A preliminary orbit for the satellite of dwarf planet (136472) Makemake

Based on archival Hubble Space Telescope observations from 2015 to 2019, this paper presents a preliminary circular orbit for Makemake's satellite with a period of approximately 18 days and an edge-on inclination that suggests the potential for mutual events between the dwarf planet and its moon.

The Gist

Imagine the dwarf planet Makemake as a lonely dancer spinning in the deep, dark ballroom of the Kuiper Belt. For years, astronomers thought it was dancing solo. But in 2015, the Hubble Space Telescope spotted a tiny partner, a moon nicknamed MK2, spinning around it.

However, there was a problem. The discovery team took the photos but never finished the dance choreography. They had the steps, but they hadn't written down the rhythm or the exact path the moon was taking.

This paper is like a detective stepping in to finish the puzzle. Here is the story of what the author, Daniel Bamberger, found, explained in simple terms:

1. The Detective Work: Cleaning Up the Photos

The author went into the "archives" (a digital library of old telescope photos) and found 13 snapshots taken between 2015 and 2019.

• The Problem: In these photos, Makemake is so bright it's like a giant spotlight blinding the camera. The tiny moon, MK2, is like a firefly trying to hide behind that spotlight.
• The Solution: The author used a clever trick. He took all the photos, stacked them on top of each other to create a "ghost" image of just the bright planet, and then subtracted that ghost from the individual photos. Suddenly, the firefly (MK2) popped out clearly in 12 of the 13 pictures.

2. The Dance Floor: Mapping the Orbit

Once the moon was visible, the author had to figure out how it was dancing. He measured where the moon was in each photo and connected the dots.

• The Rhythm: He found the moon completes one full circle around Makemake every 18 days. That's the beat of the dance.
• The Distance: The moon stays about 22,250 kilometers away from the planet. That's roughly the distance from New York to London, but in space!
• The Tilt: This is the most exciting part. The orbit is tilted almost perfectly sideways relative to our view from Earth. Imagine a hula hoop being spun by a dancer. If you look at it from the side, it looks like a flat line. That is what we are seeing here.

3. The Big Surprise: The "Eclipse" Season

Because the orbit is so flat (edge-on), something special is happening right now.

• The Analogy: Imagine two coins spinning on a table. If you look from the side, sometimes one coin will pass directly in front of the other, blocking the light.
• The Event: Because of the flat orbit, MK2 is likely passing in front of (or behind) Makemake right now, or will very soon. These are called mutual events.
• Why it matters: When the moon blocks the planet, or vice versa, the total brightness of the system dips slightly. By watching these dips, astronomers can measure the exact size of both the planet and the moon, and even see if they have dark spots or bright patches on their surfaces. It's like using a shadow puppet show to measure the size of the puppets.

4. What We Learned About the Dancers

By knowing how fast the moon is dancing and how far away it is, the author could calculate how heavy the whole system is.

• The Weight: The combined mass suggests the planet and moon are made of rock and ice, with a density similar to a very dense, icy rock.
• The Timing: The math suggests these "eclipse" events should be happening around 2023 to 2027. Since the last photos were taken a few years ago, we might be in the middle of this special season right now!

The Bottom Line

The author is saying: "Hey, we have all this data, and the math suggests a rare cosmic event is happening now. We need to point our best telescopes (like the James Webb Space Telescope) at Makemake immediately to catch these eclipses before the season ends."

It's a race against time to catch a celestial shadow play that happens only once every few decades.

Technical Summary

1. Problem Statement

The dwarf planet (136472) Makemake was confirmed to possess a satellite, designated S/2015 (136472) 1 (nicknamed MK2), following its discovery in April 2016. However, despite the initial discovery and a subsequent characterization announcement in 2018, the archival Hubble Space Telescope (HST) data collected between April 2015 and February 2019 remained largely unanalyzed for precise orbital determination.

The primary motivation for this study is twofold:

1. Data Utilization: To unlock the potential of existing, unused HST data to constrain the satellite's orbit.
2. Timing Criticality: To determine if the orbital geometry suggests an imminent or ongoing series of mutual events (eclipses and occultations) between Makemake and MK2. Such events are crucial for refining physical parameters (sizes, densities, surface features) but are only observable for a limited window. The paper posits that this window may be occurring between 2023 and 2027, necessitating immediate analysis to coordinate observing campaigns.

2. Methodology

The author performed a re-analysis of archival HST Wide Field Camera 3 (WFC3) data consisting of 13 observation sets taken over a four-year span (2015–2019).

• Image Processing:
  • The author aligned 40-second exposure sets and created median stacks.
  • These stacks were subtracted from individual exposures to remove the glare of the primary body (Makemake), enhancing the visibility of the faint satellite.
  • Twelve of the thirteen processed image sets yielded a visible detection of MK2; one set was discarded because the satellite was too close to the primary to be resolved.
  • Longer exposures (70s and 180s) were excluded from this specific orbital analysis as they were deemed more suitable for photometric characterization.
• Astrometry:
  • Offsets ($\Delta x, \Delta y$) of MK2 relative to Makemake were measured in pixels.
  • These offsets were rotated to account for the HST's orientation relative to celestial north and converted to projected physical distances (km) using the known pixel scale (0.04 arcseconds) and the Earth-Makemake distance at the time of observation.
• Orbital Fitting:
  • The derived positions were fitted to a circular orbit model.
  • The fit assumed a measurement precision of 0.7 pixels in both x and y directions.
  • The analysis assumed the satellite was stationary within each observation set (given the multi-week orbital period, trailing was negligible).

3. Key Contributions

• First Independent Orbit Determination: This paper provides the first published preliminary orbit for MK2 derived from the full set of available archival HST data, filling a gap left since the discovery announcement.
• Identification of Mutual Event Window: By calculating the orbital inclination, the author identifies that the orbit is nearly edge-on, strongly suggesting that a series of mutual events is either currently ongoing or imminent (predicted within a 7 ± 2 year window relative to January 2018).
• Call to Action: The paper serves as a critical alert to the astronomical community to coordinate observing campaigns (potentially using the James Webb Space Telescope) to capture these events before the window closes.

4. Results

The best-fit circular orbit yields the following parameters:

• Orbital Period: $18.023 \pm 0.017$ days.
• Semi-major Axis: $22,250 \pm 780$ km.
• Inclination: $83.7^\circ \pm 1.0^\circ$relative to the line of sight (where$90^\circ$ is perfectly edge-on).
• Orbital Orientation: The direction (prograde vs. retrograde) remains ambiguous based on current data.

Physical Implications:
Assuming Makemake has a radius of $715 \pm 15$ km and the system mass is concentrated in the primary:

• System Mass: $(2.69 \pm 0.20) \times 10^{21}$ kg.
• System Density: $1.76 \pm 0.17$ g/cm³.
  • Note: The author notes that using only the 2018 data available at the time of the original discovery team's work yields a density of $2.1 \pm 0.4$ g/cm³, consistent with Parker et al. (2018). The lower density in this full dataset analysis reflects the updated orbital constraints.

Mutual Events:

• The near-edge-on inclination implies that mutual events (eclipses/occultations) occur twice per orbit.
• The maximum duration of such an event is estimated at approximately 140 minutes.
• These events are comparable in significance to the Pluto-Charon mutual events observed between 1985 and 1990, offering a unique opportunity to map surface features and refine size constraints.

5. Significance

This paper is significant because it transforms a decade-old discovery into an actionable scientific opportunity. By demonstrating that the orbit is nearly edge-on, the author highlights a narrow temporal window (2023–2027) where high-value astrophysical data can be collected.

The results suggest that without immediate follow-up observations (e.g., by JWST or ground-based adaptive optics), the chance to observe these mutual events—and thereby determine the precise densities, sizes, and surface heterogeneity of both Makemake and MK2—may be lost. The paper effectively bridges the gap between archival data mining and active observational planning for Trans-Neptunian Objects.