The reddest planet ever seen explains the spiral arms of recently forming star systems

A giant planet twice the mass of Jupiter has been detected after previous searches failed to find it. The planet was expected to unfurl swirling arms within the protoplanetary disk around a very young star, but was missed because its light is at unexpectedly long wavelengths. Just as the discovery of the planets solves the mystery of the spiral arms, its redness creates a new one, with some interesting possible explanations under investigation.

Point a fairly powerful telescope at the sky and sometimes you’ll find an object with a bright center surrounded by spiral arms. Normally this will be a galaxy with a structure similar to that of our own Milky Way. If you’re operating in the infrared, however, you may be observing a star so young that it’s still surrounded by its protoplanetary disk, the gas and dust from which its planets will eventually form.

Of the 29 suitably young stars close enough for detailed study, eight have distinct spiral arms within their disks. These have been attributed to the gravitational influence of giant planets that formed before the rest of the system merged. Proving this was difficult, however, even for MWC 758, a star system 500 light-years away in Taurus. Now, however, a Nature Astronomy article announces the discovery of MWC 758c.

The arms of spiral galaxies form without giant gravitational fields to guide them, but protoplanetary disk models indicate that seemingly similar structures should require a planetary initiator. This then affects how smaller objects come together. Jupiter probably shaped the Earth and other inner planets this way. A possible planet, MWC 758b, was discovered in 2018, but subsequent observations have failed to confirm it. The lack of a shepherd planet has been such a worrying problem that several articles have been written trying to figure out where it is hiding after a decade of observations.

The discovery was finally made using the interferometer of the Large Binocular Telescope (LBT) at Mount Graham, Arizona. The LBTI is unusual among ground-based telescopes in that it can see well in the infrared, as well as optical wavelengths.

In visible light, a planet like this is so obscured by its star that we have little hope of seeing it. However, in infrared, the difference is less dramatic. This allowed us to spot a number of newly formed giant planets still warm from their formation.

To see MWC 758c, however, the authors had to observe wavelengths longer than those required for other emerging planets. It doesn’t get bright until it’s even more infrared than any planet we’ve seen before, and its discoverers want to know why.

“We propose two different models of why this planet is brighter at longer wavelengths,” said Dr. Steve Ertel of the University of Arizona in a statement. “Either this is a planet with a colder temperature than expected, or it’s a planet that’s still warm from its formation, and appears to be shrouded in dust.” Dust absorbs shorter wavelengths than longer ones and makes objects look redder than they are.

If MWC 785c is cold (relatively speaking, the light is consistent with being around 200C (392F)) it would upset much of what we think we know about planet formation. It could also offer the possibility of studying the chemistry of carbon in a planet cold enough to have clouds of water in its atmosphere.

Being surrounded by dust would be less surprising, but interesting in a different way. Some of the surrounding dust could end up settling on MWC 758c and make it bigger, but some could go on to form moons like Jupiter’s big four. It would be exciting to be able to watch the process in action.

JWST has been given time in 2024 to look into MWC 758 and hopefully resolve the matter.

The paper is published in Nature Astronomy.