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The Sun's hot bubble: what eROSITA measured on Tue Oct 29, 2024, and what is still inferred

A paper published on Tuesday, October 29, 2024 used eROSITA X-ray data to study the hot, thin gas around the Sun in the western half of the Galactic sky. The temperatures are measured. The supernova origin, the 14-million-year age and the Centaurus tunnel are not measured by this paper, and the tunnel is only a possible hint.

What is the Local Hot Bubble?

Fact: Our Sun sits in a region of space where the gas is very thin and very hot. Astronomers call it the Local Hot Bubble. Hot gas like this glows in soft X-rays, which are X-rays with low energy. [1]

Fact: how wide it is. STScI says the empty region around Earth is about 1,000 light-years wide. That number comes from the STScI press release about the Local Bubble, not from the eROSITA paper. [5]

Two names, one neighborhood. The Local Bubble is the cavity in the gas and dust around the Sun. The Local Hot Bubble is the hot gas that X-rays show in that space. Yeung and colleagues compare the two. They find the shapes match well in some directions, and not in others. [1] [4]

Why it is hard to see. The Sun's own wind can also make soft X-rays. This is called solar wind charge exchange, or SWCX. For years it made the hot gas hard to measure. The MPE release says scientists now agree that both sources are real. [3]

What happened on Tuesday, October 29, 2024

Fact: the paper came out. On Tuesday, October 29, 2024, the journal Astronomy & Astrophysics published the eROSITA paper online. It appears in volume 690 as article A399. Crossref, the DOI registry, lists that date. [1] [2]

Fact: the same day. The Max Planck Institute for Extraterrestrial Physics (MPE) posted its news release about the paper on Tuesday, October 29, 2024. The free arXiv copy was submitted one day later, on Wednesday, October 30, 2024. [3] [1]

Fact: who did it. A team led by Michael Yeung of MPE used the first eROSITA all-sky survey. eROSITA is an X-ray telescope that watches from an orbit outside Earth's geocorona, the thin outer cloud of gas around Earth. [3] [1]

Fact: peer review. The paper was peer reviewed. It is open access under a CC BY 4.0 license. [1] [2]

Measured: what eROSITA saw

Measured: the sky region. The team studied the western Galactic hemisphere, which is half of the Milky Way's sky. They did not map the whole sky. They split it into about 2,000 regions and studied the X-ray spectrum of each one. [1] [3]

Measured: a cleaner view. The survey ran while the Sun was near the low point of its activity cycle. That means less SWCX from the Sun's wind. The paper models the data without any SWCX part. [1]

Measured: the south is hotter than the north. Far from the flat disk of the Milky Way (more than 30 degrees above or below it), the hot gas in Galactic south has a temperature of about 121.8 electron-volts (eV). Galactic north is about 100.8 eV. These are the paper's fitted values. Our conversion, using 1 eV = 11,604.5 K, gives about 1.41 million K in the south and 1.17 million K in the north. [1]

Measured: hot gas and dust. The strength of the hot-gas glow is higher in directions where there is little dust near the Sun. The paper describes a clear anti-correlation between the two. That means when one goes up, the other goes down. [1]

Measured: closer to the Milky Way's flat disk, it gets hotter. At low latitudes the fitted temperature rises toward the disk, most of all toward the center of the galaxy. The paper says the fits are less secure there. [1]

Inferred: what the data do not show directly

Assumed: the density. X-rays do not give the gas density on their own. The paper assumes a steady density of about 0.004 free electrons per cubic centimeter (4×10⁻³ cm⁻³). This number comes from earlier shadowing studies, in which clouds on the bubble's wall were used to probe the gas. [1]

Derived: the pressure. With that assumption, the average thermal pressure is about 10,100 cm⁻³ K (units of particles per cubic centimeter times kelvin). The paper says this is lower than in typical supernova remnants and wind-blown bubbles. It depends on the density assumption. [1]

Inferred: the shape in 3D. The 3D model of the bubble, reaching farther toward the poles of the galaxy, is also built on the steady-density assumption. [1]

Inferred: supernovae made it. The eROSITA paper cites other work. That work says dozens of supernovae may have made and kept up the bubble. The paper does not test this itself. [1]

Unclear: why south is hotter. The paper says the cause of the north-south difference is not known. A computer model it cites predicts a pattern along a different direction. The authors call their own ideas speculation. [1]

Possible: a hot tunnel toward Centaurus

Possible, not shown: In one spot in the sky, toward the constellation Centaurus, the hot gas looks stronger where dust is thin. The paper says one "can see a hint of an extended tunnel, possibly connecting to the Loop I superbubble." It also calls this "a possible channel filled by hot plasma." [1]

Context: a stronger case elsewhere. The paper calls a different channel, the Canis Majoris tunnel on the flat disk of the Milky Way, "the most convincing evidence" of hot gas filling a channel. Centaurus is a weaker case. It stays a hint, not a result. [1]

Careful: The MPE news release uses stronger words than the paper. It says "discovery" and "potentially joining." We follow the paper. The paper warns that this region sits at the edge of another large X-ray feature, the eROSITA bubbles. That makes the fits harder. It says a closer study is ongoing. We found no result from that study. [1] [3]

Open question: Is Loop I even a real, separate bubble? The paper says its nature or existence has become unclear since the eROSITA bubbles were found. [1]

How old is the bubble? One number per claim

Inferred: about 14 million years. Zucker and colleagues traced the motions of young stars and gas clouds near the Sun. They report a burst of star birth and then supernovae near the bubble's center, "beginning approximately 14 Myr ago." Myr means million years. This is the 2022 Nature study, not the eROSITA paper. eROSITA did not measure this age. [4]

Estimate: how many supernovae. In the STScI release, Zucker says about 15 supernovae have gone off over millions of years to make the bubble we see. [5]

A separate idea about the heat pattern. The MPE release says the north-south temperature pattern could fit computer models of supernovae "in the last few million years." That is a different claim, from a different kind of evidence. [3]

When did the Sun enter the bubble?

Estimate: about 5 million years ago. João Alves, a co-author of the 2022 Nature study, is quoted in the STScI release. He says the Sun's path through the galaxy took it into the bubble about five million years ago. [5]

A different team, a different number. A 2023 computer study by Schulreich and colleagues finds that the Solar System entered the bubble about 4.6 million years ago. We list it as its own claim. We do not average the two. [8]

Coincidence: Gabriele Ponti, an eROSITA co-author, says in the MPE release that the Sun seems near the middle of the bubble purely by chance, as it moves through the Milky Way. [3]

A separate clue: iron-60 on Earth

Measured: iron-60 in deep-sea samples. Iron-60 is a rare form of iron that supernovae make. Wallner and colleagues found it in deep-sea crusts and sediments from several oceans. It arrived in two periods, about 1.5 to 3.2 million years ago and about 6.5 to 8.7 million years ago. They say this points to several supernovae or massive-star events in the last ten million years, up to about 100 parsecs (about 326 light-years) away. [6]

Modeled: where the supernovae were. Breitschwerdt and colleagues modeled the trajectories of the stars. Their work finds two supernovae 90 to 100 parsecs away, one about 2.3 million years ago and one about 1.5 million years ago. These are model results. [7]

Model: a link to the bubble. Schulreich and colleagues ran a computer model of 14 supernovae in a nearby group of stars, the Scorpius-Centaurus association. It matches the iron-60 peak 2 to 3 million years ago. It says the older peak may come from a different, neighboring bubble. [8]

What we do not say: The eROSITA paper has only one short phrase about radioactive atoms in Earth's crust, and it cites the work above. It does not link the bubble to iron-60. This page does not link the bubble to any climate change, extinction or other event on Earth.

Limits and open questions

Half the sky: The eROSITA results are for the western Galactic hemisphere only. [1]

The density assumption: The pressure and the 3D shape depend on one steady density. A different density would change them. [1]

The Sun's wind, again: The 2024 paper leaves a full treatment of the heliospheric SWCX to a later paper. On Thursday, April 16, 2026, Dennerl and colleagues published such a paper in Science. Its title says it determines the Solar System's contribution to the soft X-ray sky. We have not read it. We do not say it confirms or changes the bubble's temperatures. [1] [9]

Poor fits in some places: Toward the inner galaxy, the paper says it is hard to tell instrument flaws from real changes in the gas. [1]

More to come: The authors say a deeper study of the Centaurus region is ongoing. A second paper in the series, about the eROSITA bubbles, was published online on Thursday, October 8, 2026 (A&A 714, A85). We have not read it. [1] [10]

Outside comment: On Saturday, November 14, 2026 we checked the titles of the 26 papers that OpenAlex lists as citing the eROSITA paper. None looked like a rebuttal. We did not read them in full.

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Sources were checked on 2026-11-14. Text on this page is original. Quoted figures are attributed and linked to the source. General education only, not for safety or operational decisions. Disclaimer.