Even as I’ve been writing about the need to map out regions just outside the Solar System, I’ve learned of a new study that finds (admittedly scant) evidence for a Planet 9 candidate. I won’t get into that one today but save it for the next post, as we need to dispose of the New Horizons news first. But it’s exciting that a region extending from the Kuiper Belt to the inner Oort is increasingly under investigation, and the very ‘walls’ of the plasma bubble within which our system resides are slowly becoming defined. And if we do find Planet 9 some time soon, imagine the focus that will bring to this region.
As to New Horizons, there are reasons for building spacecraft that last. The Voyagers may be nearing the end of their lives, but given that they were only thought to be operational for a scant five years, I’d say their 50-year credentials are proven. And because they had the ability to hang in there, they’ve become our first interstellar mission, still reporting data, indispensable. Now we can hope that New Horizons carries on that tradition, since so far it has proven equally robust and remains productive.
Image: This is Figure 1 from the paper we’ll be discussing today (citation below). Caption: The trajectories of the five spacecraft currently leaving the solar system: Pioneer 10 and 11 (orange and light green, respectively), Voyager 1 and 2 (gray and green, respectively), and New Horizons (NH, red) are shown projected onto the plane of the ecliptic, along with several planet orbits (black) and the direction of the flow of interstellar hydrogen atoms (purple arrows). The locations where great-circle scans of interplanetary medium (IPM) Lyα [a uniquely useful spectral line of hydrogen] were made with the NH Alice UV spectrograph are indicated (red), including the all-sky Lyα map described here, which was executed during 2023 September 2–11 at a distance from the Sun of 56.9 au. Credit: Gladstone et al.
To understand the image, we have to talk about ultraviolet Lyman-alpha (Lya) emissions, which happen when an electron in hydrogen transitions from the second energy level (n=2) down to the ground state (n=1). It is the transition that results in the production of a Lyman-alpha photon with a wavelength of about 121.6 nanometers. Since we’ve been talking about the interstellar medium lately, it’s helpful to know that photons in this far-ultraviolet part of the spectrum are absorbed and re-emitted by interstellar gas. They’re useful in the study of star-forming regions and molecular hydrogen clouds.
These emissions are at the heart of a new paper using data from New Horizons that has just appeared from the spacecraft’s team, under the guidance of Randy Gladstone, lead investigator and first author. Says Gladstone:
“Understanding the Lyman-alpha background helps shed light on nearby galactic structures and processes. This research suggests that hot interstellar gas bubbles like the one our solar system is embedded within may actually be regions of enhanced hydrogen gas emissions at a wavelength called Lyman alpha.”
So what we have in these Lyman-alpha emissions is a wavelength of ultraviolet light that is an outstanding tool for the study of the interstellar medium, not to mention our Solar System’s immediate surroundings within the Milky Way. The beauty of having New Horizons in the Kuiper Belt is that along the way to Pluto, the spacecraft was bankrolling Lya emissions with its ultraviolet spectrograph, charmingly dubbed Alice. Developed at SwRI, Alice was turned to the task of surveying Lya activity as the craft continued to travel ever farther from the Sun. One set of scans mapped 83% of the sky.
Image: The SwRI-led NASA New Horizons mission’s extensive observations of Lyman-alpha emissions have resulted in the first-ever map from the galaxy in Lyman-alpha light. This SwRI-developed Alice spectrograph map (in ecliptic coordinates, centered on the direction opposite the Sun) depicts the relatively uniform brightness of the Lyman-alpha background surrounding our heliosphere. The black dots represent approximately 90,000 known UV-bright stars in our galaxy. The north and south galactic poles are indicated (NGP & SGP, respectively), along with the flow direction of the interstellar medium through the solar system (both upstream and downstream). Credit: Courtesy of SwRI.
The method is about what you would assume: The New Horizons team could subtract the Lya activity from the Sun from the rest of the spectrographic data so as to get a read on the rest of the sky at the Lyman-alpha wavelength. What the team has found is a Lya sky about 10 times stronger than was expected from earlier estimates. And now we turn back to the issue of that hot bubble of gas – the Local Bubble – we talked about last time. 300 light years wide, the hot ionized plasma of the bubble was created by supernovae between 10 and 20 million years ago. The Sun resides within the Bubble along with low-density clouds of neutral hydrogen atoms, as we saw yesterday.
New Horizons has charted the emission of Lyman-alpha photons in the shell of the bubble, but the hydrogen ‘wall’ that has been theorized at the edge of the heliosphere, and the nearby cloud structures, show no correlation with the data. It is in the Local Interstellar Medium (LISM) background that the relatively bright and uniform signature of Lya is most apparent, evidently the result of hot, young stars within the Local Bubble shining on its interior walls. But as the authors note, this is currently just a conjecture. Further work from the doughty spacecraft may be in the cards. From the paper:
The NH Alice instrument has been used to obtain the first detailed all-sky map of Lyα emission observed from the outer solar system, where the Galactic and solar contributions to the observed brightness are comparable, and the solar contribution can be reasonably removed….A follow-up NH Alice all-sky Lyα map may be made in the future, if possible, and combining that map with this map could result in a considerable improvement in angular resolution. Finally, the maps presented here were obtained using the Alice spectrograph as a photometer, since its spectral resolution is too coarse to resolve the details of the Lyα line structure. However, there are instruments capable of resolving the Lyα line profile (e.g., J. T. Clarke et al. 1998; S. Hosseini & W. M. Harris 2020) which could possibly study this emission in more detail, and thus (even from Earth orbit) provide a new window on the LISM and H populations in the heliosphere.
Thus we learn something more about the boundary between our system and the interstellar medium in a map that should provide plenty of ground for new investigations at these wavelengths. And we’re reminded of the tenacity of well-built spacecraft and their continuing ability to return solid data well beyond their initial mission plan. Where and when the next interstellar craft gets built remains an open question, but for now New Horizons seems capable of a great deal more exploration.
The paper is Gladstone et al., “The Lyα Sky as Observed by New Horizons at 57 au,” The Astronomical Journal Vol. 169, No. 5 (25 April 2025), 275. Full text.
