Astronomers spot a new dwarf planet candidate beyond Pluto

Astronomers have spotted a new dwarf planet candidate in the solar system, beyond the orbit of Pluto, which they provisionally named ED138.

If future observations confirm the discovery, ED138 will join a group currently made up of five dwarf planets that have been recognized by the International Astronomical Union, or IAU: Ceres, Pluto, Haumea, Makemake and Eris.

The finding is the result of detections made by the Vera C. Rubin Observatory, a new telescope that sits atop a 2,682-meter-tall (8,800-foot-tall) mountain in Chile and houses the world’s largest digital camera. The telescope started operating in 2025 and is capable of imaging the entire sky every three days.

NSF–DOE Vera C. Rubin Observatory/NOIRLab/SLAC/AURA/R. Proctor

Matthew Holman, a Smithsonian senior astrophysicist and lecturer in astronomy at Harvard University, ran a computer program he codeveloped to comb through data collected by Rubin earlier in 2026, scanning billions of detections of moving objects. When ED138 popped up, Holman said he first reacted with “excited disbelief,” and immediately checked for a false positive or an error with the computer program.

To confirm the observation, Holman, who is also a Rubin Operations team member, worked with his colleague Mario Jurić, the Rubin Solar System lead scientist and a professor in the department of astronomy at the University of Washington. Jurić looked for more observations of ED138 in Rubin’s catalog, and found 20 additional ones dating back to March 2026. Eventually, using other sources, the researchers were able to plot the orbit of the object as far back as 10 years.

However, Holman warns, ED138 isn’t officially a dwarf planet yet: “At this point, it’s still a candidate. We really need to know how physically big it is, and that will take a few months, probably.”

NSF–DOE Vera C. Rubin Observatory/NOIRLab/SLAC/AURA

ED138’s diameter is estimated to be between 600 and 1,500 kilometers (370 and 930 miles), which would make it bigger than the smallest of the recognized dwarf planets, Ceres. It currently orbits the sun at a distance of about 46 astronomical units — with each unit representing the average distance between Earth and the sun. That means ED138 is farther away than Pluto and also a trans-Neptunian object, or TNO — a class of objects that orbit the sun at a distance greater than Neptune’s.

In the night sky, ED138 is not visible with the naked eye but “is bright enough that really capable amateurs can see it if they know where to look,” according to Holman.

Part of the reason scientists hadn’t discovered ED138 before is because it’s in the southern hemisphere, and therefore very difficult to observe from the northern hemisphere, Holman noted.

“That already rules out whole sets of telescopes that might have found this, and there aren’t as many telescopes searching the sky in the southern hemisphere,” he said.

Another factor that kept ED138 hidden is that it is on the same plane as our galaxy. “For the last 20 years, the object has kind of been looping its way across the Milky Way, from our point of view ,” Holman explained. “People don’t typically search for things in the Milky Way, because there’s so many stars that it’s really hard to find moving objects.”

The new dwarf planet candidate joins the Rubin telescope’s growing list of detected objects — largely asteroids and much smaller TNOs. According to Holman, there might be “a handful” more objects the size of ED138, and Rubin should eventually find those, too.

If ED138 is confirmed as a dwarf planet, astronomers will first give it a permanent number and then an actual name, although the process could take time, and the details of who exactly will get to choose the name are still unclear.

“People have already emailed me some suggestions, and some of them are already taken by other objects,” Holman said. “I don’t know. It’ll be fun. Maybe I’ll get to suggest something, but I’m not going to hold my breath, because this is a big, big project.”

The finding is significant for the field, said Michele Bannister, an associate professor at the University of Canterbury in New Zealand, who was not involved with the discovery of ED138.

The object is “scientifically really cool because it’s a trans-Neptunian object and it’s also physically quite large, but still a lot fainter than you can see with the naked eye,” said Bannister, who studies the small worlds of the solar system.

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The size of the object is just within the range where we can start to think of it as a world, she added, and other telescopes will now pick up the baton to find out more about it.

“Maybe if we ever fly a spacecraft there, we’ll find it has a nitrogen glacier like Pluto,” Bannister said. “That would be great to see. But at the moment, all we know is that it’s big, and the rest is going to be speculation for a little bit, about what its physical characteristics really are.”

天文学家在太阳系中发现了一颗新的矮行星候选者，位于冥王星轨道之外，他们暂时将其命名为 ED138。

如果未来的观测证实了这一发现，ED138 将加入目前由五颗矮行星组成的群体，这五颗矮行星已被国际天文学联合会（IAU）认可：谷神星、冥王星、妊神星、鸟神星和阋神星。

这一发现源于维​​拉·鲁宾天文台的探测。该天文台是一架位于智利一座海拔2682米（8800英尺）山顶的新型望远镜，配备了世界上最大的数码相机。该望远镜于2025年投入使用，能够每三天对整个天空进行一次成像。

美国国家科学基金会-能源部 维拉·C·鲁宾天文台/NOIR实验室/SLAC/AURA/R·普罗克特

史密森学会资深天体物理学家、哈佛大学天文学讲师马修·霍尔曼运行了他参与开发的计算机程序，梳理了鲁宾在2026年初收集的数据，扫描了数十亿条移动物体的探测结果。当ED138出现时，霍尔曼说他最初的反应是“激动得难以置信”，并立即检查是否存在误报或计算机程序错误。

为了验证这一观测结果，霍尔曼（他也是鲁宾天文台运行团队成员）与他的同事马里奥·尤里奇合作。尤里奇是鲁宾太阳系首席科学家，也是华盛顿大学天文系的教授。尤里奇在鲁宾天文台的星表中查找了更多关于ED138的观测记录，并找到了20条新的记录，最早可追溯到2026年3月。最终，研究人员利用其他数据源，绘制出了该天体近10年的轨道轨迹。

然而，霍尔曼警告说，ED138目前还不能被正式认定为矮行星：“目前它仍然是一个候选者。我们确实需要知道它的物理大小，这可能需要几个月的时间。”

美国国家科学基金会-能源部 维拉·C·鲁宾天文台/NOIRLab/SLAC/AURA

ED138的直径估计在600到1500公里（370到930英里）之间，这将使其比已知的最小矮行星谷神星还要大。它目前绕太阳运行的轨道距离约为46个天文单位——每个天文单位代表地球到太阳的平均距离。这意味着ED138比冥王星更远，而且也是一颗海王星外天体（TNO）——这类天体绕太阳运行的轨道距离比海王星更远。

霍尔曼表示，ED138 在夜空中肉眼不可见，但“亮度足够高，如果真正有能力的业余爱好者知道在哪里看，就能看到它”。

霍尔曼指出，科学家之前没有发现 ED138 的部分原因是它位于南半球，因此很难从北半球观测到它。

“这已经排除了所有可能发现这一现象的望远镜，而且南半球的望远镜数量也不多，”他说。

ED138之所以一直难以被发现，另一个原因是它与我们的银河系位于同一平面上。“从我们的角度来看，过去20年来，这个天体一直在银河系中绕圈运行，”霍尔曼解释说。“人们通常不会在银河系中寻找天体，因为那里恒星太多，很难找到移动的物体。”

这颗新发现的矮行星候选者加入了鲁宾望远镜不断增长的已探测天体名单——名单上主要是小行星和更小的海王星外天体。霍尔曼表示，可能还有“少数”与ED138大小相当的天体，鲁宾望远镜最终应该也能发现它们。

如果 ED138 被确认为矮行星，天文学家将首先给它一个永久编号，然后再给它一个正式的名字，尽管这个过程可能需要一些时间，而且究竟由谁来选择名字的细节仍然不清楚。

霍尔曼说：“已经有人给我发邮件提了一些建议，但有些建议已经被其他物品占用了。我不知道。这会很有趣。也许我能提点建议，但我不会抱太大希望，因为这是一个非常庞大的项目。”

新西兰坎特伯雷大学副教授米歇尔·班尼斯特表示，这一发现对该领域意义重大。她并未参与 ED138 的发现。

“这个天体在科学上非常酷，因为它是一个海王星外天体，而且它的体积也相当大，但仍然比肉眼能看到的要暗淡得多，”研究太阳系小型天体的班尼斯特说。

十年来，科学家们一直在激烈争论“第九行星”是否存在。一些人说，证据正在不断积累。

她补充说，这个天体的大小刚好在我们能够将其视为一个世界的范围之内，其他望远镜现在将接过接力棒，进一步了解它。

“也许如果我们将来派探测器去那里，会发现它像冥王星一样有氮气冰川，”班尼斯特说。“那将是一件很棒的事情。但目前，我们只知道它很大，至于它真正的物理特征，还需要一段时间的推测。”