A paper published in the Astrophysical Journal examines rogue Neptune-like planets, separated from their stars. These planets are detected using gravitational lensing, rather than occultation, brightening stars as they pass by. Large surveys from wide FOV telescopes capture these unique, short-lived instances over long periods of observation. Astronomers Sam Hadden and Yanqin Wu of the University of Toronto conducted simulations to determine whether these Neptunes were more likely to be ejected or simply detached from their host stars.
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
In this episode, we're going to discuss a paper published in the Astrophysical Journal by Sam Hadden of the Canadian Institute for Theoretical Physics and Yanqin Wu of the University of Toronto. The paper has the terrific title of "Free-Floating or Merely Detached". It's referring to Neptune-sized planets not orbiting stars or so-called rogue planets.
If these planets are not near stars and don't emit their own light, how are they being detected?
The answer is by using gravitational lenses, which are sort of like gravitational magnifying glasses. When something with lots of gravity like a planet or star goes in front of a background object, in this case we're talking about other stars, its gravity can act like a lens, bending light we would not normally receive onto a path where we do see it. The extra light then makes the background star brighter, but since both objects are moving, as everything in the universe is, this alignment is brief — perhaps lasting a few hours. The background star's brightness increases as the alignment gets closer and then decreases back to its regular value as the objects move away from alignment.
Now you might be thinking to yourself, "You recently did an episode on occultations where an object blocked starlight. Why is this one making the star brighter?"
An occultation is by an object in our own solar system — usually an asteroid-like object, so its gravity is too small to lens the background star — and since it's in our own solar system, it effectively has some size. Even though Neptunes and stars are much larger than asteroids, outside of our own solar system they are effectively points. There is also the focal length of the lens to consider, and since objects in our own solar system are so much closer than background stars, the alignment distances don't work for focusing the light. So as points from our distance — presuming one object is roughly twice as close as the other, so they are really far apart and unrelated to each other — a close alignment is extremely rare.
The way we find them is not by looking at individual stars, hoping something goes between us, but by doing extremely large surveys.
Dedicated surveys include OGLE, MOA, and KMTNet. These surveys have very wide field of view telescopes that look at very dense star regions, usually either the center of our galaxy or a local galaxy called the Large Magellanic Cloud. They just stare at these regions night after night, year after year. As a gravitational lens passing a distant background object, lensing surveys can measure the mass of the planet by the strength of the lensing effect, but they don't know the planet's size or its distance from its host star or its orbital period since they don't repeat. Usually the planet's host star also lenses the background star, so while we don't know the orbital period we do know they are together.
OGLE, MOA, and the other lensing surveys have now detected about a dozen Neptune-massed planets where only the planet lenses the background star, and this raises the question, "Are those planets free-floating or merely detached?"
In this case, detached really means having a distant orbit, sufficiently distant that its host star doesn't lens the background star. Prior to the Hadden and Wu article, it has been presumed that the Neptunes are free-floating and that generates issues. Based on the rarity of a lensing alignment, Hadden and Wu calculated that there would be two free-floating Neptunes for every star in our galaxy.
How would these Neptunes become free-floating?
They likely lack the gravity to form directly from an interstellar cloud of gas, which means they must form in a disk around a host star. We also observe that protoplanetary disks — where planets form — rarely extend past 40 astronomical units or AU and it is observationally rare to form planets at distances between 10 to 40 AU. Even though in our own solar system we do have two Neptune-massed planets in that range. So what Hadden and Wu presume is that the planets form in a regular way, but then interact with another planet, either ejecting a Neptune or moving it so far away that even though it still orbits its host star they consider it detached.
To test this, they did computer simulations.
Jupiter in our own solar system has more mass than all the other planets combined and so it would be a natural for scattering Neptune-sized planets. But we also observationally know that Jupiter-sized planets are rare. So Hadden and Wu mostly concentrated on systems with multiple Neptunes. They randomly placed the planets and allowed those systems to evolve for 100 million orbits. What they found were systems where the Neptune became detached, but not wholly separated from its host star, roughly equally with those completely ejected. This would mean that half of the Neptune-only lenses are not really rogue planets, but merely extremely distant from their host star.
Does that solve the too many Neptunes problem?
That is still to be determined.