 367 b to an Earth-like planet with its rocky mantle stripped away. This could have important implications for the formation of GJ 367 b. We believe that the planet might have formed like the Earth, with a dense core made mainly of iron, surrounded by a silicate-rich mantle. A catastrophic event could have stripped away its rocky mantle, leaving the dense core of the planet naked.

Alternatively, GJ 367 b was born in an iron-rich region of the protoplanetary disc”, explains Elisa Goffo. While observing GJ 367 b, the team discovered two additional low-mass planets that orbit around the star GJ 367 in 11.5 days and 34 days, respectively. These three planets and their star comprise an extrasolar system.

GJ 367 b was first found with the Transiting Exoplanet Survey Satellite (TESS), a space telescope operated by NASA. TESS uses the transit method to measure the radii of exoplanets – among other properties. In order to precisely measure the mass of GJ 367 b and confirm that the planet has a very high density, the KESPRINT researchers at the University of Turin and at the Thüringer Landessternwarte acquired nearly 300 radial velocity measurements using the HARPS spectrograph, a high-precision instrument installed at the 3.6 meter telescope operated by the European Southern Observatory (ESO) at La Silla Observatory, Chile.

“Thanks to our intensive observations with the HARPS spectrograph we discovered the presence of two additional low-mass planets with orbital periods of 11.5 and 34 days, which reduce the number of possible scenarios that might have led to the formation of such a dense planet”, says Davide Gandolfi, Professor at the University of Turin. “While GJ 367 b might have formed in an iron-rich environment, we do not exclude a formation scenario involving violent events like giant planet collisions.”

Artie Hatzes, director at the Thüringer Landessternwarte, underscores the relevance of this discovery: “GJ 367 b is an extreme case of an exoplanet. Before we can develop viable theories of its formation, we must precisely measure the planetary mass and radius. We expect an extrasolar system to consist of several planets, so it was important to search for and to find other planets orbiting in the system – to study its architecture.”

Ends…

Planet with longest orbit detected

Newswand: Astronomers have found a planet with long orbit which opened new vistas in the research of the Universe and exoplanets.

Of the more than 5,000 planets known to exist beyond our solar system, most orbit their stars at surprisingly close range. More than 80 percent of confirmed exoplanets have orbits shorter than 50 days, placing these toasty worlds at least twice as close to their star as Mercury is to our sun — and some, even closer than that.

Astronomers are starting to get a general picture of these planets’ formation, evolution, and composition. But the picture is much fuzzier for planets with longer orbital periods. Far-out worlds, with months- to years-long orbits, are more difficult to detect, and their properties have therefore been trickier to discern.

Now, the list of long-period planets has gained two entries. Astronomers at MIT, the University of New Mexico, and elsewhere have discovered a rare system containing two long-period planets orbiting TOI-4600, a nearby star that is 815 light years from Earth.

The team discovered that the star hosts an inner planet with an orbit of 82 days, similar to that of Mercury, while a second outer planet circles every 482 days, placing it somewhere between the orbits of Earth and Mars.

The discovery was made using data from NASA’s Transiting Exoplanet Survey Satellite, or TESS — an MIT-led mission that monitors the nearest stars for signs of exoplanets. The new, farther planet has the longest period that TESS has detected to date. It is also one of the coldest, at about -117 degrees Fahrenheit, while the inner planet is a more temperate 170 degrees Fahrenheit.

Both planets are likely gas giants, similar to Jupiter and Saturn, though the composition of the inner planet may be more of a mix of gas and ice. The two planets bridge the gap between “hot Jupiters” — the toasty, short-orbit planets that make up the majority of exoplanet discoveries — and the much colder, longer-period gas giants in our solar system.

“These longer-period systems are a comparatively unexplored range,” says team member Katharine Hesse, a technical staff member at MIT’s Kavli Institute for Astrophysics and Space Research. “As we’re trying to see where our solar system falls in comparison to the other systems we’ve found out there, we really need these more edge-case examples to better understand that comparison. Because a lot of systems we have found don’t look anything like our solar system.”

Hesse and her colleagues, including lead author Ismael Mireles, a graduate student at the University of New Mexico (UNM), have published their results today in Astrophysical Journal Letters.

Patch work

TESS monitors the nearest stars for signs of exoplanets by pointing at a patch of the sky and continuously measuring the brightness of stars in that sector for 30 days, before swiveling to the next patch. Scientists use “pipelines,” or algorithmic searches, to comb through the measurements for dips in brightness that could have been caused by a planet passing in front of its star.

In 2020, one pipeline picked up a possible transit from a star in the northern sky, close to the constellation Draco. The star was categorized as TOI-4600 (a TESS Object of Interest). The initial transit was studied in detail by the TESS Single Transit Planet Candidate Working Group, a team of scientists at MIT, UNM, and elsewhere who look for signs of longer-period planets in single-transit events.

“For missions like TESS, where it only looks at each region of the sky for 30 days, you really need to stack up the number of observations to be able to get enough data to find planets with orbits longer than a month,” Hesse notes.

The group looked for the star in other sectors of TESS data and eventually identified three more transits, similar to the first. From these four events, the scientists were able to determine that the source was a planet — TOI-4600b — with a relatively long 82-day orbit. The team also picked up a fifth transit, though it was out of sync with the other signals. They wondered: Could the transit be from another star temporarily eclipsing the first? Or could it be a second orbiting planet?

Giants in the sky

In 2021, when Mireles joined the group, he took up where the team left off, looking for more observations from TESS that would explain the last, puzzling transit.

“With each sector of data that came down, I would look to see if there was a second transit, and in the first five sectors, there wasn’t,” Mireles recalls. “Then, in July of last year, we saw something.”

Actually, they saw two things: one transit that appeared in the same 82-day cycle, which further confirmed the existence of a long-orbiting planet; and a second transit, which was detected 964 days after the previous, out-of-sync transit. These last two transits were similar in depth, or the amount of light that was dimmed, suggesting that both were produced by a single object that was orbiting the star, either every 964 days, or every 482 days. After all, the team reasoned, TESS simply could have not been looking in the star’s direction to catch the planet crossing at the 482-day mark. The team used a model to simulate what a planet would look like with both orbital periods, and concluded that the 482-day orbit was more likely.

Ends…