The Week in Space and Physics: A Rare Hot Neptune
The Week in Space and Physics: A Rare Hot Neptune
On the origins of a rare exoplanet, a pair of Japanese space missions, Polaris Dawn and burping black holes
All things, the Bible says, came from dust. Modern astronomers may tell you something similar. The Earth, as their version of creation runs, emerged from a cloud of cosmic dust some four billion years ago. The other planets, from Mercury to Jupiter, were born likewise; building themselves up piece by piece from dust grains and pebbles.
This theory is sometimes known as pebble accretion. It envisions planets forming slowly, over millions of years, until they grow big enough to dominate their orbital regions. As a model for the formation of the solar system it seems to work. Physicists have used it to explain how the gas giants grew so large, how the Earth got its water and even how planets have formed around other suns.
Still, this picture occasionally runs into trouble. Take Hot Jupiters, for example, a type of gas giant that orbits close to a star. Many of them swing around their stars in a matter of days, reaching temperatures of thousands of degrees as they do. How they came to be is hard to explain, since growing giant planets close to a star is hard.
Perhaps, some theories run, these planets formed further out, like the gas giants in our solar system did. Only later did they swing inwards, ending up in perilous orbits locked close to their stars. Yet astronomers have found that most Hot Jupiters are large planets, and smaller ones - the size of Neptune, say - are vanishingly rare.
This may be because Hot Neptunes quickly boil away under the heat of a nearby star, whereas Hot Jupiters have the mass to survive for far longer. Those Hot Neptunes we have found seem to be either young - and so have not boiled away yet - or odd in other ways, suggesting unusual origin stories.
In one recent discovery, astronomers reported finding an extremely dense Hot Neptune. This density, they say, means the planet must be mostly solid or liquid; making it a planet more akin to Earth than a gas giant. Such a world would not boil away under the heat of a star - ensuring its survival - but, equally, such a large rocky world should not have formed under the normal rules of planet birth.
The astronomers behind the discovery rule out pebble accretion, arguing that it is not possible for such a big and heavy planet to form in this way. Neither is the planet big enough to have formed through a sudden collapse, like a star would have. Something rarer and far more catastrophic must have happened, they speculate.
Had a group of three gas giants formed close to the star, they say, their orbits would eventually have become unstable. Some of these planets would have swung outwards, sending the solar system into chaos. But one giant could have moved inwards, smashed into several smaller planets, and then ended up in a close orbit around the star.
The result would have been a large, rocky world somewhere close to the size of Neptune, just as seen around this distant star. More observations, of course, will be needed to confirm if this story is true. But the very existence of this planet seems to highlight just how chaotic the process of planet formation can be - and how lucky we are to have ended up in such a stable, well-behaved, solar system.
X-Ray Telescopes and the Moon
Japan last week launched a pair of space missions. The first, XRISM, will become one of the most powerful X-ray telescopes currently operating. The other, SLIM, will test out precision landing techniques on the Moon. If it works, it will also be the first Japanese probe to reach the lunar surface.
That would make Japan the fifth nation to successfully land on the Moon, following India’s touch down in August. SLIM also aims to demonstrate the ability to make precise and autonomous landings on the Moon. Engineers have equipped the probe with advanced cameras and navigation gear to guide its passage to the surface. It should, they hope, land within one hundred meters of a pre-selected point.
The ability to do this, Japanese scientists say, will mean the difference between landing where we want to, and landing where we can. At present, most missions are designed to land in fairly easy terrain, where the risks of encountering unexpected obstacles is low. Yet some of the most interesting areas of the Moon and Mars present more challenging landscapes. If we want to explore them, we will first need to perfect our ability to land spacecraft on faraway worlds.
As for XRISM, astronomers are hoping the new telescope will fill a looming gap in existing and planned X-ray telescopes. Both major X-ray observatories currently in operation - Chandra and XMM-Newton - are now more than two decades old. ATHENA, the next powerful telescope planned, is unlikely to launch before 2035.
Since either Chandra or XMM-Newton is likely to stop working in the next few years, XRISM thus provides a welcome boost for X-ray astronomy. It also carries a microcalorimeter, a long-awaited instrument that precisely measures the energy of X-rays. No working microcalorimeter has ever been operated in space - despite several attempts - but researchers are anticipating exciting results from the one onboard XRISM.
Neither spacecraft will immediately start work, however. SLIM is expected to take around four months to reach the Moon, as it will follow a fuel-efficient but slow orbit. Touch down, if it comes, will be in early 2024. XRISM is likewise expected to take about three months to commission, meaning science operations will not begin before January.
Polaris Dawn Delayed
When Jared Isaacman announced Polaris, an ambitious series of private space missions, in 2022, he was confident of a quick start. The first mission - targeting a high altitude orbit in SpaceX’s Dragon capsule - was planned for the end of 2022. A second flight would follow, possibly involving a visit to Hubble, before a final mission aboard Starship.
That last flight, of course, is dependent on the development of Starship, for which the timeline remains highly uncertain. Yet the first flight, named Polaris Dawn, is also tricky, since Isaacman wants the crew to carry out a spacewalk during the mission. No other private spaceflight has ever been performed, and SpaceX has no previous experience with handling them.
That means SpaceX has had to develop spacesuits for the mission. They will also need to prepare procedures for the spacewalk, an operation that is inherently dangerous. Even worse, the Dragon capsule will need to be depressurised to allow the spacewalk to take place, which means the entire crew will be exposed to the vacuum of space.
Perhaps it is no surprise, then, that the Polaris Dawn mission has been repeatedly delayed. At the end of 2022, Isaacman said it could take place in summer 2023. Now it has once again been pushed to next year. Exactly what is holding things up is unclear - Polaris have given little public statement - but it appears it is taking longer to develop the necessary hardware and carry out training than expected.
Burping Black Holes
Every now and then astronomers spot a star falling into a black hole. These are known as “tidal disruption events”, a nice term for a star being violently ripped apart by the gravity of a black hole. Such events usually create a bright flash of radio waves, which attracts the attention of telescopes.
Now, however, an astronomer has found that tidal disruption events often seem to turn back on after a few months or years. Observations of two dozen such events showed bursts of radio waves reappearing long after the star had been ripped apart.
Why this is happening is unclear. Yvette Cendes, the lead astronomer on the paper, wrote that it cannot be explained by other stars falling into the black hole, nor by other known effects like relativistic jets. New research, she thinks, will be needed to understand exactly what happens to stars in the years after they are torn apart by black holes.