Have Two Lonely Trans-Neptunian Objects Found Each Other?


Who says there’s no romance in the outer solar system? A new study has identified 2004 TT357 as a body that may be made up of two separate objects in contact with each other.

Identification Challenges

A large fraction of both the near and distant small bodies in our solar system are predicted to be in binary systems. When these systems are nearby and have large separations, we can use telescopes like Hubble to observe them and identify the two separate components. But when binaries are far away and have small separations, Hubble doesn’t have the resolution to tell that they’re not a single object.


Comet 67P/Churyumov–Gerasimenko is an example of a suspected contact binary in the inner solar system. [ESA]

Contact binaries are objects consisting of two lobes in contact with each other, like the bi-lobed, peanut-shaped comet 67P/Churyumov–Gerasimenko. These systems occur when two objects gravitate toward each other until the point where they touch.

Though Hubble can’t recognize distant contact binaries because the components are too close together, we can potentially identify them from their characteristic light curves. But this is a challenging process, and so far we’ve only found one confirmed trans-Neptunian-object (TNO) contact binary and one candidate — despite predictions that 10–30% of TNOs could be contact binaries.

Now, new observations from the 4.3-m Lowell Observatory Discovery Channel Telescope, presented in a study led by Audrey Thirouin (Lowell Observatory), have resulted in the identification of a potential new TNO contact binary.

light curve

Light curve for 2004 TT357. The double-peaked light curve (with a rotational period of 7.79 hr) is plotted over two cycles. [Adapted from Thirouin et al. 2017]

Changing Brightness

2004 TT357 is a TNO in our distant solar system orbiting with a semimajor axis of ~55 AU. Thirouin and collaborators’ observations of this object allowed them to construct a light curve for 2004 TT357 that revealed a ~7.8-hour period and a significant amplitude variation. The authors explore three potential causes for this variation: the object has a varying albedo; it has an elongated, ellipsoidal shape; or it is a contact binary.

Asteroids and TNOs tend to have albedo variations of no more than 10–20%. Since 2004 TT357’s light curve varies by ~80%, it seems unlikely that the albedo is the cause of the variability. And while the authors show that we can’t rule out 2004 TT357 having an elongated ellipsoid body, the morphology of the light curve favors the explanation that this object is a contact binary.

A Dense Binary

Hubble observations

Hubble observations of 2004 TT357, which (unsurprisingly, given the distance) reveal no evidence of the object being a binary. [Thirouin et al. 2017]

If 2004 TT357 is a contact binary, what can we learn about it? Its components most likely have a very unequal mass ratio in which one lobe is only 45% the mass of the other, according to the authors’ estimates. Its density could be quite high — 2 g/cm3 — which means it likely has a rocky composition.

Future observations of this system at different observing angles may allow us to confirm whether 2004 TT357 truly is a contact binary. For now, we’ll just have to hold out hope that two lonely TNOs have found each other in the vastness of space.


Audrey Thirouin et al 2017 ApJ 844 135. doi:10.3847/1538-4357/aa7ed3

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